Re: [GnomeMeeting-list] Differing behaviour of mixed sound channels using GM

Am Mittwoch, 6. Oktober 2004 23:41 schrieb Doug Beattie:
> I was wondering if anyone has seen (actually heard) this problem
> and knows how to deal with it.
> My friend and I were trying to mix both the microphone and line inputs
> from TV cards (V4L) so we could discuss about a program being viewed
> as it was airing.
> I found that if I set KAMix capture to Mix that he could hear me,
> my line input, but that he would also get his own voice with a delayed
> feedback. Very annoying. If I went to Mic only on my end his voice
> is not delayed.
> When he set his KAMix to Mix I would hear his line input, but not
> his voice nor would I hear my voice as feedback.
> Any thoughts on how to set KAMix properly for this to work correctly?
> Doug

Assuming you have some cheap Soundblaster/Onboard Sound ...

These boards normaly have 2 possibilities what to record:

1. A single source, eg. Mic, Line in, CD in ...
2. Playback mix

Neither solves your problem. In the second case, you create a feedback loop, 
as you have had. Your friends voice is travelling through the net 
(delay ...), gets played back by the soundcard by the pcm device, is recorded 
the same time as part of the playback mix, and travels back to him, so he 
will hear his voice with two times network delay.

The only possibility I can think of is using the tv cards adc for recording 
the tv sound (yes, most tv cards have their own adc, almost nobody uses it, 
but it works. For philips saa713x based ones, you have to use the right 
module parm, for bt8x8 you have to load some additional module, tvaudio IIRC) 
and the soundcard for the microphone and mix the two sounds together in 
software. It may be possible to do this with some magic in the alsa config, 
but the alsa mailing list is a better place for asking this.


Stefan Brüns  /  Kastanienweg 6 - Zimmer 1206  /  52074 Aachen
mailto:lurch gmx li
   phone: +49 241 169-4206     mobile: +49 160 3797725

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