[opus] Alleged bug in Silk codec

[Marcello Caramma (mcaramma)]

Hi Jean Marc,

please find attached the audio file (mono 16khz). I shortened it to about
10 seconds. I also add 2 patches that worked for me. Further info that
might help:

- The problem seems to be related to silk_burg_modified not reaching the
maximum gain, so the actual filter order is 16 rather than 2 (which is
what would be expected with a sine wave).
- The problem seems to happen when rshifts >= 3
- when pre-scaling the signal to be < 16384 the problem goes away (patch
scale_burg_in.diff)
- When calculating C0 and rshifts based on a 64 bits correlation instead
of using silk_sum_sqr_shift the problem also goes away (patch
new_C0_calc.diff)

I suspect that for very high prediction gain the fixed point
implementation becomes very sensitive to numerical error, but I am not too
sure why the new versions work better.
I favour the version with the new C0 calculation, as it avoids rescaling
the input.
I also played around with a version (not attached) that prescales the
input by rshifts/2 - this might be considering as it simplifies the code.

Regards,

Marcello

PS: I am using 1.1 but the same issue is present with the latest code well.


On 13/06/2014 06:05, "Jean-Marc Valin" <jmvalin at jmvalin.ca> wrote:

>Hi Marcello,
>
>Thanks for the report. It's hard to debug this without the actual file.
>Can you please post the sweep_in.raw file you used?
>
>Cheers,
>
>	Jean-Marc
>
>On 11/06/14 04:46 AM, Marcello Caramma (mcaramma) wrote:
>> Hi,
>> 
>> Apologies if this is a known issues, but I have found what I believe is
>> a bug in the fixed point implementation of the Silk codec and could not
>> find any mention on this in the archives.
>> 
>> The bug can be easily reproduced with the fixed point demo program
>> (./configure ‹enable-fixed-point ‹disable-float-api && make) using the
>> following command:
>> 
>> ./opus_demo voip 16000 1 23000 sweep_in.raw sweep_out.raw
>> 
>> Where sweep_in.raw is a 30 seconds full scale frequency sweep from 0 to
>> 8kHz sampled at 16kHz.
>> 
>> The first 6 seconds of audio after transcoding sound Ok. After that
>> artefacts are introduced all the way to the end of the file.
>> The floating point version does not have the issue (even though the
>> quality is subjectively worse roughly from the same point).
>> 
>> I believe I narrowed down the problem to the file burg_modified_FIX.c ­
>> if I make sure the input signal is scaled down to 14 bits before
>> processing the coefficients of the predictor are calculated correctly
>> and no artefact is introduced.
>> 
>> Is anyone experiencing the same problem or has a proper fix for this? (I
>> can work around the bug with input scaling for now).
>> 
>> Thanks and best regards,
>> 
>> Marcello Caramma
>> 
>> 
>> _______________________________________________
>> opus mailing list
>> opus at xiph.org
>> http://lists.xiph.org/mailman/listinfo/opus
>> 

-------------- next part --------------
A non-text attachment was scrubbed...
Name: scale_burg_in.diff
Type: application/octet-stream
Size: 2181 bytes
Desc: scale_burg_in.diff
Url : http://lists.xiph.org/pipermail/opus/attachments/20140613/d45e2ca2/attachment-0003.obj 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: new_C0_calc.diff
Type: application/octet-stream
Size: 2368 bytes
Desc: new_C0_calc.diff
Url : http://lists.xiph.org/pipermail/opus/attachments/20140613/d45e2ca2/attachment-0004.obj 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: sweep_short_m16k.raw
Type: application/octet-stream
Size: 319224 bytes
Desc: sweep_short_m16k.raw
Url : http://lists.xiph.org/pipermail/opus/attachments/20140613/d45e2ca2/attachment-0005.obj