Re: [codec] #6: Echo and Ultra-low Delay
"codec issue tracker" <trac@tools.ietf.org> Mon, 24 May 2010 14:18 UTC
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Subject: Re: [codec] #6: Echo and Ultra-low Delay
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#6: Echo and Ultra-low Delay ------------------------------------+--------------------------------------- Reporter: hoene@… | Owner: Type: defect | Status: closed Priority: major | Milestone: Component: requirements | Version: Severity: Active WG Document | Resolution: fixed Keywords: | ------------------------------------+--------------------------------------- Changes (by hoene@…): * status: new => closed * resolution: => fixed Comment: [Stephen]: 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). [Benjamin]: In my experience, the biggest problem is in complex conferences with many users at different latencies, and endpoints consisting of multiple- microphone multiple-speaker systems scattered across a room and frequently repositioned. Anyway, I don't think it's very contentious that (a) on high latency links (e.g. long distances), AEC will always be needed in speakerphones, (b) the perceived audio quality in the presence of AEC is improved by very low round-trip acoustic latency, and (c) AEC may be retuned at very low round-trip acoustic latency for further improvement in perceived audio quality. Very high audio quality may not yet be a major concern in this market, but I think this working group is going to change that. [Jari]: 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? [Stephen]: 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). [Benjamin]: "Do you mean that there is additional processing on the receiving side for the echo returning from the B user side?" Only in the user's head. The psychoacoustics of echo are very different depending on the echo time. This means that the perceived echo-canceler fidelity depends on the acoustic round-trip delay. It also means that the AEC should be retuned depending on the round-trip delay. [Marshall]: 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. [Stephen]: I agree with Jari's assessment, though I am not sure exactly what you mean by "retuning". The far-end user will perceive the fed-back echo in various ways, and that does depend on the round-trip time. However, the job of the AEC is to remove the echo, so there is no fed-back echo to hear. And the algorithms which remove echo do not depend on the round- trip delay to the far end. At least not the good ones. Perhaps more substantively, I do not think this AEC discussion actually matters in this WG context. We are not working on an AEC, we are working on an Internet Codec. Even if (for argumentation purposes) you accept the idea that somehow the AEC needs to be tuned to the round-trip delay, the round-trip delay varies enormously depending on the connection, and this round-trip time in general is not even discoverable (esp. if gateways or SBCs are used). Nothing we are doing in this group will change any of the above. As far as sidetone goes, I do not understand why that keeps coming up either. For the speakerphone use case eliminating AECs from both ends requires two conditions: (a) the round trip delay has to be very low (30 ms or less, from all delay sources) (b) there has to be sufficient attenuation on the loop. The first condition has been brought up repeatedly. For general internet WAN connections it is clearly not met, and it is in fact difficult to meet even on more local connections. The second condition has not been discussed much. But if you do have low delay but do not have enough attenuation, what you get is feedback, and not sidetone. Since users have control over the speaker volume, the second condition (in general) also can not be guaranteed. All speakerphones that I know of are either half-duplex or use AECs. This is true even for PSTN phones used in local-loop circuit-switched calls, which is the lowest delay telephony connection that there is. So for telephony at least, it is clear that AECs are needed. So I do not think continued debate on the value of sidetone is productive. I agree with Michael's comment "achieving low enough latencies for sidetone perception should not be a goal of the wg" CONCENSUS: Achieving low enough latencies for sidetone perception is out of scope. -- Ticket URL: <http://trac.tools.ietf.org/wg/codec/trac/ticket/6#comment:2> codec <http://tools.ietf.org/codec/>
- [codec] #6: Echo and Ultra-low Delay codec issue tracker
- Re: [codec] #6: Echo and Ultra-low Delay codec issue tracker
- Re: [codec] #6: Echo and Ultra-low Delay codec issue tracker