How do I convert a G.726 ADPCM signal into a PCM signal?

Question

I usually look to SoX or Window's built in audio libraries for this stuff, but it appears that neither have G.726 codecs.

So I have a sequence of bytes that I know are encoded as G.726 although the bit-rate and whether it is mu-law or A-law is not known at this time (experimentation will determine those parameters), and I need to decode them into a normal PCM signal.

So I downloaded the reference implementation from the ITU-T (ITU-T Recommendation G.191) but I'm kind of confused on how to use the G726_decode function. According to the documentation inp_buf and out_buf need to have the same length smpno and both buffers are 16-bit buffers. This seems to me like a step is missing; otherwise no compression is accomplished by using G.726. According to the Wikipedia page on G.726 sample size depends on bit rate (from 2 to 5 bits). Am I supposed to do the decompression into samples myself? So if I assume maximum compression (2 bit samples) then each byte will produce 4 samples.

Example:

char b = /* read the code from input */
short inp[4], output[4];
inp[0] = b & 0x0003;
inp[1] = b & 0x000C >> 2;
inp[2] = (b & 0x0030) >> 4;
inp[3] = (b & 0x00C0) >> 6;
G726_state state;
memset(&state, 0, sizeof(G726_state));
G726_decode(inp, output, 4, "u", 2, 1, &state);
/* ouput now contains 4 PCM samples */

Or am I missing something completely?

Answer 1

Looks like ffmpeg actually isn't able to do this, as I thought it surely would be able to... however, while I was googling I did find this post to the ffmpeg mailing list which offers a solution.

Basically, there is a separate program called g72x++ which seems to be able to decode the audio to raw PCM for you.