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Versions: 00 draft-ietf-avt-rtp-midi-format

INTERNET-DRAFT                                              John Lazzaro
October 1, 2001                                           John Wawrzynek
Expires: April 1, 2002                                       UC Berkeley



MWPP: A resilient MIDI RTP packetization for network musical performance

                <draft-lazzaro-avt-mwpp-midi-nmp-00.txt>


Status of this Memo


This document is an Internet-Draft and is subject to all provisions of
Section 10 of RFC2026.

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                                Abstract

     This memo describes the MIDI Wire Protocol Packetization (MWPP).
     MWPP is a resilient RTP packetization for the MIDI wire protocol;
     it is specialized for low-latency applications such as network
     musical performance. MWPP defines a multicast-compatible recovery
     system for gracefully handling lost and late packets during the
     performance.

     In a network musical performance system, incoming MWPP streams
     control audio synthesis software running on each network host. This
     software might use the MPEG 4 Structured Audio standard as a
     normative framework for audio synthesis. To support MPEG 4
     Structured Audio in an interoperable fashion, this memo describes
     how to transport MWPP via the generic MPEG 4 RTP packetization.




Lazzaro/Wawrzynek                                               [Page 1]


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1. Introduction

The MIDI standard [1] defines a wire protocol to interconnect electronic
musical instruments into a distributed real-time system, using short
coaxial "MIDI cables" for the physical layer.

When the MIDI standard first came into use in the early 1980s, most
electronic musical instruments used special-purpose analog and digital
circuitry to generate sound. In that era, a typical use of MIDI was to
control the sound synthesizers of several musical instruments from one
piano keyboard. In those MIDI systems, general-purpose computers did not
participate in audio processing; computers were relegated to the tasks
of recording, routing, and playing back MIDI data into the instruments.

Today, personal computers are capable of executing complex sound
synthesis software algorithms in real-time. If a computer has a MIDI
input jack, a musician can use a personal computer as a software-based
musical instrument, by connecting a MIDI controller (such as a piano
keyboard) to the computer. State of the art software synthesizers
provide low total latency (piano key press to speaker cone movement) and
low temporal jitter, and deliver a quality playing experience to the
performer.

If a personal computer acting as a real-time instrument is connected to
the Internet, and if a second similarly-configured computer is also
connected to the Internet at a different location, the two computers
could exchange MIDI data, and turn both local and remote MIDI data into
sound. If the nominal end-to-end latency is sufficiently low, musicians
using these systems can engage in a network musical performance [5].

Note that the programmable nature of software synthesis is essential for
network musical performance. With programmable synthesis, it is easy to
configure each network host to produce identical audio in response to
the same MIDI data stream, creating a sense of telepresence.

This memo describes the MIDI Wire Protocol Packetization (MWPP), a
resilient RTP [4] packetization for the low-latency transmission of the
subset of the MIDI wire protocol that is useful for real-time
performance. In this framework, each network host sends MWPP RTP packets
coding the MIDI events of its local player to remote players, and
receives the MWPP RTP packets of the remote players. Each sender-
receiver transport pair acts as a virtual MIDI extension cable.

Network musical performance systems work well when the nominal total
latency between the participating musicians is reasonably short [5].
This memo does not address the nominal latency issue; we assume a system
using MWPP has a sufficiently low nominal total latency to support the
application.



Lazzaro/Wawrzynek                                               [Page 2]


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MWPP is designed for use over UDP and other unreliable datagram
transport: the design goal is graceful recovery from lost and late UDP
packets, without using packet retransmission. MWPP also supports
reliable transport such as TCP: it includes features to minimize
bandwidth overhead when used with TCP.

Sending the MIDI wire protocol over unreliable transport is not trivial.
The MIDI standard defines a set of commands, that reflect the gestures
musicians make in playing their instruments ("NoteOn" command to start a
new note, "NoteOff" command to end the note, etc).  Gestural commands
make MIDI data streams very compact, but also very fragile: a single
lost "NoteOff" command could result in a sound that sustains
indefinitely long. MWPP defines a recovery system that ensures these
sort of catastrophic command losses do not indefinitely impact a
performance. The MWPP recovery system uses domain knowledge about how
MIDI is used to control software synthesizers.

The recent MPEG 4 Audio standard includes a normative decoder, MPEG 4
Structured Audio [2], that defines a language and run-time environment
for software-based electronic musical instruments. The standard
normatively supports real-time MIDI control of instruments, using a
subset of the MIDI wire protocol command set encoded into real-time
streaming units (midi_event chunks in SA_access_units).

MWPP relates to MPEG 4 Structured Audio in two distinct ways:

  o  MWPP normatively includes the Structured Audio standard [2].
     [2] describes the interaction of a MIDI wire protocol
     command stream and a software synthesizer, and defines the
     subset of the MIDI protocol that is useful for software
     synthesizers. We adopt these conventions through normative
     inclusion, rather than recapitulate them in this memo, and
     use them to define MWPP operation.

  o  Because we adopt Structured Audio MIDI semantics for MWPP,
     the payload of the MWPP is an appropriate resilient coding for
     MPEG 4 Structured Audio. However, to maximize interoperability,
     MPEG 4 Structured Audio streams should use the "RFC-generic"
     MPEG 4 Systems packetization [7], not the RTP packetization for
     MWPP defined in this memo. Therefore, this memo also includes
     a way to use the payload of MWPP together with RFC-generic,
     modeled after the CELP and AAC definitions for RFC-generic
     defined in [8].

The remainder of this memo describes MWPP in detail, and assumes a
working knowledge of the MIDI standard [1] and the MIDI-related sections
of the Structured Audio standard [2].




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Readers unacquainted with [1] and [2] may read Section 6 of [5], which
provide sufficient detail to understand this memo.  See [3] for a
software implementation of MWPP.

The bulk of the memo describes MWPP for RTP transport; in Sections 9 and
12, we describe RFC-generic transport of MWPP for MPEG 4 applications.
However, we refer to semantics of MPEG 4 Structured Audio throughout the
memo, as the normative framework for describing the interaction between
a software synthesizer and the MIDI wire protocol. Rather than refer to
the "software synthesizer," we refer to the "Structured Audio decoder."
We use the normative language of Structured Audio to describe the
interaction between the software synthesizer and the MIDI wire protocol.


