ADPCM decoding in C

Question

When I try to decode IMA ADPCM to 16-bit signed PCM I have half-garbaged PCM

I try to decode and encode IMA ADPCM in WAV file(22050 samples/s, 4 bit/sample, 36(!) byte(?) align(?!), 1 channel).

int16_t decodeImaAdpcmSampleUIS(struct AdpcmState* state, const uint8_t sample){
int diff;
int8_t si=state->stepindex;
int step=i_step_table[si];
int cur=state->current;

diff=step>>3;
if( sample&0x04 ) diff += step;
if( sample&0x02 ) diff += step>>1;
if( sample&0x01 ) diff += step>>2;
if( sample&0x08 ){
cur -= diff;
if(cur<-32768)
    cur=-32768;
}else{
cur += diff;
if(cur>32767)
    cur=32767;
}
//predictor: cur, state.current
//step_index: si, stepindex
si+=ima_index_table[sample&0b111];
if(si < 0)
    si = 0;
if(si > 88)
    si = 88;
state->stepindex=si;

return state->current=cur;
}

void decodeImaAdpcm(uint8_t* src, int16_t* dst, size_t srcLen){
    struct AdpcmState state={0,0};

    for(size_t i=0; i<srcLen; i++){
        *(dst++)=decodeImaAdpcmSampleUIS(&state, (*src)&0xf);
        *(dst++)=decodeImaAdpcmSampleUIS(&state, (*src)>>4);
        src++;
    }
}

Full project with WAV files: https://drive.google.com/open?id=1xuxwXj3Y_QhPDWhrQY7nmz8ycBE1cgyL

ADPCM WAV files converted to PCM WAV contains some garbage, but ffmpeg converts it normally. PCM WAV to ADPCM WAV conversion currently not implemented.

Answer 1

This happens because in WAV ADPCM is not stored as continuous stream of encoded sound. Instead it is stored as blocks, where each first 4 bytes of block are initial ADPCM decoder state.

For example: WAV.align=36;//Block size is 36 bytes, block structure is --|current(2 bytes)|step index(2 bytes)|data(WAV.align-4(36-4=32) bytes)|--