Re: [Ekiga-list] speakerphone mode

[Alan Lord <alanslists gmail com>]

Stefan Bruens wrote:
<snip />
> One thing to understand is that there are two different types of echo 
> cancellers - line echo and acoustic echo.
>
> Line echo exists when using analog interface cards. As analog equipment is 
> connected via a single pair of wires, both near and far end signals are 
> travelling on the same wires. Inside the handset/telephone/speakerphone, 
> there is something called a "hybrid", which decouples both signals, but not 
> perfectly.
>
> As the signals are travelling as an electromagnetic wave, almost at the speed 
> of light, the echo path is quite short (for example, Inhouse, 50m of wire, 
> 2/3 speed of light: 2*50m *3/2 / 3*10^8m/s = 0.5*10^-6s: half a microsecond, 
> Branch exchange, 2*2km, 20 microseconds). So line echo is not a problem for 
> voice applications, only for data (modems).
>
> Acoustic echo is quite different, as the signal comes from the loudspeaker, 
> travels with the speed of sound (~300m/s -> 3ms/m), is reflected by the walls 
> several times and reaches the microphone. As the echo patch is much longer, 
> an echo canceller with many more filter taps is needed, able to compensate 
> echo "tails" 500ms or even 4000ms long.
>
> OSLEC is an line echo canceller, able to cope only with very short echo 
> pathes. It may be able to compensate acoustic echos resulting from 
> microphones and speakers built into the same housing, which are thus strongly 
> coupled on a very short path (<10cm).
>
> Bye,
>
> Stefan

Cool - thanks for the very clear explanation. Now I think understand 
what he is talking about when David mentions trying to see if will work 
with 128ms tails.

And maybe it make my idea not such a good one after all. But the strange 
thing was before I tried OSLEC that the echo on our analogue line was 
quite "long" e.g. several 10s of milliseconds - very audible delay 
between original and echoed speech...

perhaps we are getting a bit Off Topic but very interesting nevertheless.

Thanks again,

Alan

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