2. Sending MWPP RTP packets

This section describes sending MWPP RTP packets. We describe MWPP for
use over unreliable datagram transport without sender proxies.  Reliable
transport and sender proxies are described in Section 10 of this memo.


 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X|  CC   |M|     PT      |        Sequence Number        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           Timestamp                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             SSRC                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             CSRCs                             |
|                              ...                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 1 -- RTP header

An MWPP packet begins with a standard RTP header (Figure 1).  MWPP does
not use header extensions or the marker bit. As is standard, each MWPP
RTP packet sent has its sequence number incremented by one modulo 65536.

MWPP packets encode MIDI commands that are scheduled for execution at a
particular moment in time on the sender's Structured Audio decoder.  The
RTP timestamp field encodes the moment of execution.

The timestamp clock is set by the SDP rtpmap attribute srate (see
Sections 11 and 12 for details). For simple uses of MWPP, this srate
value is identical to the Structured Audio global srate parameter, which
codes the audio sampling rate.  For example, if srate has a value of



Lazzaro/Wawrzynek                                               [Page 4]


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44100Hz, two MWPP packets coding MIDI commands that are executed 2
seconds apart on the sender's SA decoder have RTP header timestamps that
differ by 88200.

The timestamp is a monotonically increasing function of the sequence
number, as expressed in modulo arithmetic.  As is standard in RTP, the
timestamp field is initialized to a randomly chosen value.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|R|    LEN    |          MIDI Command Payload ...             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Recovery Journal ...                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 2 -- MWPP payload


The MWPP payload follows the RTP header. The MWPP payload has two
variable-length sections: a MIDI command section, and a recovery
journal.

The MIDI command section encodes the MIDI commands executed on the
sender's SA decoder at the moment encoded by the RTP timestamp.  The
MIDI command section has a one-octet header, followed by the MIDI
command payload.

The header codes the length (in units of octets) of the data field, via
the 6-bit value LEN. A LEN value of 0 is legal, and codes an empty
payload.  The header also contains two reserved R bits. All reserved
bits in this memo are named R: reserved bits MUST be set to zero by
senders and ignored by receivers.

The MIDI command payload must contain zero or more complete MIDI
commands. The first MIDI command in the MIDI data field MUST have a
status octet, but subsequent commands MAY use the running status data
compression scheme [1].

The MIDI command payload MUST only contain the MIDI commands that are
legally coded in the midi_event chunk of SA_access_unit real-time
streaming units of Structured Audio (i.e. all MIDI commands except the
MIDI System command (0xF)). Note that all commands in the MIDI command
payload are scheduled for the same moment in time; the 320 us/octet
serial delay of a MIDI cable is not emulated.

The MIDI command section of the MWPP payload is followed by the recovery
journal. Information encoded in the recovery journal enables the



Lazzaro/Wawrzynek                                               [Page 5]


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receiver to gracefully recover from the loss of all RTP packets sent
since an earlier RTP packet, called the checkpoint packet.

The growth in size of the recovery journal is limited in two ways.

  o  The recovery journal encodes the minimal session history
     that is needed for recovery, not a trace log of all MIDI
     commands sent since the checkpoint packet.

  o  A sender monitors the "last extended sequence number received"
     field of RTCP RR reports [4], and advance the checkpoint packet
     to reflect the known state of all receivers. This mechanism is
     multicast compatible, and does not require a change in the RTCP
     mechanism as described in [4].

Detailed information about the format of the recovery journal appears in
Section 5.



3. Receiving MWPP RTP packets

In this section, we describe how receivers process RTP MWPP packets.  We
assume that the RTP sender and receiver use a transmission channel with
sufficiently low nominal latency to support the application, but that
transient network disturbances may result in lost packets, and in
packets received with significantly longer latencies that the nominal
latency.

When a new RTP packet arrives, the receiver first examines the timestamp
field, and classifies the packet as either "ontime" or "late." To
perform this classification, the receiver typically maintains a model of
the latency of the channel (see Appendix B of [5] for an example latency
model, that is implemented in [3]).

The receiver then examines the RTP sequence number, and classifies the
situation as:

  o Normal. The extended packet sequence number of new RTP packet is
    one greater than the extended sequence number of the last RTP
    packet number received.

  o Sequence Break. The extended packet sequence number of new RTP packet
    is greater than in the normal case. This classification also applies
    if the new RTP packet is the first RTP packet received in the session.

  o Out of Order. The extended packet sequence number of new RTP packet
    is less than in the normal case.



Lazzaro/Wawrzynek                                               [Page 6]


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The most common occurrence is for packets to be normal/ontime. In this
case, the receiver schedules all commands in the MIDI command section of
the MWPP payload for execution on the local SA decoder.  The details of
scheduling are implementation-specific: simple decoders (such as [3])
may execute MIDI commands as soon as they arrive, but other approaches
are possible.

If a packet is normal/late, the MIDI command section of the MWPP payload
is executed as in the normal/ontime case, except for MIDI NoteOn
commands with non-zero velocity, which are discarded. These semantics
prevent "straggler notes" from disturbing a performance, quiets "soft
stuck notes" immediately, and updates all other MIDI state in an
acceptable way.

If a packet is a sequence break packet, the receiver first processes the
recovery journal section of the payload, as described in Section 5. This
processing may result in the execution of one or more MIDI commands to
gracefully recover from the packet loss.  After processing the recovery
journal, the receiver processes the MIDI command section of a sequence
break packet as if it were a "normal" packet.

If a packet is an out of order packet, its MIDI command and recovery
journal sections are ignored.


4. Sender Addendum: Guard Packets

One shortcoming of MWPP as presented in Sections 2 and 3 is that senders
are not required to maintain a minimum sending rate for RTP packets.
This can cause problems if a packet that encodes a MIDI NoteOff event is
lost. Until another packet is sent, the recovery journal mechanism
cannot function to quiet the stuck note.

MWPP senders may address this problem by sending RTP packets with empty
MIDI command sections at regular intervals: the recovery journal section
of these "guard packets" serves to quiet stuck notes and update the MIDI
state of the receiver's SA decoder. Guard packets also serve to prevent
intermediaries (such as Network Address Translators) from timing out
their services.

The use of guard packets by senders is implementation-dependent (but see
Section 14).


