[net.audio] David Mohler is completely correct

ark@alice.UUCP (Andrew Koenig) (08/17/85)

David Mohler is obviously in sole possession of the absolute truth.
Anything he says is fact, because he says so, and anything that
anyone says that contradicts him is "wasting time," because he
says so.  And of course, he must be right, because he says so.

There is obviously no point in trying to discuss anything with
someone who knows everything, so I won't.


			--Andrew Koenig

jj@alice.UUCP (08/17/85)

Just as an aside, I'm interested in those filters that
Mohler has invented that have no ringing.  It's odd,
but I've written a few papers on filter design, a few more
on audio, etc, and all of the filters I've seen worked because
they had "ringing".  Funny thing, a filter with no ringing 
passes just perfectly well as NO FILTER.

Check out Rabiner and Gold, Oppenheim and Schaeffer, or any
other basic text.


-- 
SUPPORT SECULAR TEDDY-BEAR-ISM.
"I see a dark cloud, On the horizon,..."

(ihnp4/allegra)!alice!jj

mohler@drune.UUCP (MohlerDS) (08/19/85)

Why is it that people read text and see what they choose to see?
I state that I am oversimplifying in my posting for those that
don't want to make a life long job out of picking a CD player.
Second, if you feed a 1KHZ square wave into an analog filter
that is say an eight pole filter and look at the output, and then do
the same thing to a digital eight pole filter (both filters set to
low pass filter at 22KHZ) if the resulting wave form doesn't exhibit
less ringing through the digital filter something is seriously
wrong! At least you had the decency to pose a technical point
rather than fly off like some others. If this doesn't clarify things I
can cite some papers and references that you can read. Why is it that some
NETTERS at your location prefer sarcastic replies rather than keeping the
net less hostile?

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

ark@alice.UUCP (Andrew Koenig) (08/20/85)

David,

I'm sorry you think I was flying off.  I'm not -- I'm agreeing with
you completely!  Everything you say is true.  Therefore, there's
really no point in my discussing anything with you.

rdp@teddy.UUCP (08/20/85)

In article <4164@alice.UUCP> jj@alice.UUCP writes:
>
>Just as an aside, I'm interested in those filters that
>Mohler has invented that have no ringing.  It's odd,
>but I've written a few papers on filter design, a few more
>on audio, etc, and all of the filters I've seen worked because
>they had "ringing".  Funny thing, a filter with no ringing 
>passes just perfectly well as NO FILTER.
>
>Check out Rabiner and Gold, Oppenheim and Schaeffer, or any
>other basic text.
>

Correct me if I am wrong (and I am sure the righteous amongst certainly
will, in a most vehement manner), but do not filters of the Bessel
persuation (or some other forms of constant time delay) succeed in
filtering without ringing?

Also, I have yet to see any evidence of a first-order filter even having
overshoot, much less ringing, or is something wrong with my scopes, 
resistors, capacitors and books?

rdp@teddy.UUCP (08/20/85)

In article <4179@alice.UUCP> ark@alice.UUCP (Andrew Koenig) writes:
>David,
>
>I'm sorry you think I was flying off.  I'm not -- I'm agreeing with
>you completely!  Everything you say is true.  Therefore, there's
>really no point in my discussing anything with you.

I have been elected by a vast majority of net.audio readers to try
to persuade David Mohler and Andrew Koenig to continue their spat
in a more appropriate news group, such as net.marital_squabbles.

What? You don't believe me? Why, I had the ballots here just a second
ago.

Wait, I'll find them... YOU KIDS STOP YOUR FIGHTING OR YOU WILL BE SENT
TO YOUR ROOMS!!!!! excuse me....

				:-)

That's what I've always liked about the audio business, cool, impersonal,
skeptical and scientifically informed discussions about equipment and 
music. Too bad I could never find any such discussions.

Dick Pierce

rdp@teddy.UUCP (08/20/85)

In article <21@drune.UUCP> mohler@drune.UUCP (MohlerDS) writes:
>Why is it that people read text and see what they choose to see?
>I state that I am oversimplifying in my posting for those that
>don't want to make a life long job out of picking a CD player.

Whew! what hostility!

>Second, if you feed a 1KHZ square wave into an analog filter
>that is say an eight pole filter and look at the output, and then do
>the same thing to a digital eight pole filter (both filters set to
>low pass filter at 22KHZ) if the resulting wave form doesn't exhibit
>less ringing through the digital filter something is seriously
>wrong!

Let's try this: an analog 8 pole Butterworth (or eliptical) filter at
22 Khz vs. a digital 8 pole 3db pass-band ripple Chebychev. If one don't
ring a whole bunch more than the other, than something is wrong.

But, then again, aren't we comparing apples and turret lathes?

How about saying something like "An analog 8 pole Butterworth filter
and a digital 8 pole butterworth filter, both at 22 Khz, both having
exactly the same cutoff characteristics (perfect components, perfect
algorithms, etc.) Then I and many others can agree that the transient
responses of these to filters are identical. But to simply compare
any n-pole analog filter with any n-pole digital filter is ludicrous.

