Re: [rtcweb] Transcoding

[John Leslie <[e-mail]>]

Espen Berger (espeberg) <[e-mail]> wrote:
> 
> Comments on transcoding based on testing in lab and research. 

   :^)

> *  In side by side comparison, single encode/decode and
>    encode/transcode/decode we can see that the untranscoded video
>    stream can be 20-35% lower bitrate and give the same visual quality...

   This tradeoff will be acceptable in many cases, IMHO.

> * Lip-sync should be within 50 ms, this is the conclusion from a
>   Norwegian based ph.d student and also recommendation from EBU...

   The EBU document discusses HDTV programming (and indeed recommends
audio no more than 5 msec early or 15 msec late). They claim to base
this on failure of lip-sync becoming "perceptable to 50% of observers".

   The 50 msec number seems plausible, but I have no basis to judge.

   Lip-sync can always be accomplished by delaying one or the other:
this gives us a trade-off (which I'd prefer to be under application
control).

> * Advanced media resilience techniques like LTRF, disposable frames
>   and more are hard to map between different video codecs, so
>   transcoding will likely reduce robustness for packet loss. 

   (discussed elsewhere)

> * Transcoding adds delay, between 60 - 200 ms  (depending on packet
>   loss and implementation).

   I have no basis to judge those numbers; but they seem high.

>   This has an impact since camera to screen latency should be below
>   300 ms to get a smooth conversation between to participants in a
>   video call. Test on people shows that glass to glass latency
>   should be below 330 ms, to avoid latency to be noticed.  

   Do you have a source for that?

   My gut-feel is that folks will tolerate at least 200 msec glass-to-
glass, but have problems when mouth-to-ear exceeds 150 msec. Alas I
don't have a source for that... :^(

> All in all we should avoid transcoding for video conferencing use
> cases to get to a good user experience. 

   I quite agree it's worth "avoiding" -- but that's not at all the
same as saying we should try to prevent it.

--
John Leslie <[e-mail]>