Conference cookie::notes$archive:cd_v1

    
    Highland :== High Pressure Sales
    
    	I've been in there two times looking at high end VCR's.  The
    sales people I've run into told me anything to get me to buy.  Extended
    warranties make lots of money for the store, and the salesman gets
    a piece of the action.  The prices at Highland can be quite good,
    but don't believe what they tell you.
    	As for which CD players is better, ignore the specs and trust
    your ears.
    				=Ralph=	

                 
    
    What do you clean a laser beam with ? 
    
    Q-tips and tape-drive solvent??
    
    Don't you know a dirty laser beam can wear out your CD's???
    
    	<<<what an unmitigated crock!!>>>
    	
    

re .-1

Come now.  I think it's obvious that the reference here is a short hand way of 
referring to the lens that emits the light.  Much the same as people refer to 
a lamp or light bulb as a light.  

All that aside, I still feel funny when a sales person insists that a unit is 
so unreliable that it behooves me to buy an extended warranty.  Now, if the 
warranty included a loner, that's a different story!

						     
							- JP

    
    How do you clean the lens anyway?  Can I buy a device and clean
    it myself?
    
    What about filters?  Is one filter really better (noticed by the
    human ear) than another?    
    
    						- Ted
    
    

    A while ago I bought a lens cleaner from Lectra City in framingham.
    The cost was $20. It is a programmed disk with a fine brush on the
    bottom of it.
    All you do is put it in, close the door, and will clean automatically
    (only if your player displays the # of tracks before play). If not,
    you hit the play button.
    It will stop by itself, and PRESTO, clean laser lens. They reccomend
    using it after every 10 hours of play.
    
    
    
    Rich
    

Re: .-2

The business with filters is real and meaningful, especially if you have
good ears.  Another item to consider in the same breath is what is called
"oversampling".  I'll talk about oversampling first.

Disclaimer: I'm not an expert in this - what follows is how it was
explained to me by a friend who is such an expert.

The important thing to remember here is that the signal being read from the
disc isn't an exact analog reproduction of the input signal.  It's a series
of digital values representing the amplitude of that analog signal at
discrete clicks of time.  It is therefore not truly "accurate" - the signal
developed in the digital section of a CD player is a voltage that varies in
steps instead of being continuously varied.  Remember your calculus: the
smaller the steps, the more nearly accurate the result.  Oversampling is
the rate at which the signal is read by the player, i.e., the size of the
steps.  The standard single-frequency sampling is in the neighborhood of 44
KHz, and the ultimate frequency response of such a machine is about 24 KHz. 
With double oversampling, the rate is about 87 KHz, and the ultimate
response of the machine is about 67 KHz.  Quadruple oversampling is
probably overkill.

Your initial response to the above numbers might be "well, 24 KHz is plenty
good enough for me."  It's not.  If you look at a Fourier transform of the
sound, you will find that the advantage of the higher sampling rate isn't
in frequency response; it lies in the ability to perform magic by looking
farther into the past to predict farther into the future.  In essence, this
gives the machine the ability to produce cleaner and more accurate
waveforms because it can adjust its digital-to-analog conversion
dynamically.  The result with double oversampling is distortion figures
below .002 percent, like almost unmeasurable and certainly unhearable.

The quality of the filter determines how effectively the signals in the
digital portion of the electronics are kept out of the analog portion.  If
the filter is poor, there will be a harshness introduced into the sound
that you hear, as the very high frequencies from the digital section alter
the so-carefully developed waveshapes in the analog section.

- Dick

Well, not really.  I'll take a swing at it.

What you have on the disk is a series  of digital samples of the
signal.  In the case of CDs the sampling rate  is 44.1kHz.  In a
CD player,    that signal  is fed   into  a  digital  to  analog
converter.  What  comes out of the d2a  converter is almost what
you want.  It is a wave form which is  a  bunch of little steps.
What the filter does is smooth out the little steps.

If you do  the math (Fourier  Analysis), you  can show that  the
discrete time sample of the signal on the CD contains all of the
information  that  the  analog  signal  had up   to  half of the
sampling frequency, 22.05kHz.   The little steps  on the  signal
comming out  of the  d2a converter are an  artifact  of  the d2a
process; the steps are extra signal  which has frequencies above
half of the sampling frequency (22.05kHz).  The filter in the CD
player removes this spurious signal.

Now about oversampling: Unfortunately, it is expensive  to build
a nearly perfect  filter.  That is  one which  lets  all of  the
signal below a  frequency through, and  traps all of the  signal
above that frequency.  [You would like the frequency response of
the filter to look like a step function.]  What  you do get is a
curve  with wiggles before   the  cutoff  frequency, and then  a
sloped cutoff.  And  then as componants in the  filter age, this
response changes (possibly becoming worse).

What you can do then is to  grab the  signal multiple times (and
add an error signal?).   It  is  then possible  to build a  less
techniccally difficult  filter, with a less  steep cutoff.  This
filter  will be  less prone  to  changing  response  over  time,
because the  values of the  components need  not be  as precise.
Also, it is likely  that the softer  filter will  introduce less
phase distortion, making the  music sound crisper (transients in
the music will be sharper).

As with anything  else,  it is   still  possible to  foul up the
execution  of an   otherwise good plan.  So   a   player with no
oversampling  may  sound  better  than  one  with   four   times
oversampling.  Like  any other   audio equipment,  your ears are
usually the things you should use to make your choice.