5. The Recovery Journal

The recovery journal section of MWPP RTP packets has a three-level
structure:



Lazzaro/Wawrzynek                                               [Page 7]


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  o Top-level header. Encodes recovery journal structure.

  o Channel journal header. Encodes recovery information for a
    single MIDI channel (a MIDI command executes on one of 16
    MIDI channels).

  o Chapters. Describes recovery information for a single
    MIDI command type. Chapters are specialized to the
    semantics of the command, as defined in [1] and [2].

In this section, we specify the format of the top-level and chapter
journal headers. Subsequent sections describe how senders and receivers
use these headers as part of the recovery system. Appendices describe
the semantics of each journal chapter.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S|A|K|R|TOTCHAN|    Checkpoint Packet Seqnum   | Channels ...  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 3 -- Top-level recovery journal format


Figure 3 shows the top-level structure of the recovery journal.  A
recovery journals consists of a 3-octet header, followed by a list of
channel journals. Channel journals encode recovery information for a
single MIDI channel.

If the A bit is set in the recovery journal header, the recovery journal
is "empty", and contains no channel journals. If the A bit is clear, the
channel journal list contains (TOTCHAN + 1) channel journals.

The recovery journal header includes an S bit. S bits appear on
structures throughout the recovery journal format, with uniform
semantics: if the S bit is set, the structure may be ignored if a
sequence break of exactly one RTP packet triggered the recovery journal
processing.

A set S bit on the recovery journal header indicates the previous sent
packet is a guard packet (lost guard packets can be ignored because
their MIDI command payloads are empty).

The 16-bit Checkpoint Packet Seqnum field codes the sequence number of
the checkpoint packet used by sender to create this journal. If this
sequence number has changed since the last MWPP packet sent, the K bit
is set, else it is clear.




Lazzaro/Wawrzynek                                               [Page 8]


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 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S| CHAN  |R|      LENGTH       |P|W|N|A|T|C|R|R|  Chapters ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 4 -- Channel journal format


Figure 4 shows the structure of a channel journal: a 3-octet header,
followed by a list of leaf elements called chapters. A channel journal
encodes recovery information for commands sent on the MIDI channel coded
by the 4-bit CHAN header field. The 10-bit LENGTH field codes the number
of octets in the channel journal, including the header.

The third octet of the channel journal header is the Table of Contents
(TOC) of the channel journal. The TOC is a set of bits to encode the
presence of a chapter in the journal. Each chapter contains information
to recover from the loss of a certain class of MIDI commands:

  o  Chapter P: MIDI Program Change (0xC)
  o  Chapter W: MIDI Pitch Wheel (0xE)
  o  Chapter N: MIDI NoteOff (0x8), NoteOn (0x9)
  o  Chapter A: MIDI Poly Aftertouch (0xA)
  o  Chapter T: MIDI Channel Aftertouch (0xD)
  o  Chapter C: MIDI Control Change (0xB)

Chapters appear in a list following the header, in order of their
appearance in the TOC. The Appendices of this memo describe the format
of each chapter, and explain how senders and receivers use each chapter.


6. Sending Recovery Journals

In this section of the memo, we briefly describe how senders create
recovery journals.

Senders maintain state about MIDI commands sent since the last
checkpoint packet, using a data structure that records the RTP sequence
number associated with each MIDI command (see [3] for a sample
implementation). We refer to this data structure as the "recovery
journal data structure" below.

To send a new MWPP packet, the sender first creates the MIDI command
section of the packet. Then, the sender traverses the recovery journal
data structure to build the new recovery journal. Typically, the data
structure elements match the structure of recovery journal chapters and
headers, so that simple memory copies act to build a new journal.



Lazzaro/Wawrzynek                                               [Page 9]


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Timing-sensitive chapter data (such as the Y bits of Chapter N, as
explained in Appendix A.3) are updated during the build process.

After the recovery journal is created, the sender inserts the MIDI
commands encoded in the MIDI command section of the new packet into the
recovery journal data structure.  Data structure elements corresponding
to the S bits of the recovery journal are updated during this insertion.

The reception of an RTCP RR packet may also result in an update of the
recovery journal data structure. The sender first examines the "last
extended sequence number received" field of the received RTCP RR packet,
and combines it with the RTCP RR data from other receivers in the
session.  If the sender determines checkpoint packet may be safely
updated from its current value, it traverses the recovery journal data
structure, pruning MIDI data wherever possible, in order to reduce the
size of future recovery journals.


7. Receiving Recovery Journals

In this section, we briefly describe how receivers parse recovery
journals.

In the case of a loss of a single RTP packet, the receiver uses the S
bits of the recovery journal to skip over channels and chapters which do
not encode information about the lost packet. For each chapter whose S
bit is clear, the receiver executes the chapter-specific recovery
algorithm described in the Appendices.

In the case of a multi-packet loss, the S bits are ignored, and each
chapter of each channel journal undergoes the recovery procedure
described in the Appendices.

The recovery algorithms for most chapters require information about MIDI
commands received in previous RTP packets. See [3] for a sample
implementation of data structures to efficiently maintain this state.

Note that receivers MUST use the LENGTH field of the chapter journal
header to traverse from chapter to chapter, and not rely on the sizes of
each chapter journal. This restriction is needed for backward
compatibility, as the R bits in the TOC may be used for new chapters in
future versions of MWPP.









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8. MWPP Startup Issues

The recovery journal mechanism depends on senders tracking the status of
receivers, by:

  o  Knowing the first MWPP RTP packet sent to a new receiver.
  o  Examining the "last extended sequence number received" field
     of RTCP RR reports.

In simple unicast applications, both mechanisms work well. In more
complex situations, such as true or simulated multicast transport,
senders may not know of the presence of a receiver until the first RTCP
packet arrives. In this case, lost packets early in a session may not be
protected by the recovery journal mechanism, because the sender
"incorrectly" moved the checkpoint packet.

Receivers can detect this situation by using the Checkpoint Packet
Seqnum field coded in the recovery journal header, as shown in Figure 3.
Note that this technique requires that receivers examine the recovery
journal of every MWPP packet received, although the K bit that marks
checkpoint updates minimizes the work per packet required.

