From: Jeff.Dairiki@mailbox.bison.mb.ca (Jeff Dairiki)
Newsgroups: sci.electronics
Subject: Followup: Help! My CD player sk-sk-sk-sk-skips
Date: 24 Sep 92 23:52:05 CST

Organization: University of Washington


Here's the long awaited followup regarding the many and
varied responses which I recieved regarding how to fix
a skipping CD player (and other random questions...)

How to fix a skipping CD player:

   The first responses which I recieved said, basically, "Go get a book,
   and read it."  (This is of course, in my slothfulness, what I was
   trying to avoid.)  Nevertheless, I did go down to the used bookstore
   and bought an eight year old book on CD repair.  After an hour of
   careful skimming, I attained the following enlightenments:
     
     o CD players are complex and dastardly instruments containing
       (as also pointed out by one respondent) at least four servo loops.
       There's a focus (lens up and down) servo, a fast tracking (lens
       radially in and out) servo, a slow tracking ('sled' in and out),
       and a spindle drive servo.
       
       Being a trained EE, and having been tought all the theory about
       feedback amplifier in school, and having studies linear and non-linear
       control in graduate school, I know _The Truth_ about servo systems:
       they may be simple on paper, but making them work involves much
       black magic.
     
     o There are lots of pots and screws to turn and play with.
     
     o The problem is most likely not in the power supply.
   
   In a related vein, it was suggested I obtain a service manual for my
   CD player.  I ordered one from Sony.  It's gonna cost $10.00.  I
   haven't seen it yet.
   
   A couple days later, more practical advice started arriving.
   
   Several people have suggested cleaning the lens.   Despite the warning
   printed in my player ("Never, never touch the lens" -- with international
   symbols saying same...), the general impression is that this is a
   regular (sometimes glass, sometimes optically coated, sometimes plastic)
   lens which may be cleaned using standard methods.  Like says, lens paper
   and lens cleaning fluid, or Q-tip and distilled water, or soft cloth
   and alchol, etc, etc... -- someone said that lens cleaning fluid is
   a mix of ether and alcohol.  One should take proper precautions to avoid
   scratching the lens, or damaging the optical coating -- one should
   also not munge to heavily on the lens -- as it is delicately suspended
   on some sort of spring, and is directly driven by _two_ servo control
   systems.  (See black magic, above.)
   
   I have of course (despite the sign in the CD player) been trying this
   remedy periodically for the two years that my CD player has been skipping.
   It hasn't been helping.

   Many people reported that the main cause of CD skippage is dirty grease
   in the 'sled' transport system.  They suggested cleaning and re-lubing
   the rails that the optical mechanism sled moves on, and the worm screw
   (or rack-and-pinion as in my CD player) too.
   
   So, I went home to clean and lube.
   
   I opened up the CD player (it took awhile, these portable CD players
   are tricky to disassemble, all sorts of hidden screws and clips...).
   I found the rack-and-pinion mechanism and played with it for awhile.
   Sure enough it seemed to be sticking.  As I'd turn the drive mechanism,
   the sled would move in fits and starts...

   By the time I had it apart and cleaned (I cleaned the workings with
   alcohol and kleenex) it was starting to get late.  I dug around in my
   tool supply to find my light machine oil, and found that I couldn't
   find any; so, not wanting to leave the thing in pieces overnight, I
   applied some 'Tri-Lube' (some kind of silicon goop I use on my sailboat
   hardware...) and buttoned 'er back up.
   
   Put in a CD (I picked the one which skipped the worst -- there are
   some tracks on it which skipped so badly, I hadn't even had a chance
   to listen to them yet...) ... Wow it works!  Whattaya know.
   
   A couple hours later, my listening glee was interrupted by a single
   innocuous skip.  I wasn't even sure it had skipped (I've become
   acclimatized...)  Later it skipped a couple more times.  "Well,
   It's much better than it was.  I can live with it", says I.
   
   Next day, get home, pop in "The Incredible Jimmy Smith", turn up the
   volume, crack a beer...  Ahhh....
   
   After 10 seconds of cool blues, the CD goes into rapid skip,
   auto scramble play mode.   Worse than ever.  Eyow.
   
   Well, I figure I used the wrong grease...  I plan on recleaning
   and re-re-lubing again.
   
   
   In the mean time, more suggestions have arrived.
   
   Many suggestions involve twiddling pots and/or screws randomly.

   I have a friend who did this.  He had some brand-X CD player which
   started skipping.  He opened it up and twiddled the five pots he found
   randomly until it started to work again.  He says that he did this
   several times, and each time his tuneup would last a month or so
   before the skips started in again...  He finally gave the unit to his
   dad.
   
   One respondant reports that one skipping CD player which didn't
   repond to lens cleaning, or to rail cleaning, was succesfully fixed
   by adjusting the little setscrew on the sled which tilts the lens
   assembly.

   Another suggestion was that the 'tracking sensitivity' pot was the
   one which needed adjustment.
   
