charles@sunybcs.UUCP (Charles E. Pearson) (06/25/84)
From ark@rabbit.UUCP (Andrew Koenig) Sun Feb 6 01:28:16 206 Charles E. Pearson says the following: > But you missed the point... > If the CD logic is sound it must reproduce a perfectly square wave > given a properly generated square wave test disk. > > The sinusoidal properties that the square wave on CDs display > is the perfect example of how the CD theory is either improperly > executed or has a basic fault. > > They ring, and you know it. > > They are not as good as they are supposed to be. Whether they are > better than analogue technology is almost irrelivant except that > mediocre examples of alalogue tech. produce better examples of what > the digital tech. must produce better. > > CD get your basics correct first... this 'off by one' attitude will > not be tollerated. Sorry, Charlie, but I've got to disagree with you on this one. If all the samples in a stream of digital samples up to some point have the value X and all the samples after that point have the value Y, you might expect the "correct" output to be perfectly square, with zero rise time. However, this is untrue in the presence of bandwidth limiting. The square wave approximation now so familiar from CD player test reports is the best possible approximation to a square wave that fits within the bandwidth limitations of the system. To put it differently, suppose you had an INPUT signal that looked like one of these ringy square wave approximations. The samples taken from this signal would be indistinguishable from those taken from a true square wave. Thus, if you get the square wave "right," you do so only at the cost of getting some other legitimate (i. e. one that fits entirely within the prescribed bandwidth) wrong. In other words, attempts to reproduce square waves result in ringing, not because the theory is wrong or improperly executed, but because that is what the theory says should happen. By the way, 'tolerated' has only one l. Andrew Koenig Boy have you got it wrong. What ever the signal some component in an audio system gets it is supposed to reproduce it exactly on output (size may change, but the shape must remain constant). Otherwise this is exactly what distortion is. The ringing characteristics of a CD, at best, could be called coloring, at worst it could be called such things as 'gringe' or possibly (stretching a point) 'garbage'. One of the early issues of IAR Hotlines discusses how one decent amplifier when given a square type wave, exhibited non-flat characteristics on their display device. This was tracked to, among other things, a capacater recharging. The deviation on their display was not unlike a calcomp plotter plotting a line at 30 degrees. Even minor deviations from the wave form are easily perceived and the ones from a CD in their propaganda magazines are massive. My friends over in psycho-accoustics tell me that although you cannot 'hear' above frequency X, you can perceive the difference(s). The difference is felt, not heard. The fact that a device can reproduce any given kind of wave form is of no use in evaluating any device, but the fact that it can NOT reproduce a given waveform is a clear indication that the device has clear and ever-present problems. The frequency that the comic book 'Audio' uses in evaluating CDs is 1KHz, not 20K or 22KHz. This Frequency and all of its children fundamentals is clearly audible. Charles E. Pearson UUCP: {allegra, seismo}!rochester!rocksvax!sunybcs!charles decvax!watmath!sunybcs!charles ARPA & CSNET: charles.buffalo@rand-relay Physical: University Computing Services 4250 Ridge Lea Road room 28 SUNY Center at Buffalo Amherst, NY 14226