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AIBO MIDI File Format
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Table of Contents
  1. Introduction
  2. MIDI Header Chunks
  3. MIDI Track Chunks
  4. MIDI Delta Time
  5. MIDI Events
  6. MIDI Running Status
  7. Example

1. Introduction

MIDI files are a flexible format for encoding music.   They can support multiple instruments on multiple tracks.    This document describes the subset supported by the ERS-110/210/220/310 AIBO models.


2. MIDI Header Chunks

MIDI files are composed of a header chunk, and one or more track chunks.   Each chunk starts with a 4-byte signature & 4-byte chunk length field.   The first chunk in a file is the header chunk.   Header chunks specify MIDI format & the time base.

struct header_chunk_type {
  unsigned int signature;    // "MThd" = header
  unsigned int chunk_len;
  unsigned short format;     // MIDI format (types 0, 1, 2 are valid)
  unsigned short tracks;     // Number of tracks.
  unsigned short division;   // Time delta units per quarter-note.
};

NOTE:  The ERS-110/210/220/310 AIBO platforms only support single track files (format 0 MIDI files).   However, Skitter can automatically convert multi-track MIDI to work on AIBO.

Format 0 is a single-track MIDI file.   The track contains all note and tempo information.  
Format 1 is a multi-track MIDI file, where the first track holds the tempo map & meta-data.  All tracks play at once.
Format 2 is also a multi-track MIDI file, where each track is a seperate sequence.


3. MIDI Track Chunks

MIDI track chunks contain a sequence of one or more <delta-time>,<event> pairs following the signature/chunk-length:

struct track_chunk_type {
unsigned int signature; // "MTrk" = track
unsigned int chunk_len;
};


4. MIDI Delta Time

The delta-time fields are a variable length value.   The MSB (bit 7) of each byte is a continuation flag if set.  The lower 7-bits are the time value.   Each additional byte adds 7-bits to the time-delta, up to a maximum of 0x0FFFFFFF.   You repeat shifting a running total by 7 bits & adding the new 7-bits while the MSB is set.  ie:

0x7F =           0x7F
0x86 0x21 = 0x321
0x84 0xC2 0x21 = 0x12121


5. MIDI Events

MIDI events control turning notes on & off, plus various effects for simulating musical instruments.  There are four types:

5.1 MIDI Channel Voice Events
The following channel voice events are used.   The values (status bytes) shown are in hexadecimal.   The range of values support up to 16 MIDI channels.  However, AIBO just combines them.
0x80 - 0x8F   Note Off.
0x90 - 0x9F   Note On.
0xA0 - 0xAF   Polyphonic pressure. Ignored by AIBO.
0xB0 - 0xBF   Controller Change. Ignored by AIBO.
0xC0 - 0xCF   Program/Instrument Change.
0xD0 - 0xDF   Channel Key Pressure. Ignored by AIBO.
0xE0 - 0xEF   Pitch Bend.

NOTE OFF/ON.   These events take two additional parameter bytes:   The "key" (pitch) and "velocity" (volume).  As a special case, a NOTE-ON event with zero volume is equivilent to a NOTE-OFF event. 
0x90 0x3C 0x7F = Turn on middle-C at maximum volume.
0x80 0x3C 0x00 = Turn off middle-C.

POLYPHONE PRESSURE.   Takes two additional parameter bytes.   The "key" (pitch) and "pressure" applied.
0xA0 0x3C 0x7F   =  Turn on middle-C at maximum volume.

CONTROLLER CHANGE.   Takes two additional parameter bytes.  The controller "number" and controller "value".
0xB0 0x12 0x34   =  Set controller number 0x12 to value 0x34. 
Controller number range is 0x00 to 0x77. Higher
values are reserved for channel mode events.

PROGRAM CHANGE.   Takes one additional parameter byte.   The new "program" (or instrument) number.
0xC0 0x47        =  Set instrument number to 0x47 (Clarinet).

CHANNEL KEY PRESSURE.   Takes one additional parameter byte.   The channel "pressure" value.
0xD0 0x7F        =  Change overall channel pressure to maximum.

PITCH BEND.   Takes two additional parameter bytes.   The LSB & MSB of the pitch value.
0xE0 0x00 0x7F   =  Change channel pitch to mid-range.


5.2 MIDI Channel Mode Events
Channel mode events overload the "Controller Change" event above.   They support optional extended features.   The messages are always three bytes in length -- the same as the controller change event.   The values (status bytes) shown are in hexadecimal.
0xB0 0x78 0x00   Turn off all sound.
0xB0 0x79 0x00   Reset All Controllers.
0xB0 0x7A 0x00   Disconnect Local Keyboard.
0xB0 0x7A 0xFF   Reconnect Local Keyboard.
0xB0 0x7B 0x00   All Notes Off.
0xB0 0x7C 0x00   Omni Mode Off.
0xB0 0x7D 0x00   Omni Mode On.
0xB0 0x7E 0x00   Mono Mode On.
0xB0 0x7F 0x00   Poly Mode On.
These are ignored by AIBO, and filtered by Skitter.


5.3 System Exclusive (Sysex) Events
These events are for including information other than MIDI events into a MIDI file.   There are two:  0xF0 & 0xF7.   Both are followed by a variable-length byte count (ie: delta-time format), then the message data.  ie:
 0xF0    0x06    0x11 0x22 0x33 0x44 0x55 0x66
SysEx <length> <six bytes of misc data>
These are ignored by AIBO, and filtered by Skitter.


