Re: TV and sound over CAT5

[Nigel Orr <nigel.orr@xxxxxxx>]

At 10:05 28/07/99 +0100, you wrote:
>the CD player had only three connections, the earth was common to both
>channels. This means that the return pair of each half of my twisted
>pairs was common - I presume this does not matter since there was
little
>I could do about it anyway,

It's not ideal, but if it works, it works ;-).  It is possible that the
loop area formed by the 2 earth/screen wires would be more prone to having
a current induced in it, so in some circumstances you might get a better
result by cutting, or putting a resistor (about 30 ohms) in series with the
screen (this should have no noticeable effect on the audio).

On a couple of occasions, I've made an 'earth-fixer' cable, with the
resistor in parallel with a short section of the screen connection.  A
couple of minutes with a pair of wire snips will sort out the ideal
connection for your system (try the full connection, then snip the wire in
parallel with the resistor, then snip the resistor wire), and you can then
make up a proper cable to suit.

>However, if I turned the gain up high (half way) on the hifi, to allow
>me to control the volume from the PC, there was an unacceptable level
of
>hum (sounded like mains) when no audio was playing.

Did it gradually increase as you would expect in turning the volume up, or
did it suddenly happen?  In general, it is best to send a high signal and
then attenuate it at the receiving end, so any added noise will be less
noticeable.  This is referred to as 'gain structure' in audio
applications...

>of cable, there was a much reduced level of hum, to the point that it
>was tolerable.

It's not an exact science in a real system.  I've recently read a couple of
books about noise and shielding, which give one an appreciation of how much
more there is to it- I'd still recommend the 'suck it and see' approach
though!

>Is the hum due to a more expensive hifi having a higher quality input,
>so it can detect the interference; a more expensive but older hifi that
>is in fact poorer quality; or perhaps simply that my wire went close to
>a source of interference? I tried to deliberately introduce
interference
>by running the cable close to mains cables, and by wrapping mains
cables
>around the cat5, but this made no discernable difference to either
hifi.

Ideally, mains cables should act as a balanced pair too- if you could
separate the live and neutral (this is just a thought experiment, unless
you are competent working on mains voltages!), and pass the cat-5 between
them, you would probably get more hum than you'll ever want to hear...

The probable difference between the hi-fi's is in their earthing- one might
have more leakage to earth, one might have a better power supply, one might
be earthed (Class 1) and one double insulated etc etc.- it's hard to say
without more detail.  Quality of inputs doesn't make a great deal of
difference to hum pickup in unbalanced systems, but it makes a _big_
difference in balanced systems- one of the tricky things in a
non-transformer system is getting the same impedance on each input wire, so
that equal noise signals are received...

>Am I likely to see an improved by moving to a balanced signal?
>(presumably yes).

Yes.

>Do I do this simply by sticking a transformer at
>either end of the 10base T cable?

At its simplest, yes- put it at the receiving end so noise picked up in the
cable can be 'sunk' by the sending end, which shouldn't let it on to the
signal so badly... sorry for the lack of detail in that explanation- it
might not even make sense, but the long boring theory behind it is sound
;-).  You could also try it at the sending end, or even both, which allows
you to use a transformer other than 1:1 to get higher signal levels or
lower impedance on the cable- which is sometimes helpful... another long
explanation available if you really need it ;-)

Nigel

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