To detect this situation, the receiver examines the Checkpoint Packet
Seqnum, and checks to see if it is consistent with the reception history
of the receiver. If an inconsistency is detected, receivers should
assume that the sender is not aware of its existence, and take
precautions to ensure that catastrophic MIDI errors do not occur (for
example, NoteOn commands could be "timed out" with a matching NoteOff
command after a suitably long period of time).


9. MWPP Transport over MPEG 4 "RFC-generic" RTP Packetization

MWPP, as described in this memo, is a stand-alone RTP packetization for
the MIDI wire protocol. Section 2 describes the packet format of MWPP
RTP packets: a standard RTP header (Figure 1) followed by the MWPP
payload (Figure 2).

For maximum interoperability, MPEG 4 Structured Audio systems should not
use this stand-alone RTP packetization. Instead, the generic RTP
packetization for MPEG 4 described in [7] ("RFC-generic") should be
used.

In this section, we describe how to incorporate the MWPP payload into
RFC-generic. In Section 12, we describe how to configure RFC-generic to
support MWPP. This section borrows heavily from the MPEG 4 Audio AAC and
CELP work described in [8].




Lazzaro/Wawrzynek                                              [Page 11]


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          +---------+-----------+-----------+---------------+
          | RTP     | AU Header | Auxiliary | Access Unit   |
          | Header  | Section   | Section   | Data Section  |
          +---------+-----------+-----------+---------------+

                    <----------RTP Packet Payload----------->

Figure 5: Data sections within an RTP packet (from [8], describing [7]).


Figure 5 shows the RFC-generic RTP packet, consisting of a standard RTP
header (Figure 1) followed by the RFC-generic payload.

The main purpose of an RFC-generic packet is to carry MPEG 4 Access
Units; this data is held in the final section of the payload (Access
Unit Data Section). The AU Header Section describes the way that Access
Units are packed into the Access Unit Data Section: for example, the
data section may contain multiple Access Units, or a fragment of a
single Access Unit. The RFC-generic packet may also contain ancillary
data, in the Auxiliary Section.

To incorporate MWPP into RFC-generic, we consider the payload of MWPP
(Figure 2) to be the Access Unit. We place exactly one MWPP Access Unit
into the Access Unit Data Section of each RFC-generic RTP packet; the
MWPP Access Unit is never fragmented. The AU Header Section and
Auxiliary Section are both always empty.

The RTP Header section of the RFC-generic packet is essentially the same
as the RTP header for MWPP described in Section 2, with several
exceptions:

  o  The marker bit is always set to 1, indicating a complete MWPP
     Access Unit in the Access Unit Data Section.

  o  A random offset for the Timestamp field should be avoided.


10. Reliable Transport and Proxies

The recovery journal adds significant overhead to MWPP. When sending
MWPP over reliable transport (TCP, or a point-to-point reliable IP link)
the recovery journal section of MWPP packets may be safely deleted
without affecting the proper operation of the system.

MWPP recovery journals may also be safely deleted if the SAOL program
running on the Structured Audio decoders uses application-layer recovery
techniques that make the MWPP recovery journal scheme redundant.




Lazzaro/Wawrzynek                                              [Page 12]


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Senders and receivers MUST use the Session Description Protocol (SDP)
[6] mechanism described in Sections 11 and 12 to indicate that an MWPP
session does not use the recovery journal mechanism.

MWPP packets without recovery journals are also used in association with
sender and receiver proxies. Sender and receiver proxies are used when
Structured Audio clients are running on thin clients, such as electronic
piano keyboards. These keyboards may have special-purpose audio
processing hardware for Structured Audio decoding, but may have simple
general-purpose processors that cannot handle the overhead of recovery
journal send and receive operations.

If the thin client has a reliable channel to a suitable host, sender and
receiver proxies may be used to offload the recovery journal processing
task. In this scheme, the thin client would send and receive MWPP
packets without recovery journals to the proxies. The sending proxy
would add recovery journals to outgoing packets, and the receiving proxy
would handle the lost and late packet processing described in Section 3.
The sender and receiver proxies MUST also handle the RTCP duties for the
thin client, because the thin client is not able to compute transport
statistics correctly.



11. Session Description Protocol

This section describes Session Description Protocol (SDP) [6]
definitions for MWPP transport directly over RTP. Section 12 describes
the SDP definitions for MWPP transport over the MPEG 4 "RFC-generic" RTP
packetization.

The MIME name for this packetization is mwpp. The SDP rtpmap attribute
is declared as

a=rtpmap: <payload> mwpp/<srate>/<krate>/<rj>

The <srate> parameter codes the audio sampling rate used for the RTP
timestamp field. Typically, this value corresponds to the srate global
parameter value of the SAOL program (see [2] subpart 5.8.5.2.1). We
specify <srate> in the rtpmap so that musicians can choose a different
local srate value without disturbing the MWPP system.

The <krate> parameter codes the control sampling rate, which typically
corresponds to the krate global parameter value of the SAOL program (see
[2] subpart 5.8.5.2.2). This memo does not refer to the krate value; we
include it in the rtpmap for possible future use, since like srate,
musicians may wish to choose a different local krate value.




Lazzaro/Wawrzynek                                              [Page 13]


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The <rj> parameter codes the presence or absence of recovery journals in
MWPP packets (see Section 10 for details). The two valid values for <rj>
are "rj" and "no-rj". If the <rj> parameter does not exist, its value is
assumed to be "rj".

For example, the following lines bind the packetization to dynamic
payload number 96, and specifies an srate of 44100 Hz, a krate of 1260
Hz, and the presence of a recovery journal in each RTP packet:

m=audio 5004 RTP/AVP 96
c=IN IP4 171.64.92.160
a=rtpmap: 96 mwpp/44100/1260/rj

Note that the packetization does not directly support multiple
16-channel MIDI Input sources. Different UDP ports should be used in
this case, each devoted to a single source:

m=audio 5004 RTP/AVP 96
c=IN IP4 171.64.92.160
a=rtpmap: 96 mwpp/44100/1260/rj
m=audio 5006 RTP/AVP 97
c=IN IP4 171.64.92.160
a=rtpmap: 97 mwpp/44100/1260/rj

Note that the SDP does not include a binary encoding of the SAOL program
to run on the decoder (StructuredAudioSpecificConfig). This memo assumes
that StructuredAudioSpecificConfig is sent out-of-band.

Finally, note that MWPP is self-framing, and so TCP transport is
possible without explicit framing.


