It's the average frequency of human voice. if you look at a conversation on a osciliscope, it averages out to 4k, so you double that and get the 8k sample rate.
-- RFC 1149 Compliant. ""John Neiberger"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > I understand all that, but what I don't remember is why there > is a 4KHz low-pass filter on voice lines. I know I've read the > reason before but I just can't recall what it was. Was it > simply arbitrary? A 4KHz upper limit is obviously sufficient > for voice quality. Did someone just pick that limit and filter > out everything above it, possibly to filter noise or something? > > Hmm...this is bugging me now. :-) > > But I can't be distracted right now, I'm trying to > study...which explains why I keep taking time out to check my > email and search the internet for MP3s of Clannad. :-) I did > just find a killer sampler of Celtic stuff. Very relaxing... > > John > > > ________________________________________________ > Get your own "800" number > Voicemail, fax, email, and a lot more > http://www.ureach.com/reg/tag > > > ---- On Tue, 26 Feb 2002, Annlee Hines ([EMAIL PROTECTED]) > wrote: > > > All right, John-- > > > > A couple of years ago (discreet cough), Cisco gave away > copies of books > > as > > promos. One was _IP Telephony_ by Gorlaski and Kolon (McGraw > Hill, > > 2000). > > GOOD BOOK. On pp 77-78 is an explanation of the Nyquist rate > and voice > > sampling: > > > > "...Thus, if an analog voice signal reaching up to 3400Hz is > to be > > sampled > > at the Nyquist rate, the sampling frequency must be at least > twice that, > > or > > 6800Hz, or samples per second. > > > > "Sampling does not have to be done at the Nyquist rate. The > Nyquist rate > > is > > a minimal requirement to reproduce the input waveform, but > sampling can > > be > > done at rates higher or lower than the Nyquist rate. If > sampling takes > > place > > at rates lower than the Nyquist rate, the result is > distortion of the > > waveform known as (italics) aliasing. Aliasing just means > that there is > > more > > than one output waveform that fits the 'connect the dots' > pattern of the > > samples. There is no aliasing ast the Nyquist rate and above." > > > > They go on to point out that, by sampling at a rate above the > Nyquist > > rate, > > you have more than the minimum required information to > reliably > > reconstruct > > the voice signal at the destination. This allows you to lose > a few > > samples > > in transit (not that such things would ever happen, of > course) and still > > have only one possible reconstruction. Sampling at 8000Hz > means there is > > a > > 4000Hz voice bandwidth (overly generous but convenient > because 4 is a > > power > > of 2 and that makes it easier to code in a binary system). > > > > And from the 8000 samples/sec, each of which sends 1 8-bit > word, we have > > the > > DS0 of 64000 bps (why only 56000 bps may be usable is a > separate issue, > > having to do with signaling on telephone links). > > > > Annlee > > ""John Neiberger"" wrote in message > > [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > > 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=36591&t=36566 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
