From: Andy Sheen <sheen@src.bae.co.uk>
          Fri, 28 Aug 92 10: 47:16 BST
Subject: Re: Questions on Digital Cable
Originator: tjk@csd4.csd.uwm.edu
Organization: University of Wisconsin - Milwaukee
Date: Fri, 28 Aug 1992 13:04:28 GMT
          id <sg.21486-0@ben.uknet.ac.uk>; Fri, 28 Aug 1992 11: 17:12 +0100
          id <16439-0@eros.uknet.ac.uk>; Fri, 28 Aug 1992 11: 17:01 +0100


p0p4018@zeus.tamu.edu (PREMTHAMKORN, PRAKIT) writes:

>> 3)  Can I use ordinary cable as digital cable?  If not, why?

and electron@waikato.ac.nz replied:

>I can't see any problem with using coax cable as long a you match the impedance
>of the input/output impedance of the equipment you are trying to connect
>together.  If the length of cable is not too long any cable will probably do.
>(the lenth of a miss-matched cable can "round off" the sharp corners of the
>digital information and could produce errors.)
>brucee

I think what you are saying is that as long as you use any *coax*, you'll be
OK. I'd generally agree, the reason for wanting a matched line (I/O/cable)
is to avoid reflections at the ends of the cable which can cause both "ringing"
on the signal and the "rounding off" depending on the length of line/frequency 
applied - basic transmission line theory.

The problem is made worse with audio equipment for a variety of reasons, not 
least:
1) In order to have "perfect" data out of the external DAC, the time interval 
between samples must be *exactly* the same for each and every sample (I have 
been reliably informed). This wold be OK, except that the clock used to 
sample the DAC is derived from the signal coming down the interconnect. 
Crappy signal from transport -> crappy output from DAC !!
2) For cheaper CD players, I *suspect* (I have not confirmed this - perhaps 
someone out in netland can) that the output circuitry is likely to be of lower
spec (to save money) thus rise/fall times/driving capabilities might not be
to the highest spec.

A good quality matched cable will have well defined inductance/capacitance/
resistance properties which a cheaper coax may not have. Additionally, as with 
good analogue cables, the construction of the cable and materials that are used
in that construction will affect these properties. The more expensive
cable *should* give a squarer signal (assuming you are not being sold hype)
- which is what you are after - as it should have better conductors and a 
better matching to the driver/receiver circuitry. 

So in summary, I know what I'd do if I were connecting digital systems together
- buy a decent quality matched coax cable. If funds didn't permit, then cheap
coax (perhaps aerial down cable) would be my next choice, but then I'd have
to attach suitable connectors at either end (if my local friendly hi-end
store wouldn't do it). I personally wouldn't use "ordinary" cable since the
side by side construction of most cables seem to increase the impedance seen
by the drivers considerably. 

I'd love to try this theory out, but I don't have access to an external
DAC to try it. Has anyone out there tried specialist coax vs cheap coax vs 
ordinary cable ? I'd love to know if what I'm saying here holds for practice
as well as theory :-)  It has for all non-audio electronics that I've been
involved in, but as I was reminded recently, logic doesn't always apply in
h-e audio :-D

Cheers

Andy Sheen                                 Email: sheen@src.bae.co.uk
FPC 267, British Aerospace PLC,            Tel:   +44-272-363547
P.O. Box 5, Filton,                        Fax:   +44-272-363733
Bristol. BS12 7QW                          #include <disclaimer.h>



Newsgroups: rec.audio.high-end