12. Session Description Protocol and MPEG 4 "RFC-generic" transport

This section describes Session Description Protocol (SDP) [6]
definitions for MWPP transport over the MPEG 4 "RFC-generic" RTP
packetization.

The MIME name for this packetization is mpeg-generic. The SDP rtpmap
attribute is declared as

a=rtpmap: <payload> mpeg-generic/<srate>/<krate>/<rj>

The definitions of srate, krate, and rj are identical to the
descriptions in Section 11. Note that srate functions as the RTP clock.






Lazzaro/Wawrzynek                                              [Page 14]


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The SDP fmpt command configures RFC-generic for MWPP transport, as shown
below:

a=fmpt: <payload> streamtype=5; profile-level-id=15; mode=SA-mwpp;

To signal SingleSL mode, we omit the ConstantSize and SizeLength format
parameters from the fmpt command.  The StructuredAudioSpecificConfig is
sent by other means, and so AudioSpecificConfig() is not used. The
values for streamtype and profile-level-id are tentative, pending a
check of the relevant standards documents.


13. Security Considerations

Cryptographic authentication of incoming RTP and RTCP packets is highly
recommended when using MWPP. Without such protections, attackers could
forge MIDI commands into an ongoing session, potentially damaging
speakers and eardrums. An attacker could also craft RTP and RTCP packets
to exploit known bugs in the client, and take effective control of a
client machine.


14. Congestion Control

MWPP has congestion control issues that are unique for an RTP audio
packetization. When used for network musical performance, the packet
rate is linked to the gestural rate of a human performer.

MWPP implementations SHOULD sense the MIDI stream for command patterns
that result in excessive packet rates, and filter these streams as part
of MWPP to reduce the packet rate.

In addition, the guard packet mechanism described in Section 4 of this
memo is a possible source of congestion control problems. Implementers
MUST ensure that the guard packet strategies of senders are well behaved
with respect to congestion control.


Appendix A.1. Chapter P: MIDI Program Change

Chapter P protects against the loss of MIDI Program Change commands,
which the Structured Audio standard uses to bind SAOL instruments to
MIDI channels. If a Program Change command is lost, notes played on a
channel will sound with the incorrect timbre, or perhaps not sound at
all.

To prepare for recovery, the receiver should store state for each
channel, to indicate the program value of the last Program Change



Lazzaro/Wawrzynek                                              [Page 15]


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command received on this channel and the Bank Select values (Coarse and
Fine) that were in effect at the time this Program Change command
executed. The Bank Select values are issued via the MIDI Control Change
command, and act to extend the range of program values. The stored state
should also include flag bits to signify the null cases of no Program
Change received, no Bank Select Coarse value received, and no Bank
Select Fine value received.


The encoding for Chapter P is shown below:

 0                   1                   2
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S|   PROGRAM   |C| BANK-COARSE |F| BANK-FINE   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The chapter has a fixed size of 24 bits. If the S bit is set to 1, the
previous packet sent did not include a Program Change command on this
channel, and the receiver can skip to the next Chapter if it is
recovering from the loss of a single packet.

The PROGRAM field indicates the program value of the last Program Change
command sent on this channel. If a Control Change command for the Bank
Select Coarse controller was sent before this Program Change command,
the C bit is set to 1, and the BANK-COARSE field is the Bank Select
Coarse controller value that was sent.  The F bit and BANK-FINE field
code the Bank Select Fine value in the same manner.

The receiver should compare the values in Chapter P with the stored
state for this channel, to determine if one or more Program Change
commands were lost. If a loss is detected, the receiver should execute
the Program Change command coded in Chapter P, and update its own
recovery state.


Appendix A.2. Chapter W: MIDI Pitch Wheel

Chapter W protects against the loss of MIDI Pitch Wheel commands. A
common use of the Pitch Wheel command is to transmit the current
position of a "pitch wheel" controller placed on the side of piano
controllers, which players can use to dynamically alter the pitch of all
depressed keys.

Structured Audio makes the current value of the Pitch Wheel available to
SAOL programmers in the MIDIWheel standard name, which programmers
typically use for continuous modification of instrument models, in a
manner similar in spirit to the original pitch bend semantics of the



Lazzaro/Wawrzynek                                              [Page 16]


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controller.  The recovery mechanisms in Chapter W are designed to
protect the Pitch Wheel data stream for these types of SAOL programs.

To prepare for recovery, the receiver should store state for each
channel, that codes the wheel value for the last Pitch Wheel command
received, along with a flag bit to signify the null case of no Pitch
Wheel command received.

The encoding for Chapter W is shown below:

 0                   1
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S|     FIRST   |R|    SECOND   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The chapter has a fixed size of 16 bits. If the S bit is set to 1, the
previous packet sent did not include a Pitch Wheel command on this
channel, and the receiver can skip to the next Chapter if it is
recovering from the loss of a single packet.

The FIRST and SECOND fields are the 7-bit values of the first and second
data bytes of the last Pitch Wheel command sent on this channel. The R
bit is reserved.

The receiver should compare the FIRST and SECOND fields in Chapter W
with the stored pitch wheel state for this channel.  If no difference is
detected, Pitch Wheel commands may still have been lost, but any
artifacts induced are transient in nature, and the receiver SHOULD take
no action.

If a difference is detected, the receiver should update its recovery
state to reflect the values of the FIRST and SECOND fields. In addition,
the receiver MAY execute a single Pitch Wheel command, or MAY plan a
series of Pitch Wheel commands spaced over time.


Appendix A.3. Chapter N: MIDI NoteOff and NoteOn

Chapter N protects against the loss of MIDI NoteOn commands, which
Structured Audio uses to launch new instrument instances, and MIDI
NoteOff commands, which Structured Audio uses to schedule instances for
termination.  If a NoteOn command is lost, notes are skipped, a
transient error. If a NoteOff command is lost, notes may sound
indefinitely, an error that may be catastrophic for sustained timbres.

Structured Audio ignores the velocity field of the NoteOff command, and
Chapter N does not protect this field. In the discussion below, our



Lazzaro/Wawrzynek                                              [Page 17]


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references to NoteOff commands include NoteOn commands with zero
velocity, which have semantics identical to NoteOff commands in
Structured Audio. Our references to NoteOn commands refer to NoteOn
commands with non-zero velocity only.

