[opus] FEC and Stereo

Jon Lederman
Fri, 27 Jan 2017 17:16:13 -0000

Thanks.

When you say that with fec enabled, the threshold is increased, do you mean the
bitrate - i.e., you need higher bitrate with fec enabled to suppress crosstalk?

Also, can you make any recommendations to decide whether to use fec or not?  We
are trying to tune the parameters appropriately.  Audio quality and reduction of
crosstalk are critical for us.

Thanks again.

Jon

Sent from my iPhone

> On Jan 27, 2017, at 11:38 AM, Jean-Marc Valin <jmvalin at jmvalin.ca>
wrote:
>
> Hi Jon,
>
>> On 27/01/17 08:31 AM, Jon Lederman wrote:
>> We are trying to use Opus in a VoIP environment for sending stereo
>> audio.  We have noticed a phenomenon where when FEC is enabled and
>> packet_loss_percentage>0, that there is a mixing of audio from the
>> left channel into the right channel and vice versa.  That is, rather
>> than hearing each channel in its pristine form as it was in the file,
>> there is a combination of right and left channels that is heard on
>> the left and right playback.
>>
>> First, is this the correct behavior?  If so, why does this occur?
>
> It is the correct behaviour, at least to some extent. Opus may
> deliberately introduces cross-talk between the channels to make the
> signal easier to code when it doesn't have enough bits to do a good
> enough job with the original stereo audio. If you increase the bitrate,
> then that cross-talk with be reduce and eventually disappear. Now the
> only difference that FEC does is that it increases the threshold for the
> cross-talk.
>
>> Finally, is there a way to prevent this other than disabling FEC
>> (which we don’t want).
>
> Well, obviously if you're not going to use the FEC, then you
shouldn't
> waste bits to encode it.
>
> Cheers,
>
>    Jean-Marc
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