mirror of
https://github.com/irmen/prog8.git
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66 lines
2.4 KiB
Lua
66 lines
2.4 KiB
Lua
adpcm {
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; IMA ADPCM decoder.
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; https://wiki.multimedia.cx/index.php/IMA_ADPCM
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; https://wiki.multimedia.cx/index.php/Microsoft_IMA_ADPCM
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; IMA ADPCM encodes two 16-bit PCM audio samples in 1 byte (1 word per nibble)
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; thus compressing the audio data by a factor of 4.
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; The encoding precision is about 13 bits per sample so it's a lossy compression scheme.
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;
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; HOW TO CREATE IMA-ADPCM ENCODED AUDIO? Use sox or ffmpeg:
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; $ sox --guard source.mp3 -r 8000 -c 1 -e ima-adpcm out.wav trim 01:27.50 00:09
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; $ ffmpeg -i source.mp3 -ss 00:01:27.50 -to 00:01:36.50 -ar 8000 -ac 1 -c:a adpcm_ima_wav -block_size 256 -map_metadata -1 -bitexact out.wav
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; Or use a tool such as https://github.com/dbry/adpcm-xq (make sure to set the correct block size)
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ubyte[] t_index = [ -1, -1, -1, -1, 2, 4, 6, 8, -1, -1, -1, -1, 2, 4, 6, 8]
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uword[] @split t_step = [
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7, 8, 9, 10, 11, 12, 13, 14,
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16, 17, 19, 21, 23, 25, 28, 31,
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34, 37, 41, 45, 50, 55, 60, 66,
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73, 80, 88, 97, 107, 118, 130, 143,
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157, 173, 190, 209, 230, 253, 279, 307,
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337, 371, 408, 449, 494, 544, 598, 658,
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724, 796, 876, 963, 1060, 1166, 1282, 1411,
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1552, 1707, 1878, 2066, 2272, 2499, 2749, 3024,
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3327, 3660, 4026, 4428, 4871, 5358, 5894, 6484,
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7132, 7845, 8630, 9493, 10442, 11487, 12635, 13899,
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15289, 16818, 18500, 20350, 22385, 24623, 27086, 29794,
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32767]
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uword @zp predict
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ubyte @requirezp index
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uword @zp pstep
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sub init(uword startPredict, ubyte startIndex) {
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predict = startPredict
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index = startIndex
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pstep = t_step[index]
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}
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sub decode_nibble(ubyte nibble) {
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; this is the hotspot of the decoder algorithm!
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cx16.r0s = 0 ; difference
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if nibble & %0100
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cx16.r0s += pstep
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pstep >>= 1
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if nibble & %0010
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cx16.r0s += pstep
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pstep >>= 1
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if nibble & %0001
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cx16.r0s += pstep
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pstep >>= 1
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cx16.r0s += pstep
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if nibble & %1000
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cx16.r0s = -cx16.r0s
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predict += cx16.r0s as uword
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index += t_index[nibble]
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if_neg ; was: if index & 128
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index = 0
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else if index > len(t_step)-1
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index = len(t_step)-1
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pstep = t_step[index]
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}
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}
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