R.

    Re:  Above discussions of filters.
    
    Recently I've seen several CD players that claim to have both digital
    AND analog filters.  Is this good?  Bad?  Does it matter at all?
    I had always thought that digital filtering is better than analog;
    if so - why both?  For that matter, if NOT, why both?
    
    Thanks,
    B
    

By the way, I saw the same lens cleaning disc mentioned in .5 at Lectra City, 
Salem, NH, and at Tech HiFi, Nashua, NH.  Both places had it for $29.95!  

						     
							- JP

re: .8

The type of filter only matters if you can (or think you can)
tell the difference in sound (with the rest of you system and)
your present set of ears.  Some people can hear the difference;
I'm not one of them...   Afterall, if you are buying a chunk
of audio electronics, you are doing it for the sound, not the
specifications...

	 Well,  I taught I knew...
	 I just read the few notes before refering to Oversampling
	 and (Digital) filtering. Either I am wrong or else I disagree.

	 The way I see it, is that 4x oversampling simply read the same
	 "sector" of the Disc 4 times and binary add them to make 18 bits 
	 instead to the normal 16. (In binary, times 2 add one bit).
	 The filter (either digital or not) then reproduce the signal
	 44 and dust thousand times per second.
	 If you have a 18 bits D/A you have 4x oversampling, if you
	 have 16 bits D/A you have 0x oversampling.

	 This reduces the "soft" errors that occurs once in a while.
	 i.e. if you get a "soft" error on the first read and none
	 on the 3 others, you will get only 1/4th of the error.
	 
	 But I doubt very much that 4x oversampling gives you
	 a better frequency response as mentionned before.
	 A CD give around 44K sample a second and oversampling
	 will NOT give you 176Khz as few people seems to think.

	 The fact that it have a Digital or Analog filter is a 
	 totally different matter.


	 Vince.

    Not quite.                                                   
    
    Oversampling is the creation of new sample points IN BETWEEN the
    ones you know about.  These new sample points are NOT gotten off
    of the CD, they are calculated on-the-fly by chips inside the CD
    player.
    
    Maybe an example will help ----
    				   \
    				   V
    
    If you don't oversample, you have a sample every .0000226 second.
    (= 1/44,100).  Your reconstruction filter has to look like a 
    step function, rolling off 90 or so dB within the band from 20 Khz
    to 22.4 KHz.
    	
    Now, let's put some CPU inside the digital path.  Specifically,
    we'll put in a custom LSI that takes two 16-bit numbers and produces
    the AVERAGE of the two.  And now, add a bit more logic to the
    data path that 
    	
    	1) feeds the DAC with the 16-bit value from .0000226 second ago.
    	2) feeds the averager-LSI chip the current 
    		sample value, and the .0000226-ago value
    	3) waits .0000113 second (half a sample time)
    	4) feeds the DAC with the value coming OUT of the averager LSI
    	5) waits .0000113 second (half a sample time)
    	6) feeds the DAC the now-.0000226-old "current" sample value.
    	7) goto (2)
         
    
    This means that the DAC sees a signal that "looks like" it came
    from an 88.8 KHz source- but contains no information above 22.4
    KHz.   The reconstruction filter now has slightly over an octave
    (from 20 KHz to 44.4KHz) to attenuate by 90+ dB.
    	         
    -------------------                                         
    
    We can stack 3 of the averager chips in arra and make a 4-way
    oversampler as follows:
    	
    	
    	
    Value 1>  --------------------------------------------> Out 1
    		\		\
    		 \		 \
    		  \		Averager -----------------> Out 2
                   \		 /
    		    \		/
    		   Averager ---<--------------------------> Out 3
    		    /           \
                   /             \
    		  /             Averager -----------------> Out 4
    		 /               /
    		/               /
    Value 2-----------------------------------------------> Out 5
                                                  
    
    
    Now the DAC sees what looks like a 176 KHz source, again with no
    information above 22.1 KHz and has two full octaves to roll off
    at 90+ dB.  It's not hard to build a 48 dB/octave filter with four
    op-amps and NO critically-matched parts.  
    
    
    Yes, I know I'm ignoring "corner effects" and that the averager
    LSI's do more than just average (don't some do a 3-point parabolic
    interpolation, using 3 16bit values to make 9 output values ?)
    But I think that the averager model is accurate enough to give
    you an idea of what's going on... information is NOT being added,
    you're just interpolating new points between the ones you know are
    there.

re: LLC-1

	The LLC-1 laser lens cleaner is $24.95 at Tower Records. It works
extremely well - CDs that mistracked even after cleaning them worked fine 
after using the LLC-1. Remember, however, to clean off the little brush with
the cleaning brush provided - kind of like wiping off your Discwasher after
cleaning a record.

re: extended warranties

	Actually, I'd go for it. When I lived in New York, I bought a Sony
D-77 portable unit. It has been in the shop repeatably in the last year
because the headphone jack was going to pieces (intermittent audio caused by
occasional jostling). When I purchased the unit, I was told that the laser in
the portable units needs replacing every 1.5-2 years, and that these run $100+.
For the $39.95 I spent on a 5-year deal, I'm willing to take the chance that
they lied - so I postpone buying a few CDs; I'll live.