doubleOhThree - amdtp

Reverse engineered AMDTP ISO stream en/decoding for the Digidesign(TM) 003R.

AMDTP is the Audio & Music Data Transmission Protocol. It is used on top of IEEE 1394 (aka Firewire) to provide communication between a computer and sound-card.

The Digidesign 003R uses a slight modification of the standard data stream format. It obfuscates the audio-data in the ISO-stream. Specifically it distributes the middle byte of the 24bit data stream across neighboring or all channels.

Playing a standard ISO-stream will add hissing noise to the audio.

The goal of this project is to provide a free/libre software solution to the encoding used by the Digidesign(TM) 003R. Integration into the ALSA subsystem of Linux is currently being worked on.

The code has been tested to work with 003R (rack mount) and should work with the 002 and 003 as well. Note that the devices only encode the playback (output). The digi_decode function is provided for self-test and academic purposes only.

003amdtp is licensed in terms of the GNU General Public License Version 2 or later.

Further information regarding the protocol can be found at (http://gareus.org/wiki/digi003). Driver development is taking place at (https://github.com/zamaudio/snd-firewire-003).

Example Usage

Excerpt from sound/firewire/amdtp.c:

static void amdtp_write_samples(struct amdtp_stream *s,
        struct snd_pcm_substream *pcm,
        __be32 *buffer, unsigned int frames)
{
  struct snd_pcm_runtime *runtime = pcm->runtime;
  unsigned int channels, remaining_frames, frame_step, i, c;
  const u32 *src;

  channels = s->pcm_channels;
  src = (void *)runtime->dma_area +
      s->pcm_buffer_pointer * (runtime->frame_bits / 8);
  remaining_frames = runtime->buffer_size - s->pcm_buffer_pointer;
  frame_step = s->data_block_quadlets;
#ifdef EXAMPLE
  // NB. this should not be static (multiple devices)
  // and also re-initialzed on open/close or stream-restart.
  // use digi_state_reset(&state);
  static DigiMagic digistate = {0x00, 0x00, 0};
#endif

  for (i = 0; i < frames; ++i) {
    for (c = 0; c < channels; ++c) {
      buffer[s->pcm_quadlets[c]] =
          cpu_to_be32((*src >> 8) | 0x40000000);
#ifdef EXAMPLE
      digi_encode_step(&digistate, &buffer[s->pcm_quadlets[c]]);
#endif
      src++;
    }

    buffer += frame_step;
    if (--remaining_frames == 0)
      src = (void *)runtime->dma_area;
  }
}