Xref: utzoo sci.electronics:10772 comp.std.internat:645 rec.video:11248 comp.graphics:10532 Path: utzoo!attcan!uunet!tut.cis.ohio-state.edu!zaphod.mps.ohio-state.edu!usc!elroy.jpl.nasa.gov!ames!eos!shelby!csli!keith From: keith@csli.Stanford.EDU (Keith Nishihara) Newsgroups: sci.electronics,comp.std.internat,rec.video,comp.graphics Subject: Re: I don't need HDTV! Message-ID: <12780@csli.Stanford.EDU> Date: 21 Mar 90 15:53:03 GMT References: <5478@okstate.UUCP> Sender: keith@csli.Stanford.EDU (Keith Nishihara) Reply-To: Neil Hunt Followup-To: sci.electronics Organization: Center for the Study of Language and Information, Stanford U. Lines: 78 minich@a.cs.okstate.edu (MINICH ROBERT JOHN) writes: >From article by bas+@andrew.cmu.edu (Bruce Sherwood): >> The analogy with audio is that a CD with frequency response out to 10 >> MHz would not sound better than one with frequency response out to 20 >> KHz, because the human ear can't hear the higher frequencies. > Well, it probably would sound a bit better. Consider this: >A 20KHz sample on CD looks something like this > * * * * * * * * * * >* * * * * * * * * * >which is just a dumb square wave. Sure, it's at high enough of a pitch >that most people wouldn't be able to discriminate between it and a pure sine I can't take this any more! You don't just feed the samples through an audio amlifier and see the square wave! You put them through a `reconstruction filter' which reconstructs the waveform. An ideal reconstruction filter, with a step function low pass frequency response at 20kHz will reconstruct the 20kHz waveform as a *perfect* sine wave. It will also reconstruct a 19.99 kHz waveform *perfectly*, notwithstanding the fact that there is a beat between the sample frequency and the frequency represented (the sample points `walk' slowly along the wave shape). So if the basilar membrane in your ear responds up to 20kHz you _will not hear_ the difference between a properly reconstructed signal from 44.1kHz samples and a signal reconstructed from 20MHz samples! Most adults' hearing is far below this limit, in any case (15kHz is considered good -- if you have often: operated heavy machinery, fired a gun, driven a car with the window open, or listened to loud music with headphones on, 8 to 12 kHz may be more like it!) Before someone asks what if the original were not a sine wave: recall that complex waveforms may be considered as a summation of sine waveforms of different amplitudes and frequencies, so in a linear system it is valid to think only in terms of the behaviour of the individual sine wave components. Of course, perfect reconstruction filters are hard build, so a 44.1 kHz sample rate permits reconstruction filters to have a finite roll off starting at 20kHz and being essentially fully cut at 22.05kHz (the limit for a 44.1 kHz filter), and *still* repro- duce all frequencies up to 20kHz *perfectly*. Now if the filter did not cut off frequencies above 22.05 kHz, some of that 20 kHz signal would appear as a 24.1 kHz signal (reflected in frequency about the Nyquist frequency). This would be undesirable. Oversampling (no one sells CD players that don't `oversample' any longer, do they?) permits some of the reconstruction filtering to be done using a digital filter. Consider 4x resampling: each sample is replicated four times in a row at 176.4 kHz. A digital filter with a cut off frequency of 20 kHz can be applied. Now when reconstructing, the analog filter still has to be flat to 20kHz, but need not be fully cut until 88.2 kHz, the Nyquist rate for the 4x oversampled signal. Since the digital filter has en- sured that there will be no frequency components in the digital signal between 20kHz and 88.2kHz, a much lower Q filter may be used, which is much easier and cheaper to design. Now what about those 18 bit players? CDs only have 16 bit sam- ples dont they? but if you use oversampling and digital filter- ing, you can `interpolate' between the original samples and sam- ple quantisation. But what does it buy you? The reconstructed signal is only as good as the orignal digital material. A good advertising gimmick, in my opinion. (What about the precision and linearity of those 18 bit A-D converters?) Neil/. Neil%teleos.com@ai.sri.com Note that our mail feed via SRI is currently dead, so that flames, questions and assertions that `my hearing is good to 37.496e29 MhZ -- medically verified' (you must be an alien) will be thrown into the bit bucket.