Re: [codec] Thresholds and delay.

[Marshall Eubanks <[e-mail]>]

Dear Stephen;

On May 12, 2010, at 6:43 AM, stephen botzko wrote:

> Acoustic echo cancelers remove the signal from the local speaker(s)  
> that is feeding back into the local microphone(s).  So if my AEC is  
> turned off, then you will hear the echo (and I will not).
>
> The echo that you hear is delayed by the round trip transport time,  
> the processing delays in the sender and receiver, and the acoustic  
> path delay between the speaker and the microphone.  However, the  
> echo that the AEC detects is only delayed the acoustic path delay  
> and whatever processing delay (buffering) the receiver has on its  
> audio capture and render circuitry.
>
> So you are correct if you are thinking that the network transport  
> time and codec delay do not impact the requirements for the AEC.
>
> There can also be path delay between the receiver output and the  
> actual speaker.  For instance in a video conferencing application,  
> the video processing in the TV itself can add delay, and the TV will  
> delay the audio to maintain sync.  So if your phone system can be  
> connected to external sound systems, then that can effect the delays  
> the AEC has to accommodate.
>
> Also, in larger rooms the acoustic path delay can be substantial  
> since sound takes about 100 ms to travel 100 feet  (341 meters per  
> second).

It is not clear to me if the CODEC charter includes stereo (not  
typically used in telephony, but reasonably common in video  
conferencing); stereo AEC is tougher than mono and I believe that  
there is some serious IPR in this area.

Regards
Marshall

>
> Regards
> Stephen Botzko
>
> On Wed, May 12, 2010 at 4:01 AM, <[e-mail]> wrote:
> Stephen&co,
>
>
> Generally I believe that AECs mainly take care of the acoustic echo  
> generated in the phone itself (operating on the microphone signal,  
> acoustic delay up to a few ms). Do you mean that there is additional  
> processing on the receiving side for the echo returning from the B  
> user side?
>
>
> Best regards,
>
> Jari
>
>
> From: [e-mail] [mailto:[e-mail]] On  
> Behalf Of ext stephen botzko
> Sent: 11 May, 2010 21:19
> To: Ben Schwartz
> Cc: [e-mail]
>
>
> Subject: Re: [codec] Thresholds and delay.
>
>
> >>>
>
>
> In the presence of echo, round-trip delay must be kept below 30 ms to
> ensure that the echo is perceived as sidetone, according to the  
> Springer
> handbook of speech processing:
> >>>
> Though true, I don't think this is a mainstream consideration.
>
> VOIP phones that are capable of speakerphone operation all have  
> acoustic echo cancelers, and those cancelers are already tuned to  
> deal with internet delays with other voice codecs.  Certainly our  
> phones and videoconferencing systems do not have problems with path  
> delays of this order (hundreds of milliseconds).
>
> From my own experience (not testing) I agree with Brian's claim that  
> 500 ms round trip is acceptable for most conversation.
>
> It does depend on what you are doing, and there are certainly tasks  
> where much lower delays are needed.
>
> Regards,
> Stephen Botzko
>
>
> On Tue, May 11, 2010 at 2:06 PM, Ben Schwartz <[e-mail] 
> > wrote:
>
> On Tue, 2010-05-11 at 12:48 -0400, Marshall Eubanks wrote:
> > As a point of order, I object to any graphs without an available  
> paper
> > behind them.
>
> I have located the first paper mentioned by Christian Hoene at
> http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=81952
> but of course it's paywalled.
>
> One test in that paper told trained subjects to "Take turns reading
> random numbers aloud as fast as possible", on a pair of handsets with
> narrowband uncompressed audio and no echo.  Subjects were able to  
> detect
> round-trip delays down to 90 ms.  Conversational efficiency was  
> impaired
> even with round-trip delay of 100 ms.
>
> Let me emphasize again that these delays are round-trip, not one-way,
> there is no echo, and the task, while designed to expose latency, is
> probably less demanding than musical performance.
>
> In the presence of echo, round-trip delay must be kept below 30 ms to
> ensure that the echo is perceived as sidetone, according to the  
> Springer
> handbook of speech processing:
>
> (http://books.google.com/books?id=Slg10ekZBkAC&lpg=PA83&ots=wc9yM9WrCs&dq=sidetone%20delay%2030%20ms&lr&pg=PA84#v 
> =onepage&q&f=false)
>
> Such low delays are clearly impossible on many paths, but for Boston  
> to
> New York City (or London to Paris), ping times can be less than 18 ms,
> making echo->sidetone conversion just barely possible for a codec with
> 5ms frames.
>
> I accept Brian Rosen's claim that a slow conversation doesn't normally
> suffer greatly from round-trip latencies up to 500 ms, but under some
> circumstances much lower latencies are valuable.  Let's make sure
> they're achievable for those who can use them.
>
> --Ben
>
> _______________________________________________
> codec mailing list
> [e-mail]
> https://www.ietf.org/mailman/listinfo/codec
>
>
>
> _______________________________________________
> codec mailing list
> [e-mail]
> https://www.ietf.org/mailman/listinfo/codec