Echo Stream Format

The Echo Stream Format is the music and sound effect format used in the Echo sound engine on the Sega Mega Drive. It contains commands and instructions that are parsed by Echo and passed on to the sound hardware. To optimize playback process, the format is headerless. While it is not space-optimized, it is performance-optimized because resources when using the Z80 are low.

Event list

Note on

Event	Description
$00nn	Note on FM channel #1
$01nn	Note on FM channel #2
$02nn	Note on FM channel #3
$04nn	Note on FM channel #4
$05nn	Note on FM channel #5
$06nn	Note on FM channel #6
$08nn	Note on PSG channel #1
$09nn	Note on PSG channel #2
$0Ann	Note on PSG channel #3
$0Bnn	Note on PSG channel #4
$0Cnn	Note on PCM channel

For $00 to $06 :

These events do a "note on" at the specified FM channel. The event is followed by a byte, which indicates which note to play. The value is as follows, where "octave" ranges from 0 to 7 and "semitone" ranges from 0 to 11:

32 * octave + 2 * semitone + 1

For $08 to $0A :

These events do a "note on" at the specified square wave PSG channel. The event is followed by a byte, which indicates which note to play. The value is as follows, where "octave" ranges from 0 to 5 and "semitone" ranges from 0 to 11:

24 * octave + 2 * semitone

$0Bnn: Note on PSG channel #4 :

This event does a "note on" at the noise PSG channel. The event is followed by a byte, which indicates what kind of noise to play. The following values are valid:

       $00 ... Periodic noise, high pitch
       $01 ... Periodic noise, medium pitch
       $02 ... Periodic noise, low pitch
       $03 ... Periodic noise, PSG3 frequency
       $04 ... White noise, high pitch
       $05 ... White noise, medium pitch
       $06 ... White noise, low pitch
       $07 ... White noise, PSG3 frequency

When using values $03 and $07, the third square wave PSG channel controls the noise frequency. You can change this frequency using the events to change the frequency of that channel (usually you'd use event type $3A).

$0Cnn: Note on PCM channel :

This event does a "note on" at the PCM channel. More specifically, it starts playback of a PCM sample. This event is followed by a byte, that specifies an index in the pointer list indicating where's the sample data. Samples are stored as EWF (Echo Waveform Format).

NOTE: FM channel #6 will be disabled. That channel will be re-enabled when PCM playback is over (either because the waveform is over or because the channel is stopped explicitly).

Note off

Event	Description
$10	Note off FM channel #1
$11	Note off FM channel #2
$12	Note off FM channel #3
$14	Note off FM channel #4
$15	Note off FM channel #5
$16	Note off FM channel #6
$18	Note off PSG channel #1
$19	Note off PSG channel #2
$1A	Note off PSG channel #3
$1B	Note off PSG channel #4
$1C	Note off PCM channel

These events do a "note off" at the specified channel.

$1C: Note off PCM channel :

This event does a "note off" at the PCM channel. This means that any PCM playback is immediately stopped. FM channel #6 is immediately enabled as well.

Set Volume

Event	Description
$20nn	Set volume FM channel #1
$21nn	Set volume FM channel #2
$22nn	Set volume FM channel #3
$24nn	Set volume FM channel #4
$25nn	Set volume FM channel #5
$26nn	Set volume FM channel #6
$28nn	Set volume PSG channel #1
$29nn	Set volume PSG channel #2
$2Ann	Set volume PSG channel #3
$2Bnn	Set volume PSG channel #4

For $20 - $26 :

These events set the volume of a specific FM channel. The event is followed by a byte, which indicates the new volume. A value of $00 is the loudest, a value of $7F is the quietest.

For $28 - $2B :

These events set the volume of a specific PSG channel. The event is followed by a byte, which indicates the new volume. A value of $00 is the loudest, a value of $0F is the quietest.

Set Frequency

Event	Description
$30nnnn	Set frequency FM channel #1
$31nnnn	Set frequency FM channel #2
$32nnnn	Set frequency FM channel #3
$34nnnn	Set frequency FM channel #4
$35nnnn	Set frequency FM channel #5
$36nnnn	Set frequency FM channel #6
$38nnnn	Set frequency PSG channel #1
$39nnnn	Set frequency PSG channel #2
$3Annnn	Set frequency PSG channel #3
$3Bnnnn	Set Noise Type PSG channel #4

For $30 to $36 :

These events set the raw frequency of a specific FM channel, without triggering a new note. Meant for note slides. The following two bytes specify the new frequency in the same format as the YM2612 expects. The first byte is register +$A4, the second byte is register +$A0.

