From: paul@gaitlab1.uwaterloo.ca (paul j guy)
Newsgroups: rec.audio.high-end
Subject: Re: Making a Denon 2560 Level 1 (and jitter)
Date: Fri, 13 Nov 1992 02:28:10 GMT
Organization: University of Waterloo, Waterloo, Ontario, Canada
Distribution: usa
NNTP-Posting-Host: 129.89.7.4
Originator: tjk@csd4.csd.uwm.edu

In article <1dtof9INN321@uwm.edu> kucharsk@Solbourne.COM (William Kucharski) writes:
>
>Well, I got enough responses that I thought a post was called for.
>
....trimmed out some stuff here

>generation of the clock for outgoing digital data.  The problem in the Denon
>units is that Denon did not design the players to be level 1 transports and
>thus did not specify the use of high precision (NPO) parts.  The upshot of
>all this is that the 16.9344 Mhz crystal may be off by hundreds of parts per
>million as it is simply trimmed by two known value capacitors.  Therefore,
>depending upon crystal and capacitor tolerances, the clock encoded into the
>digital output stream may be off by hundreds of parts per million as well.

....removed some more text, and a schematic of the crystal oscillator...

>The solution to the problem is to replace one of the capacitors trimming the
>crystal in order to provide a more accurate crystal frequency resulting in
>the generation of a clock signal with significantly less jitter.
>
...removed some more text here describing how to align the oscillator to
be within the tolerances required.
>
   I think that you will say that I am nit-picking, but apart from the
otherwise excellent description of how to get the oscillator bang-on,
you have not adressed the jitter problem (IMHO).
   To improve the jitter in a crystal oscillator will require MUCH more
than a counter, you will need to know intimate details about the 
crystal (it's cut for example), and the drive levels, circuit Q, analog
noise in the oscillator circuit, power supply noise, etc,etc. If the
manufacturer has the proper loading for the crystal (they are specified
for a certain load capacitance) then I suspect any fiddling with the
values of external capacitance will most likely agrravate the jitter
problems. I wouldn't be at all surprised if the manufacturer was a
bit slack in designing this circuitry, and the jitter may be reduced
by tweaking. 
   The only tweaks that come to mind for solving a problem like this
without having to resort to more than $20,000 worth of test equipment
are the following:
1:  Decouple the supply (where it fed to the chip) with high frequency type
decoupling capacitors, which are, I'm sorry to say, ceramic and not
polypropylene. At the frequencies we are dealing with here, and their
harmonics, geometry and lead inductance are the important issues. The
size MUST be small, and the area bound by the leads, cap body, and
circuit board must be small as posible. I have achieved good results
soldering right to the IC leads, but only to the DIP packages, I wouldn't
try soldering the high density types.
2:  take the crystal right off the board, and using some of the high
spectral purity oscillator circuits, make a discrete oscillator. The
amateur radio types can probably point you toward the better designs.
What degree of spectral purity is needed? Since jitter appears
as sidebands to the original fundamental frequency, how narrow
does it have to be? If I had to model the jitter modulating signal,
how would I characterize it? What is the frequency spectrum of the
jitter itself? Yechh..... horrible questions, I'll bet there are
no good answers to these, without money up front. If you get
on the jitter bandwagon, look in October and November issues of
RF Design, there's a 2 part article on timing recovery and jitter.
3:  Shield this circuit from other influences. More importantly,
shield the rest of the CD player from THIS circuit, 'cause I found
my CD player (NAD5100) was injecting harmonics up to 100MHz and more,
into the D/A and audio output stages. The radiated junk was not a
small signal, touching a X10 oscilloscope probe, with a ground lead
that didn't exceed 3 inches, to ANY of the signals related to this
oscillator, would clobber the reception on my FM tuner, and it
was connected to CABLE through coax, and properly terminated. The 
radiation of interference was considerably reduced when I grounded
the crystal can. I was able to reduce the leakage of this signal into
the audio chain, by at least a factor of 10 by grounding the crystal can
and carefully shielding that part of the circuit with .005 inch copper
foil. (And it seemed to make an audible difference, for what that's
worth) ....whoops I'm changing topics again! I found audible differences
when cleaning up the RF, but what about jitter? I suspect that
shielding would have very little effect, since this is probably the
biggest signal radiator in the CD player, and is not likely to be affected
by the rest of the quieter circuitry. Has anyone else tried fiddling
around with the oscillator circuits (with reducing jitter in mind)?

   And would it make any difference? Other than the ears which can
easily influenced by bullshit, sleep, coffee, time of day, and previous
noise exposure, I don't think any of us can make a clear unbiased
judgement as to whether the circuit modifications actually reduced
the jitter. Some of the changes may improve the quality of the sound,
but quantitively pinning down the effects of jitter-reducing tweaks
will need some very good test equipment, which I'm sure very few of
us can afford. The kind of money I would need to successfully tweak
my cheapo system, would pay for the best of the "class A" equipment.
Oh well....back to the ears and lots of 'psychobabble' for them
to feed on.

          -Paul

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