🚀 go-pugleaf

In article <1991Mar1.004711.15100@watcgl.waterloo.edu> cjwein@watcgl.waterloo.edu (Chris J. Wein) writes:
>In article <wilf.667759065@rigel.sce.carleton.ca> wilf@sce.carleton.ca (Wilf Leblanc) writes:
>>jbuck@galileo.berkeley.edu (Joe Buck) writes:

>>>Example of CD salespeak: pushing oversampling as an advanced technical
>>>feature.  Oversampling is simply inserting zeros ...
>>>Still, some sales critters think it's an advanced technical extra.

	It isn't, mainly because it's NOT an extra.  All CD players
oversample, and use a FIR filter (i.e. digital filtering).  The
cheapest, oldest ones use 2x oversampling, and don't advertise the
'feature'.

>>This kills me too.  Especially 8x oversampling !
>>If you want to spend enough money, you can get very near linear 
>>phase response with an analog filter.  So, you are right).

	Not exactly.  You'd have to spend money on things like thermostats
to keep a really high Q filter (for a brickwall filter), because
REGARDLESS of cost, ALL the components in an analog filter are
poorly controlled.  Typical digital frequency accuracy (if you
aren't trying hard) is 0.001%; that kind of performance is
equivalent to an analog pole with a Q of 100,000 (which is not
feasible).

>I also understand that oversampling increases the 'transition region'
>of the filter thus allowing for lower order filters. 
>
>As for the CD's that do not oversample, what type of filter is generally used?
>  [more about why this would be a difficult filter to make}

	As I mentioned above, there ARE no non-oversampling CD
players, for exactly the reasons you gave.  A steep-slope
cutoff filter with good passband linearity is going to require
a bevy of trimmed capacitors/inductors, and would (1) cost
mightily, (2) require difficult factory adjustments, (3) not
survive shipping without needing readjustment, (4) fail when
the room temperature changed.
	The last 2x-oversampled CD player I took apart had four
trimmer components (inductors) in the filter.  4x-oversampling
filters typically have merely selected components (not trimmed).
I'd trust the long-term reliability of the 4x models before 
the 2x ones.
	IMHO, 8x and 16x oversampling are simple hype; the available
capacitors and other filter components cannot be characterized
accurately for the range of frequencies involved (20 Hz to 200000 Hz)
so if I were designing a filter for 'em, I'd just use the same
design as for the 4x units.  The part of the filtering that
ISN'T hype is the digital part (the number of taps in the FIR
{Finite Impulse Response} filter is a major factor in both cost
and performance).  None of the sales info ever mentions this,
though...

	John Whitmore