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An interesting post from a cd player engineer at Phillips

205.188.198.186

Posted by paul on October 26, 1999 at 09:06:21

::p::: Dave Klein has started a new list called "High Fidelity", this post popped up during a recent discussion on modding cd players-what is real- what is crap.Here it is:

"It seems you would like me to say some more on this subject. Well you are
now being warned this gets a bit technical in places :-(

As I mentioned before all CD players now have the required buffers to
prevent miss clocking of the data. This means that all the data will be
timed to the system referance crystal. If the data is arriving to fast and
the buffer is filling up then the spin speed of the cd is reduced and vias
versa if the buffer is emptying.

What this does not take into account is that the clock is a ANALOGUE input
to the DAC. The process of reconstructing the digital information using the
clock as a time referance effectively FM demodulates any frequency jitter
that is on the clock on to the ouput.
The proof for this is complicated but if anyone is interested I am sure I
can find a referance to it somewhere.

Any jitter that is greater then 167pS in a 16 bit system is sufficiently
large to be measurable above a 16 bit noise floor and amounts less than this
may be audible below the noise floor paticularly if it has harmonic content.

167pS is not allot of jitter. A typical SPDIF receiver has 1nS of jitter on
the recovered clock, hence the requirement for crystal based phase locked
loops in high quality outboard DAC's.

This is one of the arguments why single box players, should sound better. No
SPDIF means no need to recover the clock with a phase locked loop. So you
can use the crystal referance to drive the DAC clocks.

A typical consumer grade crystal has about 50pS (This is a real
generalisation as they vary allot) jitter on its output. So should sound
much better. However a single logic gate on a good supply will introduce
about 100pS of jitter. (from measurements on 74HC244) So if you buffer the
clock output (a good idea if you want to drive it any distance accross the
PCB.) it is already at the limit in terms of required performance.

Now to the problem, note I said a single logic gate on a good supply. If the
supply is moving up and down becasue of erratic supply demands in the rest
of the system. For example to drive a spinning motor that is trying to deal
with an unbalanced disc which it is spinning at arround 500rpm (nominal
speed for cd replay). Then the supply to the buffer could be going up and
down allot, that supply disturbace is directly modulated onto the edge of
the signal passing through it. We have jitter. But this jitter is much worse
than the normal jitter from a buffer, it normaly has a white noise like
distribution, this induced jitter will have a harmonic component and thus be
much more audible.

There are lots of things that can cause the power supplies to be modulated
but some of the worst culprits are the spinning motor and the focus motor in
the mechanism as they draw high currents. They are normally run from a
seperate supply to reduce interaction with the analogue and digital
supplies. This can still happen however if there is a common return ground
or even in the transformer as drawing more current from one secondary will
tend to make the others dip slightly.

Other sources are data induced jitter from zero crossings. (As the data
crosses the zero threashold its binnary reprisentation goes from
0000000000000000 to 1111111111111111 (there should be 16 ones and zeros it
might be meant to have a sign bit I can't remeber the exact reprisentnation
but it has lots of 1's followed by lots of 0's) as this change occurs
current is drawn from the supplies.

Microphony in the crystal itself, it responds to the sound in the room.

Beat frequencies between the audio referance crystal and any other crystal
in the system (this is normally avoided by using the audio referance to run
the microcontroller as well)

Microproccesor noise, the program loop in a processor can draw current in a
cyclic manner as the main loop is repeated again and again. Depending on the
length of the main loop this can be in the audio range. (so yes changing the
software can make it sound better - I know it makes you go mad after a
while:-))

PSU induced jitter from the rectifier noise, which can have harmonics up
into the KHz region.

I am sure there are others but I can't think of them at the moment. Good
desing can minimise all of these but sometimes only at a cost.

So the practical upshot of this is, that putting bits of foil on your cd's
may or may not improve the sound depending on how well balanced the CD
already is and in what region your servos are operating. You could see this
in the same way as wheel balancing for a car. Adding mass could also
increase the inertia of the system which if it tends to "hunt" (i.e keep
going faster and then slower as the servors control loop is not well damped
for the paticular disk characteristics.) you could improve the sound.
Whatever happens you will almost certainly change the sound due to these
effects and it is very easy to interpret a change as an improvement because
that is what you want ot hear.

You could almost certainly achieve the same effect with a bit of sticky tape
as this would also mass load the CD. So this could be used as a check.

None of this requires strange energy patterns to be flowing, hence my
earlier post saying that if Chris wants to test the energy theory it should
be done by sticking the foil to the cd player and not to the disk. This is
one of the suggested methods at Peter Belts site
http://www.belt.demon.co.uk/index.html as well as treating furniture etc. I
consider this to be very unlikely, however he talks some really good techno
bable for anyone interested in reading it and apparently will send out free
samples of the foil if requested.

Sorry this is such a long post I hope it is of some interest."

I found it very interesting so I thought I would share this with the Asylum. The foil discussion concerned the application of a small piece of foil tape on the back of a cd to improve sonic quality-you can see this at Peter Belt's site.::p:::

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