To prepare for recovery, the receiver should maintain state for each
note number for an active channel. The recovery algorithms in this
section references receiver recovery state variables, using the
following nomenclature:

  vel   This variable is initialized to zero at the start of a
        session. If a NoteOn command is executed for this note,
        vel is set to the velocity value of the command. If a
        NoteOff command is is executed for this note, vel is
        set to zero.

  seq   Whenever a NoteOn or NoteOff command executes, seq is
        set to the extended sequence number of the RTP packet
        whose parsing resulted in execution of the command.

  time  Time of the last NoteOn or NoteOff command, in the
        internal time units used by the application.


The encoding for Chapter N is shown below:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 8 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|B|   LENGTH    |  LOW  | HIGH  |S|   NOTENUM   |Y|  VELOCITY   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S|   NOTENUM   |Y|  VELOCITY   | ....                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   BITFIELD    |   BITFIELD    |     ....      |   BITFIELD    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The chapter consists of a 2-byte header, followed by a list of 16-bit
note logs, followed by a list of bitfields. A note number is represented
in a note log or a bitfield if it has been used in a NoteOff or NoteOn
command since the last checkpoint packet.

If the note number last appeared in a NoteOn command, it appears in a
note log; if the note number last appeared in a NoteOff command, it
appears in a bitfield.

The 7-bit LENGTH field codes the number of note logs; zero is a valid
value, and codes an empty note log. The maximum number of note logs is
127; in the musically unlikely case of 128 concurrent NoteOn commands,



Lazzaro/Wawrzynek                                              [Page 18]


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one NoteOn command is unprotected, risking a transient (not
catastrophic) error on one note number.

The 4-bit fields LOW and HIGH determine the number of bitfield bytes
that follow the note logs. A bitfield byte codes NoteOff information for
eight consecutive MIDI note numbers, with the MSB representing the
lowest note number. A 1 in a bit position indicates a NoteOff command
has occurred for this note number since the last checkpoint packet, and
that this NoteOff command occurred more recently than a NoteOn command.

The MSB of the first bitfield byte codes the note number 16*LOW, while
the MSB of the last bitfield byte codes the note number 16*HIGH. If LOW
is less that or equal to HIGH, there are (HIGH - LOW + 1) bitfield bytes
in the chapter. To code a chapter with no bitfield bytes, senders MUST
set LOW to 15 and HIGH to 0.

If the B bit is set to 1, the previous packet sent did not include a
NoteOff command on this channel, and the receiver can skip parsing the
bitfield section of the chapter if it is recovering from the loss of a
single packet. To skip over a Chapter N, the receiver calculates the
chapter length based on the values of LENGTH, LOW, and HIGH.

We now explain note log encoding, reproduced for reference below:

 0                   1
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S|   NOTENUM   |Y|  VELOCITY   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

A note log will exist for a note number (coded by the 7-bit NOTENUM
field) if a NoteOn command has occurred for this note number since the
last checkpoint packet, and that this NoteOn command occurred more
recently than a NoteOff command. The 7-bit VELOCITY field codes the
velocity value for this NoteOn command; this field MUST not be zero.

If the S bit in a note log is 1, the previous packet sent did not
include a NoteOn command for this note number, and the receiver can skip
parsing this note log if it is recovering from the loss of a single
packet.

The Y bit helps receivers to make the "play or skip" decision for
recovered NoteOn events. Senders set Y to 1 for recovery packets sent
shortly after the arrival of a NoteOn command from a MIDI controller;
subsequent recovery packets are sent with Y = 0.






Lazzaro/Wawrzynek                                              [Page 19]


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The tables below summarizes the recovery algorithm for Chapter N.  For
each set bitfield position, the receiver executes a strategy in the
table column indicated by the recovery state variable vel for the note
number:

|--------------------------------------------------------------------
|    vel   |  Diagnosis              Suggested recovery             |
|--------------------------------------------------------------------
|   zero   | Either no note events | Do nothing.                    |
|          | have been lost, or a  |                                |
|          | series of NoteOn ->   | Update seq to the              |
|          | NoteOff events have   | current packet number.         |
|          | been lost.            |                                |
|--------------------------------------------------------------------
| non-zero | Either one NoteOff was| Execute a NoteOff              |
|          | lost or a series of   | command to end the             |
|          | NoteOff->On->Offs     | current note.                  |
|          | were lost.            |                                |
|          |                       | Update velocity to 0,          |
|          |                       | update seq to the current      |
|          |                       | packet number.                 |
|--------------------------------------------------------------------





























Lazzaro/Wawrzynek                                              [Page 20]


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For each note log, the receiver executes the strategy in the table
column the recovery state variable vel for the note number:


|--------------------------------------------------------------------
|    vel   |  Diagnosis              Suggested recovery             |
|--------------------------------------------------------------------
|   zero   | Either one NoteOn was | If Y = 0, never play           |
|          | lost, or a series of  | the new note. If Y = 1,        |
|          | NoteOn->Off->On.      | play a new note if an          |
|          |                       | analysis of the current        |
|          |                       | packet timestamp and the       |
|          |                       | estimated delay from the       |
|          |                       | sender shows the current       |
|          |                       | packet is reasonably on        |
|          |                       | time.                          |
|          |                       |                                |
|          |                       | Update vel to value            |
|          |                       | VELOCITY, and seq to           |
|          |                       | the current packet number.     |
|--------------------------------------------------------------------
| non-zero | Either no note events | Do one of:                     |
|          | have been lost, or a  |                                |
|          | series of NoteOff->   | [1] Leave the current note     |
|          | NoteOn events have    |     (with velocity vel)        |
|          | been lost.            |     playing.                   |
|          |                       |                                |
|          |                       | [2] End the current            |
|          |                       |     note but do not            |
|          |                       |     start a second note.       |
|          |                       |                                |
|          |                       | [3] End the current note       |
|          |                       |     and start a second         |
|          |                       |     note with velocity         |
|          |                       |     VELOCITY.                  |
|          |                       |                                |
|          |                       | Based on the values of         |
|          |                       | vel, VELOCITY, Y,              |
|          |                       | seq, the current packet        |
|          |                       | number, and the current        |
|          |                       | checkpoint packet number.      |
|          |                       |                                |
|          |                       | Update vel to value            |
|          |                       | VELOCITY, and seq to           |
|          |                       | the current packet number.     |
|--------------------------------------------------------------------





Lazzaro/Wawrzynek                                              [Page 21]


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The second entry is this table is complex, due to the ambiguity of
situation. A series of simple tests resolves the issue in most cases:

   TEST 1: (seq < checkpoint packet sequence number)

   If the value of seq is less than the current checkpoint packet
   sequence number, recovery is simple, since we know that the last
   NoteOn event executed is not a part of the note log, and so we know
   that a series of one or more NoteOff->NoteOn events have been lost.
   In this case, the current note should always be ended, and the
   execution of a new note should occur using the same criteria we use
   in the second entry of the table above. If seq indicates that the
   last NoteOn executed did occur during the current note log, our task
   is more difficult.