5.4 Meta Events
These events store music related information, such as track names, tempo, instruments names, etc...   The events start with 0xFF, followed by a variable-length byte count (ie: delta-time format), then the message data.   ie:
0xFF 01 len msg         Misc Text Event (any message you like)
0xFF 02 len msg        Copyright notice
0xFF 03 len msg         Track name
0xFF 04 len msg        Instrument name
0xFF 51 03 tt tt tt     Tempo in microseconds per quarter note (24-bit value).
0xFF 58 04 nn dd cc bb  Time signature (ie: 3/4 time).
nn = numerator
 dd = denominator power of 2). 1=>2, 2=>4, 3=>8, ...
 cc = MIDI clocks per tick
 bb = Number of 1/32 notes per 24 MIDI clocks.
0xFF 2F 00             End of Track (should be last event on all tracks).


6. MIDI Running Status

To reduce file size, MIDI supports the so called "Running Status".   So long as the status byte (ie: note on/off events) doesn't change, there is no need to repeat it.   Status bytes are the MIDI channel voice/mode event bytes described above.

Without Running Status:
 0x90   0x3C   0x7F     0x90   0x3D   0x6F     0x90   0x3E    0x5F
status key velocity status key velocity status key velocity

Since the subsequent status bytes doesn't change, they can be omitted...

With Running Status:
 0x90   0x3C   0x7F    0x3D   0x6F    0x3E    0x5F
status key velocity key velocity key velocity

6.1 Rules for Running Status
  • In effect while status byte doesn't change.
  • Applies to channel voice/mode events only.
  • System Exclusive (SysEx) Events cancel running status.

6.2 Running Status Efficiency


To improve running status efficiency, use note-on events with velocity 0 instead of note-off events.   This avoids needing to issue new status bytes.   For example, the following cannot use running status:

 0x90    0x3C   0x7F       0x80   0x3C   0x40       0x90   0x3E   0x5F
status key velocity status key velocity status key velocity
(note on) (note off) (note on)
However, changing the note-off event to a note-on with zero-velocity permits:

 0x90   0x3C   0x7F      0x3C   0x00      0x3E   0x5F
status key velocity key velocity key velocity
(note on) (note off) (note on)


7. Example

Chunk Headers
Meta Events & Status Bytes
Time Delta's
000000:  4d 54 68 64  00 00 00 06  00 00 00 01  00 78 4d 54  |MThd.........xMT|
000010:  72 6b 00 00  01 27 00 ff  51 03 05 b8  d8 00 ff 58 |rk...'..Q......X|
000020:  04 04 02 18  08 00 c0 00  00 90 4f 60  50 4f 00 28 |..........O`PO.(|
000030:  4f 60 50 4f 00 28 4f 60  50 4f 00 28 4f 60 50 4f  |O`PO.(O`PO.(O`PO|
000040:  00 28 4f 60  50 4f 00 28 4f 60 50 4f  00 28 4e 7f  |.(O`PO.(O`PO.(N.|
000050:  50 4e 00 28 4e 7f 50 4e  00 28 4e 7f  50 4e 00 28 |PN.(N.PN.(N.PN.(|
000060:  4e 7f 50 4e 00 28 4e 7f  50 4e 00 28 4e 7f 50 4e  |N.PN.(N.PN.(N.PN|
000070:  00 28 52 7f  50 52 00 28 52 7f 50 52  00 28 52 7f  |.(R.PR.(R.PR.(R.|
000080:  50 52 00 28 52 7f 50 52  00 28 50 7f  50 50 00 28 |PR.(R.PR.(P.PP.(|
000090:  52 7f 50 52 00 28 53 7f  50 53 00 28 53 7f 50 53  |R.PR.(S.PS.(S.PS|
0000a0:  00 28 53 7f  50 53 00 28 53 7f 50 53  00 28 52 7f  |.(S.PS.(S.PS.(R.|
0000b0:  50 52 00 28 50 7f 50 50  00 28 4e 7f  50 4e 00 28 |PR.(P.PP.(N.PN.(|
0000c0:  4e 7f 50 4e 00 28 4e 7f  50 4e 00 28 4e 7f 50 4e  |N.PN.(N.PN.(N.PN|
0000d0:  00 28 4e 7f  50 4e 00 28 4e 7f 50 4e  00 28 4e 7f  |.(N.PN.(N.PN.(N.|
0000e0:  50 4e 00 28 4e 7f 50 4e  00 28 4e 7f  50 4e 00 28 |PN.(N.PN.(N.PN.(|
0000f0:  4e 7f 50 4e 00 28 4e 7f  50 4e 00 28 4e 7f 50 4e  |N.PN.(N.PN.(N.PN|
000100:  00 28 52 7f  50 52 00 28 52 7f 50 52  00 28 52 7f  |.(R.PR.(R.PR.(R.|
000110:  50 52 00 28 52 7f 50 52  00 28 50 7f  50 50 00 28  |PR.(R.PR.(P.PP.(|
000120:  52 7f 50 52 00 28 53 7f  50 53 00 81 20 4e 7f 50 |R.PR.(S.PS.. N.P|
000130:  4e 00 28 53  7f 81 70 53  00 00 ff 2f  00 |N.(S..pS.../.   |
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