   From what I can tell from my obsolete CD repair book, the pots all
   adjust various servo loop gains and offsets -- maybe one of them
   adjust the laser-diode output level,...  In my unit I found six
   trimpots, one setscrew, and one trimcap; none of the adjustments is
   at all labeled.  Figuring that a random walk in 8-space is is doomed,
   I have decided to wait until my repair manual arrives before venuring
   into this maze.
   
   There were other suggestions.
   
   Turning the player on its side seems to have worked for some.  (I
   had already tried rotating my player -- upside-down worked best for
   me, but the improvement was minor...)

   One fellow suggested that verbal threats to the CD player might work.
   
   They didn't for me...


Despite the fact that my CD player still doesn't work, I was very
impressed by the quantity and quality of responses I recived to my query.

My gratitude extend to all who have responded, and all who have yet to
respond.

I will let y'all know if I ever get my CD player working...   

Thanks Again.

Jeff.


PS As for my other questions, regarding just what in bean city is a
one-bit D/A, and what 8x oversampling means, I recieved several helpful
replies.  Here are some edited replies -- I hope I don't offend anybody
by including your name to give you credit:


Regarding 'oversampling':

From Clifton Koch:

  It all has to do with digital filtering of the output and aliasing.  If
you sample a 1Khz tone at 20Khz, there would be no diffence in the digital
samples between the 1Khz tone and a 11Khz tone due to aliasing, so components
of both tones would be there on playback.  With a CD sampling at 44.1 Khz,
aliasing starts at 22.05Khz, so you have to filter everything out of the D/A
converter above that mark.  To get the 20-20Khz bandwith we want for music,
it takes a mighty steep analog filter to do the job, which has a lot of bad
side effects.

  In comes digital.  Oversampling to a digital filter merely means using the
same data multiple times on a digital filter.  It doesn't seem like this will
do much usefull, but mathematically, it does.  It moves the aliasing region
out by a multiple of the oversampling, allowing a very shallow analog filter
to do the anti-aliasing filtering.  That's what they mean by X times
oversampling.  The greater the number X, the less of an analog filter you need.

From Dave Platt:

The digital signal coming off of the CD must be filtered before it's
played back.  Older CD players do all of this filtering in their analog
electronics, using a "brick wall" filter...  these are expensive and
tricky to design.  More recent CD players do some of the filtering using
digital electronics...  they pre-smooth the signal using digital math by
creating "extra" samples (something like interpolation, although the
math is different) and then feed both the original and "extra" samples
to a DAC which is running several times faster than normal (e.g.  8
times faster).  The output of the "oversampled" DAC must still be run
through an analog filter, but the filter's design is much less complex
(and is hence less expensive) than that of a "brick wall" filter.

Finally, Hendrik Boshoff give this user friendly description:

Sampling may be modelled as multiplication of the signal and a periodic
series of deltas in the *time domain*. The transform of the signal is
bandlimited. The transform of the delta-series is another delta-series.
Multiplication in time maps to convolution in frequency. Therefore, the
transform of the sampled function is an infinite repetition of the
spectrum of the original signal.

You don't want to sample too fast, then you must store too much data.
Nyquist says you don't want to sample too slow, then the repeating
spectra start overlapping and therefore distorting the baseband one
you're interested in. So you sample at 43.1 kHz for a signal limited
to approximitely 20 kHz.

Having a perfect low pass filter at your disposal, recovering the
original signal from the samples requires just filtering. But real
filters are difficult to tune accurately and don't easily attenuate
60 dB in 3 kHz if cut-off is at 20 kHz.

Now for the trick: Before the D/A-stage, numerically interpolate
the samples. An easy way is just to add a sample between each
existing pair, with value their average. Do this two
times more, and you have 8x the number of samples you started with.

This does not add any information to the sampled signal (thank heavens).
So what does it do? In the frequency domain, the original band-limited
spectrum is still there. The replicas however, have shifted up in
frequency. The first one sits around 345 kHz. So you have from 20 kHz
to 325 kHz to allow your filter to cut off. This is very easy, and you
can use a sloppy and cheap filter without getting unwanted high frequency
components.


As for what a one-bit A/D is, it seems to be some kind of fancy PWM like
scheme:

Clifton Koch says:

  1 bit D/A is another method of oversampling.  Instead of PCM as an output,
you do what is called delta-modulation.  Each output sample has a value of
1 or 0, meaning the voltage should go up or down.  If you do this fast enough,
you get a very close approximation of the original wave, with very little
antialiasing circuitry needed because of the high clock rate.  1 bit D/As
usually operate well into the MHz, so the filter for above 20Khz is usually
on the order of a single 15pf capacitor.

Dave Platt says:

One-bit and low-bit DACs (such as MASH and Bitstream) are less expensive
to manufacture than multi-bit (14-, 16-, 18- or 20-bit) DAC.  They use a
low-resolution, but very precise DAC, running at very high conversion
rates (millions of times per second) and some sophisticated digital
filtering.  Multi-bit DACs use a high-resolution DAC running at a slower
speed (tens or hundreds of thousands of times per second).  It's cheaper
to build a "narrow, fast" DAC than a "wide, accurate, medium speed" DAC.

There are s

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Newsgroups: sci.electronics