In the Cvoice class I attended, my instructor answered the reason why it was 4K as there were 2 groups wanting different rates, so the compromise between them was 4k. None of my IP Telephony course books or Cisco Press books for the class re-iterate that, but I recall hearing it...
Larry Letterman Cisco Systems [EMAIL PROTECTED] -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of John Neiberger Sent: Tuesday, February 26, 2002 9:23 PM To: [EMAIL PROTECTED] Subject: Re: Re: Sample Rate [7:36566] Exactly, that's what Priscilla and I both just said. :-) What I'm trying to find out is why the original 4KHz limit on voice calls was put into place. It sounds like it was simply an arbitrary decision. 4KHz is sufficient for a telephone call and to provide clear calls that included higher frequencies might have added some technical complexities, perhaps. They also added a high-pass filter around 400Hz since most telephones can't reproduce low frequencies well and it also filters out some harmonics of 50-60Hz hum that might show up from time to time. That is concrete reason for including a high-pass filter and I wondered if there was a concrete technical reason for including the 4KHz low-pass filter. From the sounds of it there really isn't a technical issue, 4K is just a nice round number. :-) I've actually read that they limit it to around 3.4KHz, but if you sample that at 8KHz you'd be well above the Nyquist limit. John ________________________________________________ Get your own "800" number Voicemail, fax, email, and a lot more http://www.ureach.com/reg/tag ---- On Tue, 26 Feb 2002, Brian ([EMAIL PROTECTED]) wrote: > 64 kbps comes about from sampling 8 bits at 8khz, 8x8000=64000 > > > Bri > > On Tue, 26 Feb 2002, 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 [EMAIL PROTECTED] Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=36604&t=36566 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
