wjm@whuxj.UUCP (MITCHELL) (06/12/84)
<chomp, chomp, chomp> Yes, one of the problems of choosing a 44.1 KHz sampling rate for CD's is that no frequency above 22.05 KHz will be reproduced. Since a square wave is composed of all odd harmonics of the given fundamental frequency, the square wave response for CD players will not look as "good" as that from analog components that can reproduce 22KHz+ frequencies (even with 5 or 10 dB of loss compared to response at 1KHz or so). What does this mean??? 1. As I've said before, I feel the 44.1 KHz sampling rate is too low - I believe that there is useful musical information in the top octave of music (10-20 KHz) and that some of the harmonics on percussion instruments go as high as 25KHz. Personally, I can hear 25 KHz (ultrasonic alarms drive me bats) and there is information there. While I realise the need for a digital standard if CD is to be a workable system, I fear we may have been locked into a standard that limits us as recording technology improves. This is the same problem that happened with the NTSC TV standard, and is now coming back to bite us when we want to develop HDTV. 2. The square wave test is very useful, since it does give some feel for the high frequency response of the system. 3. Even though no CD player can properly handle a 20 KHz square wave, the test results provide valuable information about the effects produced by the filters in the unit, by examining the amount of ringing that takes place at the corners of the square wave. Regards, Bill Mitchell (whuxj!wjm)
rfg@hound.UUCP (R.GRANTGES) (06/12/84)
Now hold on just one darned minute, folks. I'm sure I have seen square waves from a CD test record that looked better than anything I've ever seen from a disc (except maybe with the old Weathers' pickup). Mitchell, here you are going on about 20 khz square waves. Almost any amplifier is going to mess that one up today, not to mention the small problem of how do you radiate it acoustically. We <should> be talking about 1000 hz square waves. The CD medium is inherently capable of producing -off the disc- a "perfect" square wave, where I define perfect as flat topped (in this case response to DC so even a 100 hz or 10 hz square wave would look flattopped) with "vertical" transitions (in this case, a rise time of less than one sample period -24 microseconds). True, some players will show ringing. Most of the photos I have seen show less ringing than what we have become accustomed to with phono cartridges - of which moving coil designs seem to be the worst (?). THe initial article led off with a wild claim, contrary to fact, that everyone seems to have accepted - that CD square wave response is bad. It's not. It's superb! For that matter, so is the impulse response. Somebody donate big bucks to the cause and we'll buy some test discs to take pictures at home. Meanwhile, I'll go by what I've seen printed. A well known rule of thumb is that for a square wave to look good, you need flat response and linear phase between 0.1 and 10 times the repetition rate of the square wave. With rep rates to 1000 hz, the CD medium, even with the poorest "filtering", provides almost perfect response - certainly far better than any other medium in common use. I hate this overkill, why didn't yoiu guys kill this one earlier (Quick, Henry, the Flit!) Dick Grantges hound!rfg
gregr@tekig1.UUCP (06/14/84)
Sorry Bill but I got to take issue with you on your remarks concerning CD square waves. CD square waves are remarkably good for frequencies within the audible range. In general they are much better than the most highly regarded moving coil cartridges which you can verify by looking at back issues of hi fi mags as one source. The best "looking"! square waves are produced by moving magnet cartridges, which are generally considered inferior to the moving coils by the "golden ears". So what does this tell you about drawing conclusions from looking at square waves? Should we denounce the sound of moving coil cartridges because we misinterrupt the meaning of the square wave appearance? Secondly it makes no since to talk about CD response to 20Khz square waves. The second harmonic is 40Khz and almost no speaker can reproduce it and fewer listeners can hear it. Hence reproducing 20Khz square waves is the same thing as reproducing 20Khz sine waves as far as the listener is concerned. In addition I know of no tape recording equipment that could possible record a 20 Khz square wave without virtually turning it into a sine wave, so where are you going to get any source material.