You are right, John. With digital telephony, the analog speech signal is filtered before sampling. High and low frequency components are removed. I think it was just a tradeoff. We don't expect the human voice to sound that great over the phone anyway, and by filtering we can reduce bandwidth requirements. In fact, the human voice doesn't sound too great over the phone. We've just gotten used to it. Music on hold over the phone sounds pretty bad.
Priscilla At 10:25 PM 2/26/02, John Neiberger wrote: >This is OT, but the upper limit of human hearing is actually >around 20KHz at best and usually drops to around 16KHz or so. >If your upper limit starts to drop below that you'll start to >notice that it's difficult to hear clearly. (Sorry, in my >other life I'm a sound engineer and musician.) > >I've heard that the 4KHz limit is because there is a low-pass >filter used for voice. I can't remember the exact reason, but >that information plugged into the Nyquist theorem explains--as >Priscilla mentions--why a DS0 is 64Kbps. > >Okay, time to do some serious studying once I'm through being >lazy and drinking this coffee... > >John > >---- On Tue, 26 Feb 2002, Priscilla Oppenheimer >([EMAIL PROTECTED]) wrote: > > > At 08:06 PM 2/26/02, Rafay wrote: > > >How do you describe Sample Rate.? > > > > In what context? The term is sometimes used when describing >the analog > > to > > digital process, for example when digitizing voice. Voice >produces an > > analog wave as your lungs and tongue press against the air. >An analog > > wave > > has infinite possible values. Computers can't deal with >infinity. They > > work > > with discreet numbers. The solution is to sample the analog >voice many > > times per second. Sampling means to take a snapshot. > > > > The sample rate is how often the analog wave is sampled. >Nyquist showed > > that you have to sample at twice the rate of the highest >frequency that > > may > > occur in the original data. Most humans don't output (and >can't hear) > > anything about 4 KHz. So sample 8,000 times per second (8Khz) >and the > > result will be good enough. When using a sample rate of 8,000 >KHz, if > > each > > sample is saved in an 8-bit byte, the resulting data rate is >64 Kbps. > > That's one DS0. Compression allows us to use a smaller data >rate, with > > some > > loss in fidelity. > > > > Priscilla > > ________________________ > > > > Priscilla Oppenheimer > > http://www.priscilla.com >[EMAIL PROTECTED] > > > > > > >________________________________________________ >Get your own "800" number >Voicemail, fax, email, and a lot more >http://www.ureach.com/reg/tag ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=36713&t=36566 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
