[opus] FEC and Stereo

[Jean-Marc Valin]

On 27/01/17 12:29 PM, Jon Lederman wrote:
> Thank you.  Yes, we do need both channels independent.  So, if we
> encode each channel separately, we will be sacrificing the
> compression ratio we would achieve with stereo encoding, correct?

Not necessarily. Stereo makes two assumptions:
1) It assumes the two channels are somehow correlated
2) It assumes the two channels are meant to be listened to using
headphones or speakers setup for stereo listening
If instead you have (e.g.) two different people talking in each channel
and both are meant to be listened to independently, then pretending it's
a stereo stream will actually hurt compression significantly.

> 1) If we use fec, we can reduce cross-talk but increasing bitrate.
> However, that should result in higher packet loss due to higher
> bitrate. 

Packet loss may or may not be related to the bitrate. It depends on the
network and I'm not the best person to help you about that.

> 2) If we don’t use fec, we can have no crosstalk.  However,
> then we have significant chance of packet loss.

Possibly.

> 3) We could encode
> each channel separately.  In this case, we would have no crosstalk,
> but we would have a lower compression rate/efficiency.

If you have a stereo music file, then coding independently will makes
things worse. For as I said earlier of the two channels are independent,
you'll be much better off using two separate streams.

Cheers,

	Jean-Marc

> Thanks.
> 
> -Jon
> 
>> On Jan 27, 2017, at 12:24 PM, Jean-Marc Valin <jmvalin at jmvalin.ca>
>> wrote:
>> 
>> On 27/01/17 12:16 PM, Jon Lederman wrote:
>>> 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?
>> 
>> Correct. Another effect I forgot to mention is that Using FEC will 
>> actually force SILK/hybrid rather than CELT, so it's possible that 
>> disabling FEC makes you use CELT, which tends to have wider stereo 
>> separation for lower bitrates.
>> 
>>> 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.
>> 
>> In general, I would say that if you know the decoder is able to use
>> FEC, then you should enable it in the encoder. The encoder will
>> decide by itself when to actually use it depending on bitrate and
>> other parameters. As for reducing cross-talk, I guess it comes down
>> to the *reason* you want to reduce cross-talk. Normally the encoder
>> will whatever will result in the best quality, so it must be
>> thinking that reducing cross-talk is good. Now, if your reason is
>> that your two channels carry completely independent information,
>> then the fix would be to simply code two separate channels. It's
>> hard to say more considering that you haven't said anything about
>> your signals and why the stereo separation is so important for
>> you.
>> 
>> Cheers,
>> 
>> Jean-Marc
>> 
>>> 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 _______________________________________________ opus 
>>>> mailing list opus at xiph.org 
>>>> http://lists.xiph.org/mailman/listinfo/opus
>