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The papers i read gave me the impression that the hybrids at the central offices are equipped with ECs. So they are not? Or those ECs do not provide sufficient suppression? --- "Shaw, David G (David)" <> wrote: > I will discuss V.32, since it is the first case of > this > type and all others are at least similar. > The calling modem sees answer tone (2100 Hz) and > sends a tone at 1800 Hz. > The answer modem then switches from answer tone to a > pair of tones > at 600 and 3000 Hz. After a short delay, the answer > modem > reverses the phase of the 2 tones and starts a > timer. When > the calling modems sees the phase change, it waits a > predetermined > 64 symbols at 2400 symbols per second and reverses > the phase > of the 1800, while starting a timer. When the > answer modem sees > the phase change at 1800 it now has an estimate of > the round trip > delay by subtracting the 64 symbols from the total > on the timer. > The answer modem now waits that same 64 symbols and > reverses the > phase of the two tones again. The calling modem > sees the phase > changes again and also has an estimate of the round > trip delay, again by subtracting the 64 symbols from > its timer. > The assumption here is that the echo producing > mechanism is the > hybrid in the central office closest to the remote > modem, so that > the measured delay end to end is just a little > longer than the round trip > delay I would expect to see before the remote echo > comes back. > All of this does not take into account that > possibility > of another/other echo(es) somewhere else in the > network. > > Regards, > Dave Shaw > > -----Original Message----- > From: Huo Jiaquan [mailto:] > Sent: Friday, November 21, 2003 2:10 AM > To: > Subject: RE: [echocancel] Re: LEC and NEC > Sorry, i have been working with acoustic echoes, not > very familiar with modem designs. Could u explain > 'determined through an exchange between the > end-points > during startup' in a bit more details? Or sugguest > some good ref.? > > thx > > --- "Shaw, David G (David)" <> > wrote: > > For modem, the bulk delay is determined through an > > exchange between the end-points during the startup > > of the modem. > > For voice calls, you will have to come up with a > > clever algorithm to determine the delay. > > > > Dave Shaw > > > > -----Original Message----- > > From: Huo Jiaquan [mailto:] > > Sent: Monday, November 17, 2003 8:50 PM > > To: > > Subject: Re: [echocancel] Re: LEC and NEC > > > > > > How do u determine the bulk delay for the far-end > > echo? > > _____________________________________ > Note: If you do a simple "reply" with your email > client, only the author of > this message will receive your answer. You need to > do a "reply all" if you > want your answer to be distributed to the entire > group. > > _____________________________________ > About this discussion group: > > To Join: > > To Post: > > To Leave: > > Archives: > http://www.yahoogroups.com/group/echocancel > > Other DSP-Related Groups: http://www.dsprelated.com > ">http://docs.yahoo.com/info/terms/ > http://personals.yahoo.com.au - Yahoo! Personals New people, new possibilities. FREE for a limited time. |
RE: Re: LEC and NEC
Started by ●November 24, 2003
