[FFmpeg-devel] [PATCH v3 1/2] avcodec: add adpcm_ima_ssi encoder

[Zane van Iperen]

On Sat, 11 Apr 2020 22:02:26 +0200
"Paul B Mahol" <onemda at gmail.com> wrote:

> >
> > Ping.
> >
> > Also, could someone please clarify something for me?
> >
> > When encoding, there's no inherit differences between trellis and
> > non-trellis other then a potentially more-accurate set of nibbles?
> >
> > And the decoder is none the wiser and chews on them both the same
> > way?
> >
> > If I'm right, then this looks to be a decoder issue...
> >  
> 
> You are wrong, just follow qt adpcm encoder code.
> 

I did.


Is it possible, and I don't suggest this lightly, that the trellis code
is bugged, and errors accumulate over time?

I ask this because all of the encoders (except adpcm_yamaha, which is
different anyway) have either the full sample and/or the step index
periodically written to the bitstream, which is then used to reset the
state machine, thus errors don't accumulate.

adpcm_ima_ssi doesn't have that, so it relies on the state being
accurate through the entire stream. This would certainly explain the
behaviour I'm seeing in certain cases (see below).

In both images, the top stream is encoded with '-trellis 0', and the
bottom with '-trellis 1':
- https://0x0.st/iSdS.png
  This one's obvious where the problem is.
- https://0x0.st/iSdQ.png
  This is more subtle, but the entire stream is offset slightly. It's
  more noticeable at the start and end.


For reference, here's the code I'm using. You'll see it's basically
the same as the adpcm_ima_qt code:

if (avctx->trellis > 0) {
    FF_ALLOC_OR_GOTO(avctx, buf, n * avctx->channels, error);

    for (ch = 0; ch < avctx->channels; ch++) {
        adpcm_compress_trellis(avctx, samples + ch, buf + n * ch,
                               c->status + ch, n, avctx->channels);
        c->status[ch].prev_sample = c->status[ch].predictor;
    }

    for (i = 0; i < n; i++) {
        for (ch = 0; ch < avctx->channels; ch++) {
            put_bits(&pb, 4, buf[n * ch + i]);
        }
    }
    av_free(buf);
}


Or maybe I just don't fully understand how trellis works.

Zane