sjc@mordor.UUCP (06/12/84)
>now, back to my favorite passtime of flaming CD's.... > >If they are sooooo good, them how come they naver can produce a >square square wave? >All of you out there who insist on seeing measurements and alike >before evaluating something never seem to notice that a CD has >never produced a (nearly) square wave. Either they cannot produce one >or the test disk was not properly written (and nobody is even trying >to make a proper one)[test disk that is]. >Whay you all seem to miss is that the CD square wave has a decaying >sine wave across the top and bottom of the square where it is supposed >to be flat. >Even the SHURE V15 type II can produce a more square wave. A square wave is equivalent to an infinite series of sinusoids. No matter how perfectly a CD reproduces the components below 22kHz, it cannot reproduce the components above 22kHz, so its square wave is doomed. The Shure V15 type II reproduces the components below 22kHz more poorly than does the CD, but since it reproduces some components above 22kHz (albeit even more poorly), its square wave looks "squarer". I suggest, Mr. Pearson, that you perform two experiments, using a high quality audio system. Find a signal generator which produces square waves and sine waves of the same RMS amplitude. First, record a 10kHz square wave at 0dB on a cassette and play it back, using a dual-trace scope to compare the original with the copy. Then connect the signal generator to the auxiliary input of your preamp, and alternately listen to a 10kHz square wave and a 10kHz sine wave. In the first case, I find the difference horrifying; in the second, it's almost imperceptible. Analog tape recorders have a rough time of it: not only do they have trouble reproducing high frequencies at full amplitude due to tape and head saturation, they have phase-shift problems considerably messier than those which have generated so much controversy with respect to CDs. --Steve Correll sjc@s1-c.ARPA, ...!decvax!decwrl!mordor!sjc, or ...!ucbvax!dual!mordor!sjc
charles@sunybcs.UUCP (Charles E. Pearson) (06/18/84)
[] 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.
mat@hou5d.UUCP (M Terribile) (06/19/84)
Here we go again:
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.
If you put in a 1 Khz square wave, run the output through a filter with
perfect phase characteristics which cuts off sharply at 21 kHz, and view the
result, you will see a square wave with sinusoidal type ripples on it. That's
all there is to it -- the 21st, 23rd, etc, harmonics of a 1kHz square wave are
large enough to be visible, less than 15 db down, and over 21 kHz.
It's true, some (but not all) of the filters used exhibit ringing. Oversampled
players with combination digital and analog fiters do better than straight
analog filters. The oversampling allows the corner frequences to be up
over 40hKz (hence less phase muck-up) and move much of their increased
quantitization up to the 176kHz area. The digital filtering spreads the
reconstruction distortion both backwards and forwards in time, so that
the result looks more like what they showed you when you learned about
Fourier transforms.
--
from Mole End
Mark Terribile
(scrape..dig) hou5d!mat
,.. .,, ,,, ..,***_*.ark@rabbit.UUCP (Andrew Koenig) (06/19/84)
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