Re: TV and sound over CAT5

[Raymond <Reb.barnett@xxxxxxx>]

re: Whole house MP3, intranet dev, and caller id(!)

With regard to running audio over CAT5, I experimented a little last
night. I cut a short (screened) cable used for connecting portable CD
players to hifis in half and soldered 14m of CAT5 between the two ends.
The original cable was arranged so that the earth return formed the
screen, although this ran as seperate screens for the left and right
channel of the original cable, I assumed that since the plug going into
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, and presumably the two channels have very
similar signals on them compared to external noise. (This last paragraph
confused me, so I hope you follow it!).

The upshot was that when run from the PC (upstairs) to our more
expensive hifi (downstairs), the sound quality was fine (as compared to
how it normally sounds, although I didn't perform a blind test ;-) ).
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. Interestingly, when
connected to the upstairs hifi (a cheaper Aiwa), using the same length
of cable, there was a much reduced level of hum, to the point that it
was tolerable.

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.

Am I likely to see an improved by moving to a balanced signal?
(presumably yes). Do I do this simply by sticking a transformer at
either end of the 10base T cable?

We're really quite excited by how well this worked now, since it means
that not only do we get a proper whole house (2 rooms at first)
multichanger, but it occurred to me that I can run some caller ID
software on the PC and therefore get the house to announce who incoming
telephone calls are from - and this at no extra cost other than the BT
subscription! A killer solution since that removes the desire to get a
more expensive DECT phone that supports caller ID display (I've held off
buying one for a year or more since I think it is shocking they haven't
managed to put this feature in when my mobile has had it for years).

I've also played with Visual Interdev 6.0, which seems much better than
Interdev 1.0 that I first played with, so I may end up developing in
this. Does anyone have any pointers to simplish ways of using to VB to
extract caller ID information from a modem?

Ray Barnett.

> -----Original Message-----
> From: nigel.orr@xxxxxxx [mailto:nigel.orr@xxxxxxx]
> Sent: 26 July 1999 13:52
> To: REB.Barnett@xxxxxxx
> Subject: [ukha_d] Re: TV and sound over CAT5
>
>
> At 13:03 26/07/99 +0100, you wrote:
> >I'm hoping to run output from a soundcard to the input of a hifi
> >amplifier over a CAT5 cable pair. Given this is not the full
> amplified
> >power can I expect this to work with reasonable quality, or
> do I need to
> >be more clever in sending the signal?
>
> The short answer is "quite likely it will work just fine".
> The long answer
> follows:
>
> There are 2 likely potential problems, and 2 methods of
> finding whether
> they will occur or not.  The problems are
> 1) loss of high frequency, due to high source impedance of
> the soundcard
> driving a long cable.  Probability is about 5% or maybe less,
> with modern
> equipment.
> 2) mains 'hum'- 50Hz noise induced from nearby mains cables
> or noisy mains
> equipment (compressors in fridges etc).  Probability is about
> 90% that you
> will have some sort of audible noise of this kind.
>
> The 2 methods of finding whether these will happen or not are:
> 1) Years of detailed analysis
> 2) 'Suck it and see'
>
> I'd recommend 2)- assuming you want whole house audio, you
> definitely need
> the cable, and you can add line drivers, transformers etc at
> each end to
> reduce or remove both problems.  Connect it up, if the noise level is
> acceptable and the sound quality is fine, then go with it.
> If it sounds
> muffled or there is too much background noise, look at
> solutions to that.
> If I was specifying a professional system, I'd assume there
> would be both
> problems and specify balanced line drivers, but at home you
> can 'evolve' to
> solve whatever problems you find.
>
> >Will the other cable pairs suffer
> >from induced noise even with low level signals or can I use them
for
> >other purposes?
>
> Yes they will, and yes you can, if the noise is acceptable on
> the other
> pairs ;-)
>
> > I had thought that the point of using 'twisted pairs'
> >was so that they didn't cause/suffer to much interference (but I'm
a
> >software guy, not a techie!).
>
> It's a bit of a generalisation.  Put (fairly) simply, a wire
> in a field
> (produced by other wires carrying current) will generate a
> small current.
> If you had another wire at the exact same location as the first, and
> subtracted its generated current from the first, you would
> delete all the
> noise.  However, 2 wires at the _exact_ same location isn't possible
> (although co-ax is remarkably close!), so it is found that by
> spinning the
> wires round each other (a twisted pair), you get a reasonably good
> approximation, and it's fairly cheap and manageable.
>
> This all breaks down when the source of the noise is very close to the
> cable.  The distance from the source to each wire is now markedly
> different, so they don't generate the exact same current, so
> you will get
> some 'left over' noise, and there's nothing much you can do
> about that.
> Does that make sense?
>
> So, anything on one pair in the same jacket will appear to
> some extent on
> all the other pairs on the same jacket.  Question is whether
> the resulting
> noise is acceptable to you.  If one pair is audio Left and
> one is audio
> Right, it'll probably be unnoticeable, if one pair is audio
> and one is IR
> remote data, it will probably be irritating.  If one pair was
> a microphone
> signal and one is speaker (about 60dB difference), the result would
> probably be unbearable.  If two of the pairs carry ethernet,
> the network is
> apparently likely to be unreliable if the other pairs are
> used as well.
>
> As above, it might just work in a particular situation, so at
> home you can
> try it and see if it does, but don't go installing the same
> system in other
> houses because it happened to work in yours!
>
> Nigel
>
> --------------------------------------------------------------
> ----------
> Was the salesman clueless?
> Productopia has the answers.
> http://clickhere.egroups.com/click/555
>
>
>
> eGroups.com home: http://www.egroups.com/group/ukha_d
> http://www.egroups.com -
Simplifying group communications
>
>
>
>
>
>
>

------------------------------------------------------------------------
Click Here to apply for a NextCard Internet Visa and start earning
FREE travel in HALF the time with the NextCard Rew@rds Program.
http://clickhere.egroups.com/click/449


eGroups.com home: http://www.egroups.com/group/ukha_d
http://www.egroups.com - Simplifying
group communications