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 -- The way out is open! http://www.theopensourcerer.com _______________________________________________ ekiga-list mailing list [email protected] http://mail.gnome.org/mailman/listinfo/ekiga-list
