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Playing_DAC_BGM_with_SMPS.md

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How to play DAC BGM using SMPS and Mega PCM 2

This guide will show you an easy and reliable way of implementing DAC BGMs natively using SMPS and Mega PCM 2 in Sonic 1 Github AS disassembly.

The overall process goes as follows:

  1. Add your music to Mega PCM 2's sample table;
  2. Replace or create a dummy SMPS music file which plays your BGM's sample in a DAC channel;
  3. That's it, you can now play your BGM natively using SMPS.

This method has the following advantages:

  • Your BGM is immediately available in the sound test;
  • SMPS treats it as a proper BGM: it will stop when another BGM is played and will react to fade in/fade out commands (unless it's TYPE_PCM_TURBO which excludes volume control).

1. Adding a sample for BGM

Let's assume you have my-bgm.wav file in the correct format you need to use as a BGM. Add it to your project's directory (preferably sound/dac directory in case of Sonic 1 disassembly).

Now open Mega PCM 2's sample table (SampleTable.asm) and add your sample in any free slot in the table, or append it to the end:

SampleTable:
    ;           type            pointer     Hz
    dcSample    TYPE_DPCM,      Kick,       8000                ; $81
    dcSample    TYPE_PCM,       Snare,      24000               ; $82
    dcSample    TYPE_DPCM,      Timpani,    7250                ; $83
    dcSample    TYPE_PCM,       MyBGM,      0, FLAGS_LOOP       ; $84  NOTE: sample rate is auto-detected from WAV file
    ; <...>
    dc.w    -1  ; end marker

; ---------------------------------------------------------------
    ; <...>
    incdac  MySFX, "sound/dac/my-bgm.wav"

You'll probably want your BGM to loop (otherwise it'll just stop once finishes), so add FLAGS_LOOP flag for the sample as shown above.

Make sure your file's format is supported by Mega PCM 2. WAV or raw PCM files should be in 8-bit unsigned mono format and have the supported sample rate. See Sample table format for more information.

Note

Mega PCM 2 verifies your sample table during boot. For WAV files, it will error out if the format is not supported. If you have MD Debugger installed and you followed Mega PCM 2 installation guide precisely, you should see a very detailed error description pointing to the problematic sample. See Troubleshooting for more information.

2. Wraping sample into SMPS music file

For simplicity, let's assume you want to replace GHZ BGM (sound id $81) with your new DAC BGM. In Sonic 1 Github AS disassembly, it's located in sound/music/Mus81 - GHZ.asm file.

Open this file and replace its contants with the following:

Mus81_GHZ_Header:
    smpsHeaderStartSong 1
    smpsHeaderVoice     Mus81_GHZ_Voices
    smpsHeaderChan      $01, $00
    smpsHeaderTempo     $01, $03

    smpsHeaderDAC       Mus81_GHZ_DAC

Mus81_GHZ_DAC:
    dc.b    $84         ; play BGM sample
    smpsStop

Mus81_GHZ_Voices:

Replace $84 below Mus81_GHZ_DAC: with your sample id, if it's different.

Now, when you build the ROM and go to GHZ, it should play your new DAC BGM instead.

Warning

Your DAC BGM can still be interrupted by DAC SFX, so make sure you don't play them.

Warning

Your BGM won't support fade out effect if it has TYPE_PCM_TURBO or FLAGS_SFX flag. "Fade in" effect after 1-up jingle also isn't supported by SMPS for continously playing samples (only newly requested drums will be picked up in normal BGMs).