[Ffmpeg-devel] MP3 decoding from RTP stream. Getting "invalid new backstep"

[Martin]

Hi,

>>>the best way:
>>>1) Return a value that indicates a problem
>>>2) Set a flag (which one?) in AVCodecContext that indicates a problem
>>>3) Changing the buffer values to 0? Is 0 the right value for silence?
>>
>>Zero is not a good value as it will cause pops or clicks if the previous
>>sample has a large value.
> 
> 
> hmm zeroing a block before the MDCT shouldnt introduce any big
> discontinuities i think though iam not that good at MDCT related stuff ...
> one alternative might be to simply use the coeffs fromm the previous
> block, if that fails too google/citeseer would be the right place to
> look for audio error concealment
> 
> [...]
> 

I'm still not able to solve the problem with writing zero to the buffer
I tried like this in the function mp_decode_frame :

right before returning:

  if (discard_frame) {
    //dicard frame is set if an backstep or overread occurs.
    for(i=0;i<nb_frames;i++) {
      for(ch=0;ch<s->nb_channels;ch++) {
        int j;
        for(j=0;j<SBLIMIT;j++) {
          s->sb_samples[ch][i][j] = 0;
        }
      }
    }
  }



This didn't worked as expected. Also I found another problem caused by
this point in the huffman_decode() function:

  if (pos > end_pos2 && last_pos){
                /* some encoders generate an incorrect size for this
                   part. We must go back into the data */
                s_index -= 4;
                skip_bits_long(&s->gb, last_pos - pos);
                av_log(NULL, AV_LOG_INFO, "overread, skip %d enddists:
%d %d\n", last_pos - pos, end_pos-pos, end_pos2-pos);
                s->frame_distorted = 1;

                break;
            }


My idea was now to introduce a new flag into the MPADecodeContext:
int frame_distorted;

Now I set this flag to 0 at the start of decode_frame()
and set it to 1 when backstep or overread occurs. At the end of
decode_frame() I check the flag and return -1 if it is set. This is
working for me. If I send a patch with the new flag, would you include it?

Martin