   TEST 2: (vel != VELOCITY)

   If vel doesn't equal VELOCITY, we know that the NoteOff corresponding
   to the last NoteOn executed was lost. In this case, the current note
   should always be ended, and the execution of a new note should occur
   using the same criteria we use in the second entry of the table
   above.

These two tests leave the (vel == VELOCITY) case to consider. This case
is not a rare event, since many MIDI devices do not implement velocity
sensing, and generate all NoteOn's with the same velocity value.

    TEST 3: (Y == 1)

    If Y is 1, the NoteOn in the note log occurred recently. If the
    receiver executed the last NoteOn recently (which was can tell by
    the time value for the last executed note), we know the note log
    represents the last executed note, and the correct action is to
    let the note continue to play. If the last note was not recently
    executed, it should be terminated, and an execution of a new note
    should occur using the same criteria we use in the second entry of
    the table above.

These three tests leave the following case unresolved: Y == 0 (the
NoteOn in the note log didn't occur recently) and vel == VELOCITY (the
last executed note and note log note are ambiguous).  In this case,
letting the last executed note continue to play, to be turned off by a
forthcoming NoteOff command, is an acceptable result.








Lazzaro/Wawrzynek                                              [Page 22]


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Appendix A.4. Chapter A: MIDI Poly Aftertouch

Chapter A protects against the loss of MIDI Poly Aftertouch commands.
This command supports piano keyboard controllers that have individual
pressure sensors under each key, that generate a continuous signal
whenever the key is depressed. Keyboard controllers that include these
sensors send a stream of Poly Aftertouch commands for the duration of
each key event. Because multiple keys may be down at once, the Poly
Aftertouch command specifies a note number (0-127) as well as a pressure
value (0-127).

SAOL programmers may access the last aftertouch value for each MIDI note
in the MIDItouch[128] standard name array. Programmers typically use
MIDItouch[] for continuous modification of instrument models parameters.
The recovery mechanisms in Chapter A are designed to protect the
aftertouch data stream for these types of SAOL programs.

To prepare for recovery, the receiver should store state for each note
on each channel, that codes the pressure value of the last Poly
Aftertouch command received, along with a flag bit to signify the null
case of no Poly Aftertouch command received.

The encoding for Chapter A is shown below:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 8 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S|  LENGTH     |F|   NOTENUM   |R|  PRESSURE   |F|   NOTENUM   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|  PRESSURE   |  ....                                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The chapter consists of a 1-byte header followed by a list of 16-bit
note logs. Note logs exist for note numbers whose pressure value has
been changed by a Poly Aftertouch command since the last checkpoint
packet, and let receivers recover from the loss of those commands. Only
one note log may exist in the note list for a particular note number.

The 7-bit LENGTH field codes the number of note logs minus one; the
expression (1 + 2*(LENGTH + 1)) yields the number of bytes in the
chapter. The maximum chapter length of 257 bytes protects the worst-case
situation of Poly Aftertouch commands occurring for all 128 MIDI notes
since the last checkpoint packet.

If the S bit is set to 1, the previous packet sent did not include a
Poly Aftertouch command on this channel, and the receiver can skip to
the next Chapter if it is recovering from the loss of a single packet.




Lazzaro/Wawrzynek                                              [Page 23]


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For each note log, the 7-bit NOTENUM field identifies the MIDI note
number of the log, and the 7-bit PRESSURE field indicates the pressure
value of the last Poly Aftertouch command sent.

If the F bit is set to 1, the previous packet sent did not include a
Poly Aftertouch for this note, and the receiver can skip to the next
note log if it is recovering from the loss of a single packet.

If the F bit is 0, the receiver should compare the PRESSURE value with
the stored pressure value for the note; if these values are different,
the receiver should update its recovery state to reflect the value of
the PRESSURE field.  In addition, the receiver MAY execute a single Poly
Aftertouch command, or MAY plan a series of Poly Aftertouch commands
spaced over time.


Appendix A.5. Chapter T: MIDI Channel Aftertouch

Chapter T protects against the loss of MIDI Channel Aftertouch commands.
This command supports piano keyboard controllers that use a single
pressure sensor for the entire keyboard. Keyboard controllers that
include this sensor send a stream of Channel Aftertouch commands
whenever at least one key is depressed. Unlike the Poly Aftertouch
command, the Channel Aftertouch command does not specify a note number,
only a pressure value (0-127).

Structured Audio makes the pressure value of the last Channel Aftertouch
command available to SAOL programmers in all array positions of the
MIDItouch[128] standard name array, which programmers typically use for
continuous modification of instrument models parameters. The recovery
mechanisms in Chapter T are designed to protect the aftertouch data
stream for these types of SAOL programs.

To prepare for recovery, the receiver should store state for each
channel, that codes the pressure value for the last Channel Aftertouch
command received, along with a flag bit to signify the null case of no
Channel Aftertouch command received.

SAOL programmers may access the last aftertouch value received via in
the MIDItouch[128] standard name array; all array positions contain the
same value. Programmers typically use MIDItouch[] for continuous
modification of instrument models parameters. The recovery mechanisms in
Chapter T are designed to protect the aftertouch data stream for these
types of SAOL programs.







Lazzaro/Wawrzynek                                              [Page 24]


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The encoding for Chapter T is shown below:

 0
 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|S|   PRESSURE  |
+-+-+-+-+-+-+-+-+

The chapter has a fixed size of 8 bits. If the S bit is set to 1, the
previous packet sent did not include a Channel Aftertouch command on
this channel, and the receiver can skip to the next Chapter if it is
recovering from the loss of a single packet.