> At least you had the decency to pose a technical point
>rather than fly off like some others. If this doesn't clarify things I
>can cite some papers and references that you can read. Why is it that some
>NETTERS at your location prefer sarcastic replies rather than keeping the
>net less hostile?
See note on hostility, above.
>

Who the hell are you talking about. "you" and "your" has no definite number
attached, so I guess that means me, or us, or them, or what?

Dick Pierce

spp@ucbvax.ARPA (Stephen P Pope) (08/22/85)

     Sorry, Dave, since you clearly know quite a bit about
signal processing, but there are some misstatements in
your recent posting.
     First off, a 14-bit quantizer, oversampled at 4x, will
give you the resolution of a 15 bit quantizer, not a 16
bit quantizer.  Each time you double the sample rate, half
as much quantization noise energy lies in-band as previously
did, so you get a 3dB S/N improvement.  A 4x oversampling
gives you a 6dB improvement, which is the same improvement
you would get from adding ONE bit to your quantizer.
A 16-bit quantizer has 12dB better S/N than a 14 bit quantizer.
     Also, you are commingling two partially related issues --
D/A implementation, and reconstruction filter design.
All consumer CD players process operate at a 44 kHz sample
rate (for obvious reasons) but some interpolate up to
a higher sample rate, filter digitally, D/A convert at this
higher sample rate, and then filter the analog signal.
The advantage of doing this is that the sharp cut-off you
need at about 20 KHz is implemented by an EXACTLY LINEAR PHASE
digital filter, while the analog filter does not have a
lot of phase shift in the audio band since its rolloff occurs
much higher.
     The implication (I'm not sure you stated it explicity)
that a CD player that implements its reconstruction filter
in this way has a higher S/N than one that has an all-analog
reconstruction filter is completely false.  The statement
that the D/A implementation might be more linear is true,
however, the D/A conversion could be performed after 
interpolation to a higher sample rate even with an all-analog
reconstruction filter.  
     To summarize:

You can eliminate most of the phase shift in a CD player's
reconstruction filter by using a sharp-cut-off, linear-phase
digital filter followed by a gradual-cut-off analog filter.
As a side benefit, this possibly allows a more linear and/or
lower cost D/A implementation; but the same D/A implementation
could be used without the digital filter.

steve pope (ucbvax!spp)

mohler@drune.UUCP (MohlerDS) (08/23/85)

Steve,
	Thanks for the pleasant and accurate reply, it is refreshing that
not everyone uses flames to clarify or dispute an article.

	I must appologize for the haste in which my initial points were
made, they suffered seriously as a result. I agree that a 16 bit quantizer
improves S/N by 12db and that 4x oversampling only yields a 6db improvement
over a simple 14 bit quantizer. My point was that you are close to 16 bit
performance, but it is far more accurate, as you say, to state clearly that
you have 15 bit performance with respect to noise! 

If however, you use noise
shaping ( per the philips scheme ) where the 28 bit output of the transversal
filter is rounded off to 14 bit data, then the 14 LSB's are delayed by
one sampling period, reversed in sign, and summed with the next sample
the result is a 7db improvement in the average quantization error and noise
for low frequency signals (below 22.05KHZ). The philips scheme also uses
a 3rd order low pass bessel filter that is 
3db down at 30khz. All of this means that
from DC to 22.05 KHZ you have a 1db better S/N with the philips 14 bit
system than a non-oversampled 16 bit system. 
This I contend is virtually the same as
16 bit performance with respect to signal, noise and linear amplitude, and
was my original point!

I also agree that the noise performance is not based on the filter type
(analog or digital), but instead the size ( number of bits ) of the
quantizer, the oversampling ( if any ), and the noise shaping ( again, if
any ). My other point was since this lets you use a filter that is easier
to implement in the consumer electronics price range and since it allows you
to use an ADC that is cheaper (in some cases) than a 16 bit ADC that it
seems to me, the better approach. The cost of a 24th order filter (The 
theoretical 96th order transversal filter, 
altered by the fact that since data only
arrives at the sampling rate which means that 3 out of 4 of the numbers
multiplied in the filter would be zero, therefore the filter can be simplified
to 24 elements.) done digitally, both in terms of power and dollars
will be less expensive than an analog filter.

Thanks again for the comments!

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

mohler@drune.UUCP (MohlerDS) (08/23/85)

Steve, 
	One last point, since the 24th order transverse filter is a digital
implemented filter and the output is what allows you to do noise shaping,
the performance is better than an analog filter could do using a 14 bit
ADC. Which as you and I have pointed out can be cheaper or more linear than
a 16 bit ADC. 

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

mohler@drune.UUCP (MohlerDS) (08/23/85)

Mr. Pierce,
	For clarification of my points, please see my 3 most recent postings
or call me, as jj was so kind as to do! As for hostilities, your article is
hostile too! I am told by people who have dealt with nut.audio longer than
I, that the net is an inherently hostile place and if you can't stand the
heat shut the xxxx thing off or read selectively - so I will. 

	I still think if people were quicker to submit a technical question
rather than flame away, more information would be exchanged and alot less
bad blood would result. I will accept from your dubious article the accurate
criticism that I was unclear and that many people misunderstood my article
but as for the other slams, you become just one more case of flaming without
much point!

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