[Speex-dev] Sound card problem in acoustic echo cancellation

Steve Underwood steveu at coppice.org
Wed Jun 9 18:27:09 PDT 2010

On 06/10/2010 03:13 AM, limaoquan2000 wrote:
> Then why ONE sound card have different capture and playback rate?
> It must be ONE single physical clock generator which is used by both 
> ADC and DAC
> in the sound card, isn't it?
It seems some cards use a PLL for their ADC, so they can lock to an 
incoming SPDIF signal, but always use a local crystal clock source for 
their DAC. These cards do not have their ADC and DAC synchronised. It 
seems like Skype's speakerphone works OK on these machines, so maybe 
they do something tricky to measure the slip rate and resynchronise in 
software. On the other hand, they might just fall back to a crude echo 
suppression scheme. Skype seems to do a reasonable job, though.

> If you are a hardware engineer. Will you design two different physical 
> clock for
> ADC and DAC seperately?
> What on earth causes this problem? Who knows its intrinsic real reason?
> Isn't there any other solutions?
> For example, I have also tested Microsoft MSN and Skype. It seems that AEC
> function of those softwares are all no problem. Their AEC are designed 
> by GIPS.
> I guess that must be some solutions.
> The solution of GIPS is only echo suppression?

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