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On Mon, 9 Feb 1998, Paul Khoury wrote: > On Mon, 09 Feb 1998 17:06:15 -0700 (MST), billy@MIX.COM wrote: > > >> >22kHz is a bit high for a 44.1kHz sampling rate - the high frequency > >> >limit is one half the sampling rate... > >> > > >> I don't quite understand (for a change. <g> ). Could you explain that > >> again, please? > > > >In the world of digital sound recording and transmission there is > >what's called (I believe) Shannon's law (many years since I was in I always thought it was Nyquist's theorem. Shannon is usually more associated with coding information inside a signal. > >school) which says the highest frequency that can be digitized is > >one-half the digital sampling rate. Thus a 44.1kHz sample rate > >system can only go up to 22.05kHz. Then there are anti-aliasing > >low-pass filters to enforce this, but they are not perfect so there's > >some gradual decline in frequency response before hitting the actual > >cutoff point. Since in this case you are only 50Hz away in practice > >there will not be much left of your signal after going through this > >filter. > > > >For what it's worth any decent analog tape recorder running at > >15 ips (inches per second) or faster would record this quite well. > >Or if you could sample at 48kHz you might have a chance of getting > >it that way too. I don't know what software might be available > >for the Thinkpad at that sample rate though. I believe the default DOS audio software that came with the 701 is capable of 48kHz. It's been ages since I booted to DOS so I could be wrong. > >The next step in commercial systems looks like a 96kHz sample > >rate - if this eventually finds its way into notebook computers > >you'd have no trouble at all, but that's probably a year or > >two away. > > I'm just trying to record a train passing by for a small project, > so if I choose 22KHz, is that the sample rate, or the other rate? > > Or do you think 11KHz might work fine as well? That depends on what you want to record. If it's just an audio effect or looking at a doppler shift, 22kHz (44kHz sampling rate) is above the hearing range for most people (depending on age and gender, most people limit out at around 14kHz to 20kHz). If you've never heard a 16-20kHz sound, it is *very* high pitched. 22kHz sampling should be fine. You may even be able to get away with 11kHz (depending on if you're listening for the whistle or the engine roar). OTOH, if you want to do a sophisticated analysis of the sound of a train passing, you're going to need the higher frequencies a 44, 48, or 96kHZ sampling rate will provide. The fundamental tone of any sound (e.g. a whistle) is its lowest frequency. All the harmonics and overtones that give each different sound its individual character are higher in frequency, often several octaves (one octave is twice the frequency). Also oversampling makes it easier to run a Fourier transform (to get it into frequency space so you can tell which "notes" make up the whole thing). Boy does this bring back memories... :-) -- John H. Kim kim@mak.com