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http://forums.adobe.com/thread/448445
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Travis, have you managed to find a work around? I have been thinking about a server side solution. It basically involves decoding the Speex stream to RAW, running an AEC algorithm, re-encoding and sending the data on its way. Its just a thought and I not sure on the additional latency it will add.
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server side AEC post encoding:  http://lists.xiph.org/pipermail/speex-dev/2007-July/005935.html
For AEC, the following conditions must be met:
1) No codec must be used in the echo path (maybe G.711 is OK)
2) There must not be any drift in the sampling clocks
3) There must not be any audio samples lost on the echo path
The entire path from AEC to loudspeaker and from mic back to AEC must be free of any non-linearity, codec, drift
It turns out the Adobe does have AEC. They use it on Adobe Connect Pro http://www.adobe.com/products/acrobatconnectpro/. The AEC is delivered via a Flash plugin (yes they exist) through Flash Player Express Install (see http://www.flashmagazine.com/news/detail/plugins_for_the_flash_player/). The Express Install mechanism has been used since Flash 7 to auto update Flash and it is used in install AIR.
Adobe's Mysterious System.Product and Connect Pro:  http://www.lingolesson.com/misc/adobe-s-mysterious-systemproduct-and-connect-pro/
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http://stackoverflow.com/questions/1860522/echo-problem-with-speakers-in-flash-red5-application-for-two-way-video-conferenci
The solution you're looking for is called echo cancellation. Unfortunately today there are only two options:
1) do the echo cancelling in Flash. Adobe has the ability to do this embedded in flash, but unfortunately they only allow it to be used when running against Adobe Connect's conferencing server. Lame, but true.
2) your other option is to decode the audio from all participants in a server, mix the audio there,