The 7-bit PRESSURE field indicates the pressure value of the last
Channel Aftertouch command sent. The receiver should compare the
PRESSURE value with the stored pressure value for the note; if these
values are different,the receiver should update its recovery state to
reflect the value of the PRESSURE field.  In addition, the receiver MAY
execute a single Channel Aftertouch command, or MAY plan a series of
Channel Aftertouch commands spaced over time.


Appendix A.6. Chapter C: MIDI Control Change

Chapter C protects against the loss of MIDI Control Change commands.  A
Control Change command alters the 7-bit value of one of the 128 MIDI
controllers. Most MIDI controllers are meant to be used as continuous
parameters (for example, controller 7 is the Main Volume control), but
some parameters have special semantics.

Structured Audio makes the current value of MIDI controllers available
to SAOL programmers in the MIDIctrl[128] standard name array, that
programmers typically use for continuous modification of instrument
model parameters. The recovery mechanisms in Chapter C are designed to
protect the Control Change data stream for these types of SAOL programs.

In addition, a Structured Audio decoder implements special semantics for
the Bank Select Coarse and Fine, Sustain Pedal, All Notes Off, and All
Sound Off controllers. The recovery mechanism in Chapter C, in
conjunction with Chapter P, protects the special semantics of these five
controllers.

To prepare for recovery, the receiver should store state for each
channel about the Control Change data stream. For the All Notes Off and
All Sound Off controllers, the receiver should keep a count, module 128,
of the total number of Control Change commands received.





Lazzaro/Wawrzynek                                              [Page 25]


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For all other controllers, the receiver should store the value of the
last Control Command received, along with a flag bit to signify the null
case of no Control Change command received. Note that this state should
not reflect any changes to MIDIctrl[] made by assignment statements by
SAOL code.

The encoding for Chapter C is shown below:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 8 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S|  LENGTH     |F|  CONTROLLER |R| VALUE/COUNT |F| CONTROLLER  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| VALUE/COUNT |  ....                                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The chapter consists of a 1-byte header followed by a list of 16-bit
controller logs. A controller log exist for All Notes Off and All Sound
Off controllers if a new Control Change command has been received for
the controller since the last checkpoint packet. For all other
controllers, a controller log exists for controllers whose values have
been changed by a Control Change command since the last checkpoint
packet. Only one controller log may exist in the controller list for a
particular controller number.

The 7-bit LENGTH field codes the number of controller logs minus one;
the expression (1 + 2*(LENGTH + 1)) yields the number of bytes in the
chapter.

If the S bit is set to 1, the previous packet sent did not include a
Control Change command on this channel, and the receiver may skip to the
next Chapter if it is recovering from the loss of a single packet.

For each controller log, the 7-bit CONTROLLER field identifies the
controller number. For most controllers, the VALUE/COUNT field codes the
value of the last Control Change command sent for this controller.

However, if the controller log codes the All Notes Off or All Sound Off
controllers, the VALUE/COUNT field codes the total number of Control
Change commands received for the lifetime of the session.  If this value
exceeds 127, modulo arithmetic is used, but the value 0 is skipped.

If the controller log codes the Sustain Pedal controller, zero is used
to code pedal release. To code pedal depression, the the VALUE/COUNT
field codes the total number of pedal depressions that occur during a
session.  If this value exceeds 127, modulo arithmetic is used, but the
value 0 is skipped.




Lazzaro/Wawrzynek                                              [Page 26]


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If the F bit is set to 1, the previous packet sent did not include a
Control Change command for this controller, and the receiver can skip to
the next controller log if it is recovering from the loss of a single
packet.

If the F bit is 0, the receiver should compare the VALUE/COUNT field
with its stored state for the controller.

For the All Notes Off and All Sound Off controllers, and the Sustain
Pedal controllers coding a depressed pedal, if the stored modulo count
for the controller does not match the VALUE/COUNT field, the receiver
should update its state for this controller, and execute the semantics
of the lost command. Note these commands happen sufficiently
infrequently that the ambiguity of modulo comparisons should not affect
the recovery process.

For all other controllers, if the recovery state does not match the
VALUE/COUNT field, the receiver should update its recovery state to
reflect the value of the VALUE/COUNT.  In addition, the receiver MAY
execute a single Control Change command, or MAY plan a series of Control
Change commands spaced over time.


Appendix B. Author Addresses

John Lazzaro (corresponding author)
UC Berkeley
CS Division
315 Soda Hall
Berkeley CA 94720-1776
Email: lazzaro@cs.berkeley.edu

John Wawrzynek
UC Berkeley
CS Division
631 Soda Hall
Berkeley CA 94720-1776
Email: johnw@cs.berkeley.edu


Appendix C. References

[1] MIDI Manufacturers Association. The complete MIDI 1.0
detailed specification, 1996. http://www.midi.org

[2] International Standards Organization. ISO 14496 MPEG-4,
Part 3 (Audio) Subpart 5 (Structured Audio) 1999.




Lazzaro/Wawrzynek                                              [Page 27]


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[3] Sfront source code release, includes a Linux networking
client that implements the MIDI RTP packetization.
http://www.cs.berkeley.edu/~lazzaro/sa/

[4] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson.
RFC 1889: RTP: A transport protocol for real-time applications,
1996.

[5] John Lazzaro and John Wawrzynek. A Case for Network
Musical Performance. The 11th International Workshop on Network
and Operating Systems Support for Digital Audio and Video
(NOSSDAV 2001) June 25-26, 2001, Port Jefferson, New York.
http://www.cs.berkeley.edu/~lazzaro/sa/pubs/pdf/nossdav01.pdf

[6] M. Handley and V. Jacobson. RFC 2327: SDP: Session Description
Protocol.  1998.

[7] Internet Engineering Task Force. RTP Payload Format for MPEG-4
Streams.  Work in progress, draft-ietf-avt-mpeg4-multisl-02.txt.

[8] Internet Engineering Task Force. Use of "RFC-generic" for MPEG-4
Elementary Streams with no SL layer. Work in progress,
draft-ietf-avt-mpeg4-simple-00.txt.


Appendix D. Expiration Notice

This document expires April 1, 2002.























Lazzaro/Wawrzynek                                              [Page 28]


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