[mythtvnz] RG6 cable and mains power cable fed through same junction
box - interference likely?
Toby Mills
mythtvnz@lists.linuxnut.co.nz
Fri, 18 May 2007 15:21:53 +1200
Matt,
you will find that network cable and coax are fairly immune to EMI as
they operate at frequencies well beyond the 50Hz power frequency.
You would have to be drawing huge amounts of current to interfere with
these signals significantly and in most commercial installations, coax
and network happily coexist alongside TPS and in the same cable trays.
However, audio is a different story as 50Hz is a particularly nasty
frequency that will give you a nasty buzz coming through your sound
system, especially if you have mismatched grounds.
There are a couple of options for fixing this...
1) keep your audio cables a long way away from your power network and
coax (audio is much more suceptable and I 300mm is still close enough to
pick up noise).
2) make sure the audio devices all have a common earth (usually this
means plugging everything into the same power point) or ensuring you
house has a common ground stake.
3) go to balanced audio connections.
The third is the least common but by far the most effective. You can buy
balancing transformers from dick smith or if you are an audiphile there
are thousands of options.
I use this technique to pipe audio signal from my house out to my garage
out to amplifiers that feed various speaker zones in the house.
Balanced cables use 2 conductors and a shielded ground (standard RCA
cables just have two conductors one of which is ground), the two
conductors carry identical copies of the audio signal except that one of
them is out of phase with the other.
You use a balancing transformer to flip the phase at the source.
What happens is that any interference introduced along the length of the
cable is introduced into both conductors equally.
You use another balancing transformer at the other end of the cable
which flips the out of phase signal back over again then combines the
two conductors back into one plus ground.
In the process of this, you effectively cancel out any interference that
was introduced along the length of the cable.
The result is crystal clear sound with zero interference and you can use
it to run audio signal for hundreds of meters without any interference.
If you find you have a lot of noise in your signal, this is a foolproof
and fairly easy way of fixing it permanently.
You need to run a different sort of audio cable, just standard
microphone cable is fine which you could also buy from mr smith, and you
would need one cable for each of the left and right signal plus 4
balancing transformers.
Of course if its only a short run you should be fine, but it pays to
test it before you seal everything up, I have seen plenty of cases where
someone has wired their house up then hooked it all up to discover a
horrible hum that comes through everything.
Cheers
Toby
Matt Poff wrote:
> Cheers,
>
> Yeah I checked with PDL and they recommended separation of 300mm so I
> think I'll have to get the Gib saw out. In that case I might install a
> five socket comms gang plate with a splitter for TV/FM, an RG6 and
> then two audio connectors for future satellite speakers in a home
> theatre set up.
>
> What's the best common wall connector to carry +/- audio signal using
> just one connection per lead? I don't really want to add four RCAs to
> a gang plate when I can get away with two of something else.
>
> Matt
>
> Noel & Di wrote:
>> Hads
>> It's important to maintain segregation of services - ie an insulated
>> insert in the double flush box to keep the coax and TPS apart -
>> otherwise all conductors & cables must be insulated for 230V to
>> comply with the regs. The old 70's style point with integral ribbon
>> socket is contrary to the current regs because it provides inadequate
>> segregation of LV (230V) and ELV wiring systems. Normally a minimum
>> separation of 100mm between power & data/Av with avoidance of
>> parallel cable runs is recommended to minimize EMI.
>> Noel
>>
>> Hadley Rich wrote:
>>
>>> On Fri, 18 May 2007 09:52:21 Matt Poff wrote:
>>>
>>>
>>>> I'm thinking of feeding some RG6 cable (just standard stuff - not
>>>> ultra
>>>> shielded) up into a existing 1970s side-by-side two socket flush box
>>>> (replacing the old 1970's face plate with two vertical PDL face plates
>>>> side-by-side, one with the two power sockets, the other with coax
>>>> TV/FM
>>>> and RG6 connectors).
>>>>
>>>> Anyone know if I'm likely to have any problems with interference
>>>> locating the signal cables this close to mains power cables? The comms
>>>> cables come through the floor and the power cable comes from above so
>>>> they are only in close proximity in the flush box.
>>>>
>>>
>>> Should be fine.
>>>
>>> hads
>>>
>>> P.S Please start a new thread when starting a new subject. It messes
>>> up peoples mailboxes.
>>>
>>>
>>>
>>
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>
>