Echo uses the following frequency values for each semitone:

       C  - 644 | E  - 810 | G# - 1021
       C# - 681 | F  - 858 | A  - 1081
       D  - 722 | F# - 910 | A# - 1146
       D# - 765 | G  - 964 | B  - 1214

For $38 to $3A :

These events set the raw frequency of a specific square wave PSG channel, without triggering a new note. Meant for note slides. The following two bytes specify the new frequency, the first byte containing the four least significant bits (LSB aligned), and the next byte containing the six most significant bits (LSB aligned too).

Echo uses the following frequency values for each semitone:

          |Oct.0|Oct.1|Oct.2|Oct.3|Oct.4|Oct.5
       ---|-----|-----|-----|-----|-----|-----
       C  | 851 | 425 | 212 | 106 |  53 |  26
       C# | 803 | 401 | 200 | 100 |  50 |  25
       D  | 758 | 379 | 189 |  94 |  47 |  23
       D# | 715 | 357 | 178 |  89 |  44 |  22
       E  | 675 | 337 | 168 |  84 |  42 |  21
       F  | 637 | 318 | 159 |  79 |  39 |  19
       F# | 601 | 300 | 150 |  75 |  37 |  18
       G  | 568 | 284 | 142 |  71 |  35 |  17
       G# | 536 | 268 | 134 |  67 |  33 |  16
       A  | 506 | 253 | 126 |  63 |  31 |  15
       A# | 477 | 238 | 119 |  59 |  29 |  14
       B  | 450 | 225 | 112 |  56 |  28 |  14

For $3B :

This event works like event $0Bnn, but it doesn't trigger a note attack (i.e. instrument envelope won't be reset, note won't start playing if it wasn't already).

Set Instrument

Event	Description
$40nn	Set instrument FM channel #1
$41nn	Set instrument FM channel #2		$42nn	Set instrument FM channel #3
$44nn	Set instrument FM channel #4
$45nn	Set instrument FM channel #5
$46nn	Set instrument FM channel #6
$48nn	Set instrument PSG channel #1
$49nn	Set instrument PSG channel #2
$4Ann	Set instrument PSG channel #3
$4Bnn	Set instrument PSG channel #4

For $40 to $46 :

These events are used to set the instrument to be used by a specific FM channel. The event is followed by a byte, which has an index in the pointer list that indicates where's the FM instrument data. Instruments are stored as EIF (Echo Instrument Format).

For $48 to $4B :

These events are used to set the instrument to be used by a specific PSG channel. The event is followed by a byte, which has an index in the pointer list that indicates where's the PSG instrument data. Instruments are stored as EEF (Echo Envelope Format).

Channel locking (SFX only!)

Event	Description
$E0	Lock FM channel #1 [SFX ONLY]
$E1	Lock FM channel #2 [SFX ONLY]
$E2	Lock FM channel #3 [SFX ONLY]
$E4	Lock FM channel #4 [SFX ONLY]
$E5	Lock FM channel #5 [SFX ONLY]
$E6	Lock FM channel #6 [SFX ONLY]
$E8	Lock PSG channel #1 [SFX ONLY]
$E9	Lock PSG channel #2 [SFX ONLY]
$EA	Lock PSG channel #3 [SFX ONLY]
$EB	Lock PSG channel #4 [SFX ONLY]

These events are used to "lock" specific channels. When a channel is locked, BGM can't mess up with it. All channels used by a SFX must be locked before they're used. Channels get unlocked when the SFX stream is over. To lock the PCM channel, you need to lock FM channel #6 (event $E6). Echo will behave accordingly.

Set FM parameters

Event	Description
$F0	Set parameters for FM channel #1
$F1	Set parameters for FM channel #2
$F2	Set parameters for FM channel #3
$F4	Set parameters for FM channel #4
$F5	Set parameters for FM channel #5
$F6	Set parameters for FM channel #6

These events set up some miscellaneous parameters for the FM channels. The event is followed by a byte, which speficies the new parameters to be used. The format of this byte is as follows:

       Bit 7 ..... 1 to enable left speaker, 0 to mute it
       Bit 6 ..... 1 to enable right speaker, 0 to mute it
       Bit 5-0 ... Must be 0