From: snail@lsl.co.uk
Newsgroups: uk.misc
Subject: Re: Record shops (LPs)
Date: 18 Dec 92 17:47:53 GMT
Distribution: uk
Organization: Laser-Scan Ltd., Cambridge

In article <20778@acorn.co.uk>, steve@acorn.co.uk (Steve "daffy" Hunt) writes:
> T R Stickland (bsstrs@gdr.bath.ac.uk) wrote:
> : The problem wil digital technology is taht you cannot record everything.
> : So you get a nearly perfect copy of part of your original signal only.
> : These ommisions are what analogue fans find unaaceptable.  On CDs (don't
> : know if this is down to the format or common to all digital technology)
> : you particularly miss out on high frequency information.  Cymbals expecially
> : sound bad, for example.
> 
> Yes and in fact CD players gently roll off the high frequency
> information before the limit imposed by the sampling freqency to avoid
> coming to an abrupt ceiling.
> 
> Although you cannot actually *hear* sound above 20-odd KHz, its
> absense supposedly alters our subjective perception of the audible
> lower frequencies.  (Like all my understanding of hi-fi this was
> culled from a comic, so is probably at best a gross
> oversimplification!)

OK folks, simple lesson in smapling, and then a real DSP expert can come in
and fix up the few bits I get wrong.

Shannon's Sampling Theroem states that, at the very least, the sampling
frequency must be two times that of the highest frequency to be sampled.
If you don't do this you get an effect known as aliasing where a frequency
'appears' in the sound, which is a a frequency which is the difference between
the sampling frequency and the frequency being sampled. I experimented with
aliasing as a musical effect pedal I built - unusable, horrible, bleugh!

Given this 44.1KHz instantly comes down to 22.05 KHz bandwidth. However this
alos means that a sine wave at 22.05 KHz would at best be represented by a
triangle wave and at worst by a flat line. As the frequency being smapled
decreases (with sampling rate staying constant) the effect accuracy of the
digital representation increases. Realistically you need to sample at 5 times
the highest frequency you want to sample. Most people will tell you a value of
3 is adequate. Since most of the info is down in the sub 5 or 6 KHz band this
is true. But a simple guitar note has many subtle higher order harmonics which
all add up to a nice sound to listen to. Remove these and it doesn't sound as
nice. It gets more complex when you add other instruments and all the modern
sound processing effects (chorus, distortion, flanger, echo, phasing etc).
Read any current Neil Young interview and you'll see he hates CD for reasons
like this, although you need to read between the lines (he doesn't
explain it very well).

I'd say a realistic sampling rate to get the full bandwidth of sound up to
20KHz would need to be > 100KHz, pref 150KHz. Luckily most peoples ears
don't hear that far up the range. Most can hear to about 15KHz tho!

15/44 > 2.5 times.

I reckon 44.1KHz gives you not bad reproduction up to about 10K. After that
forget it. We haven't even got onto quantisation distortion, and the fact that
the D/As should be 18bit logarithmic ones (yu can get them). CDs are only 12
bit (unless changed in the last year or so), even if the player is 16bit
(just throw the bottom 4 bits away). Having
said that a logarithmic D/A should handle quatisation errors nicely.
-- 
snail@lsl.co.uk      

"Washing one's hands of the conflict between the powerful and the powerless
 means to side with the powerful, not to be Neutral."
                                                     Quote by Freire.
                                                     Poster by OXFAM.