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139 lines
5.5 KiB
Lua
139 lines
5.5 KiB
Lua
adpcm {
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; IMA ADPCM decoder. Supports mono and stereo streams.
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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 like so (example):
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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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; And/or use a tool such as https://github.com/dbry/adpcm-xq (make sure to set the correct block size, -b8)
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;
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; NOTE: for speed reasons this implementation doesn't guard against clipping errors.
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; if the output sounds distorted, lower the volume of the source waveform to 80% and try again etc.
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; IMA-ADPCM file data stream format:
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; If the IMA data is mono, an individual chunk of data begins with the following preamble:
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; bytes 0-1: initial predictor (in little-endian format)
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; byte 2: initial index
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; byte 3: unknown, usually 0 and is probably reserved
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; If the IMA data is stereo, a chunk begins with two preambles, one for the left audio channel and one for the right channel.
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; (so we have 8 bytes of preamble).
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; The remaining bytes in the chunk are the IMA nibbles. The first 4 bytes, or 8 nibbles,
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; belong to the left channel and -if it's stereo- the next 4 bytes belong to the right channel.
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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 @requirezp predict ; decoded 16 bit pcm sample for first channel.
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uword @requirezp predict_2 ; decoded 16 bit pcm sample for second channel.
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ubyte @requirezp index
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ubyte @requirezp index_2
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uword @requirezp pstep
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uword @requirezp pstep_2
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sub init(uword startPredict, ubyte startIndex) {
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; initialize first decoding channel.
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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 init_second(uword startPredict_2, ubyte startIndex_2) {
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; initialize second decoding channel.
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predict_2 = startPredict_2
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index_2 = startIndex_2
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pstep_2 = t_step[index_2]
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}
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sub decode_nibble(ubyte nibble) {
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; Decoder for nibbles for the first channel.
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; this is the hotspot of the decoder algorithm!
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; Note that the generated assembly from this is pretty efficient,
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; rewriting it by hand in asm seems to improve it only 5-10%
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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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predict -= cx16.r0s
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else
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predict += cx16.r0s
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; NOTE: the original C/Python code uses a 32 bits prediction value and clips it to a 16 bit word
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; but for speed reasons we only work with 16 bit words here all the time (with possible clipping error)
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; if predicted > 32767:
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; predicted = 32767
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; elif predicted < -32767:
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; predicted = - 32767
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index += t_index[nibble]
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if_neg
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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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sub decode_nibble_second(ubyte nibble) {
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; Decoder for nibbles for the second channel.
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; this is the hotspot of the decoder algorithm!
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; Note that the generated assembly from this is pretty efficient,
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; rewriting it by hand in asm seems to improve it only 5-10%
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cx16.r0s = 0 ; difference
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if nibble & %0100
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cx16.r0s += pstep_2
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pstep_2 >>= 1
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if nibble & %0010
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cx16.r0s += pstep_2
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pstep_2 >>= 1
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if nibble & %0001
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cx16.r0s += pstep_2
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pstep_2 >>= 1
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cx16.r0s += pstep_2
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if nibble & %1000
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predict_2 -= cx16.r0s
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else
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predict_2 += cx16.r0s
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; NOTE: the original C/Python code uses a 32 bits prediction value and clips it to a 16 bit word
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; but for speed reasons we only work with 16 bit words here all the time (with possible clipping error)
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; if predicted > 32767:
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; predicted = 32767
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; elif predicted < -32767:
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; predicted = - 32767
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index_2 += t_index[nibble]
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if_neg
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index_2 = 0
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else if index_2 >= len(t_step)-1
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index_2 = len(t_step)-1
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pstep_2 = t_step[index_2]
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}
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}
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