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From: mohler@drune.UUCP (MohlerDS)
Newsgroups: net.audio
Subject: Re: cd players? (answers)
Message-ID: <19@drune.UUCP>
Date: Fri, 16-Aug-85 22:35:03 EDT
Article-I.D.: drune.19
Posted: Fri Aug 16 22:35:03 1985
Date-Received: Tue, 20-Aug-85 04:39:55 EDT
References: <226@geowhiz.UUCP>, <4159@alice.UUCP>
Organization: AT&T Information Systems Laboratories, Denver
Lines: 88

Andy,
In your note you fail to mention the key differences and reasons
for digital filter - oversampling vs. analog filter - non oversampling!
I am replying to add some facts to that question and ask for the netters
to talk more productively on the net about CD players - you know,
discussion where we all learn rather than some of us wanting to have the
last word - wouldn't that be nice!

The digital filter - oversampling approach allows you to spread the 
quantization noise over 4x the bandwidth yielding 1/4 the effective
noise! It also allows you to use a cheaper or 
better quality 14 bit DAC instead of
a more expensive or lower quality 16 bit DAC for approximately the same
performance! 

A digital filter won't cause all kinds of ringing like
an analog filter will! This is a key issue since a lower cost
system has much more potential for preamp distortion due to slew-rate
or speed related distortions than a high priced system, making it even
more important that a low budget buyer of a CD player pick a digital -
oversampled CD player! For those that aren't familiar with speed induced
distortions: the higher the input level and frequency the better the preamp
will have to be to avoid distorting the signal. 

Also an active filter is
more difficult to manufacture than a digital filter! The frequency
response of an digital - oversampled unit will be flatter than an
analog - non-oversampled unit and it won't vary (as much) with
time, temperature, humidity etc.

So, you can get a better manufactured or cheaper digital - oversampled
CD player with lower noise, less risk of slew-induced distortions - no
ringing, and flatter frequency response (other than with a one clock -
beat frequency avoidance scheme) or pick the analog - non-oversampled
approach!

The variation in response is audible and measurable on an analog -
non-oversampled unit as is lack of filter alignment, slew rate distortions,
noise level providing you know how to measure and listen for (using test
disc's) these problems. If you don't believe this, borrow a scope and
a test disc and look and listen since if you disagree you have either:
A) haven't measured players
B) listened and used music instead of test tones
C) don't care
D) other - I don't accept that you disagree since it has been proven! By
   myself and by Audio experts in labs that tried such tests and yes the
   levels were matched and the results were statistically significant.

Also, lets say you don't care, if it is this hard to 
hear and measure the differences,
you still get either a better built or cheaper unit with a digital -
oversampled system!! 

For the record:
1) Some of this is an oversimplification for those that simply want
   to buy a unit without making a career out of it.
2) I have spent the last 3 years working on digital signal processing
   and digital circuit design and have measurements 
   to documant what I say is true
   as does N. V. Philips!
3) I think that rather than people wasting time with this argument
   of what amounts to opinion, it would be far more productive to talk
   about facts! It would be far more informative and interesting to 
   discuss open unsolved issues like: Has anyone started looking at 
   rise time limitations in
   a CD player due to the 44KHZ sampling rate vs. the psychoacoustic
   fact that the rising edge of a wave-form (ictus) has a definite
   bearing on audible cues that make a recording sound more or less
   like live? Or has anyone started looking at the fact that the only way
   to realize the potential of the CD player is to never use a low speed
   tape medium in recording due to frequency modulation distortion, modulation
   noise and massive phase shifts induced in such units? Or has 
   anyone looked into how much the
   CD players tendency to distort lower levels more than high levels
   degrades the sound potential of a CD player (if any?). I SUBMIT THAT
   THIS WOULD BE MORE PRODUCTIVE THAN BEATING THE XXIV OUT OF THE SAME
   OLD DISAGREEMENTS!!

4) I DO NOT MEAN TO DUMP ON ANDREW, I SUGGEST THAT MANY OF THE CD PLAYER
   POSTINGS HAVE BEEN INACCURATE, OPINIONATED, OR UNINFORMED AND WE COULD
   BE LEARNING SOMETHING RATHER THAN WASTING TIME. I MERELY CHOSE
   ANDY's ARTICLE TO POINT OUT ONE OF MANY INACCURACIES AND MAKE A PLEA
   FOR BETTER MORE EFFECTIVE DISCOURSE.

			David S. Mohler
			AT&T - ISL @ Denver
			drune!mohler or druxu!mohler