Two questions to prolongue the distraction: 1. If a 4KHz signal is sampled at 8Khz, it is sampled twice per cycle. Once in the positive half and once in the negative. Doesn't this mean that the value is dependant totally on the point at which it is sampled (and if you sample it as it passes zero both times it won't be heard)?
2. If with ISDN you have 56Kb usable in US and we have 64Kb usable in UK, if I called from UK to you in US, would you still be listening after I hung up. :-) (After this question you'd probably hang up as soon as I called - I know) Gaz (Long week and 4 beers deep - Is it Friday yet?) ""Steven A. Ridder"" wrote in message news:[EMAIL PROTECTED].; > 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 > 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 > > > 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=36880&t=36566 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
