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

[Martin]

Michael Niedermayer wrote:
> Hi
> 
> On Fri, Nov 03, 2006 at 12:25:30PM +0100, Martin wrote:
> 
>>Hi,
>>
>>
>>>>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?
>>>
>>>
>>>not without understanding why setting sb_samples to 0 doesnt work
>>>
>>>[...]
>>
>>It seems to work now, with setting sb_frames to 0. I did it after the
>>synthesis filter which didn't work. How should I transmit the fact that
>>a distortion occured in huffmann_decode() to the function
>>mp_decode_frame()? Can I use the flag for this purpose?
>>I'm not sure whether I break something if I set sb_samples to 0 directly
>>at the point where the distortion occurs in huffmann_decode().
> 
> 
> huffmann_decode() should return -1 if theres an error
> 
> [...]

Thanks for the help!
So here is a patch which sets buffers to 0 if 'overread' or 'backstep'
occurs which prior resulted in distorted sound.

Martin
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