DSP (File Format)

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Revision as of 18:19, 24 January 2015 by >Aruki (→‎Header)
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The .dsp format is a common GameCube/Wii format for audio that comes with the SDK. It encodes sound using Nintendo's ADPCM codec. The same ADPCM codec is also embedded into several Retro Studios format, like AGSC; the CSMP format actually embeds the entire DSP format within it.

Header

Offset Size Description
0x0 4 Sample count
0x4 4 ADPCM nibble count
0x8 4 Sample rate
0xC 2 Loop flag; 1 means looped, 0 means not looped
0xE 2 Format; always 0
0x10 4 Loop start offset
0x14 4 Loop end offset
0x18 4 Current address; always 0
0x1C 2 × 16 Decode coefficients; this is 8 pairs of signed 16-bit values
0x3C 2 Gain; always 0
0x3E 2 Initial predictor/scale; always matches first frame header
0x40 2 Initial sample history 1
0x42 2 Initial sample history 2
0x44 2 Loop context predictor/scale
0x46 2 Loop context sample history 1
0x48 2 Loop context sample history 2
0x4A 2 × 11 Padding
0x60 End of DSP header

ADPCM Data

The ADPCM audio data is split up into multiple frames. Each frame is 8 bytes; it starts with a one-byte header, then has 7 bytes (or 14 samples) of audio data. For each frame, the bottom 4 bits are the scale value, and the top 4 bits are the coefficient index to use for the current frame.

Example C Decoding Function

vgmstream used as reference:

<syntaxhighlight lang="c">static const s8 nibble_to_s8[16] = {0,1,2,3,4,5,6,7,-8,-7,-6,-5,-4,-3,-2,-1};

s8 get_low_nibble(u8 byte) {

   return nibble_to_s8[byte & 0xF];

}

s8 get_high_nibble(u8 byte) {

   return nibble_to_s8[(byte >> 4) & 0xF];

}

s16 clamp(s32 val) {

   if (val < -32768) val = -32768;
   if (val > 32767) val = 32767;
   return s16(val);

}

void DecodeADPCM(u8 *src, s16 *dst, const DSPHeader& d) {

 s16 hist1 = d.initial_hist1;
 s16 hist2 = d.initial_hist2;
 s16 *dst_end = dst + d.num_samples;
 while (dst < dst_end)
 {
   // Each frame, we need to read the header byte and use it to set the scale and coefficient values:
   u8 header = *src++;
   u16 scale = 1 << (header & 0xF);
   u8 coef_index = (header >> 4);
   s16 coef1 = d.coefs[coef_index * 2];
   s16 coef2 = d.coefs[coef_index * 2 + 1];
   // 7 bytes per frame
   for (u32 b = 0; b < 7; b++)
   {
     u8 byte = *src++;
     // 2 samples per byte
     for (u32 s = 0; s < 2; s++)
     {
       s8 adpcm_nibble = (s == 0) ? get_high_nibble(byte) : get_low_nibble(byte);
       s16 sample = clamp(((adpcm_nibble * scale) << 11) + 1024 + ((coef1 * hist1) + (coef2 * hist2)) >> 11);
       hist2 = hist1;
       hist1 = sample;
       *dst++ = sample;
       if (dst >= dst_end) break;
     }
     if (dst >= dst_end) break;
   }
 }

}</syntaxhighlight>