Re: [codec] #16: Multicast?

[Koen Vos <[e-mail]>]

Quoting Benjamin M. Schwartz:

> Quoting Koen Vos <[e-mail]>:
>> For typical VoIP applications, Moore's law has lessened the pressure
>> to reduce bitrates, delay and complexity, and has shifted the focus to
>> fidelity instead.
>
> I think this is a typo, and you mean "lessened the pressure to  
> reduce bitrates and complexity, and has shifted the focus to  
> fidelity and delay instead".

Not a typo: codecs have become more wasteful with delay, while  
delivering better fidelity.  G.718 evolved out of AMR-WB and has more  
than twice the delay.  Same for G.729.1 versus G.729.  This is not by  
accident.

The main rationale for codec delay being less important today is that  
faster hardware has reduced end-to-end delay in every step along the  
way.  As a result, a typical VoIP connection now operates at a flatter  
part of the "impairment-vs-delay" curve, meaning that reducing delay  
by N ms at a given fidelity gives a smaller improvement to end users  
today than it did some years ago.  Therefore, the weight on minimizing  
delay in the "codec design problem" has gone down, and the optimum  
codec operating point has naturally shifted towards higher delay, in  
favor of fidelity.

I've mentioned before that average delay on Internet connections seems  
to be 40% to 50% lower now than just 5 years ago, which is just one  
contributor to lower end-to-end delay.  That doesn't mean high-delay  
connections don't exist - they do, for instance over dial-up or 3G.   
But in those cases it's still better to use a moderate packet rate  
(and bitrate), to minimize congestion risk.

The confusion may come from the fact that the trade-off between  
fidelity and delay changes towards high quality levels: once fidelity  
saturates, delay gets priority.  Even more so because such high  
fidelity enables new, delay-sensitive applications like distributed  
music performances.  This is reflected in the ultra-low delay  
requirements in the requirements document.

To summarize, the case for using sub-20 ms frame sizes with  
medium-fidelity quality is now weaker than ever, because the relative  
importance of fidelity has gone up.

koen.