Re: Balance Audio (& Video)

[Keith Doxey <keith.doxey@xxxxxxx>]

Hi John,

A normal audio signal from a CD player or the like is an unbalanced signal.
There are two conductors, Signal and Ground.
The cable is almost always a screened one with the active signal using the
centre conductor and the return path being via the grounded screen which is
hopefully preventing interference getting through to the wanted signal in
the centre.

Problems arise when 2 pieces of earthed equipment are connected together.
There are then 2 ground connections between the pieces of equipment, one
via the screen of the cable and the other via the earth leads of the
equipment and the ring main. Just because a piece of equipment only has a 2
core mains lead doesnt mean and earth loop cant exist. It can occur via
Aerial leads fed from a distribution amplifier and the audio leads to other
Hifi components that are earthed.

One of the classic symptoms of an earth loop is that even with the volume
turned down you get a hum as soon as you connect the audio lead.

Balanced audio does not use ground as part of its signal path and so is
immune to earth loops. Instead, the audio signal is sent as two equal
amplitude but opposite polarity signals. At the receiving end the
difference between the signals is taken as the active signal. Very crude
example follows.

Unbalanced signal of 5 volts gets a 1 volt signal induced into it
(Wanted) 5 + (Unwanted) 1 = 6 which is not the signal that was sent

Balanced signal sent as to equal but opposite signal +5 and -5
difference=10
(Wanted +) +5  + (Unwanted) 1 = +6
(Wanted -)  -5   + (Unwanted) 1 = -4
Difference between +6 and -4 is still 10. The signal has got through intact
and the interference is cancelled or to put it another way, both wires had
+1 volt induced, the difference between +1 and +1 equals a big fat zero.
No interference and you can run for hundreds or thousands of metres over
unscreened twisted pair cable.

Before the days of digital audio thats how all broadcast audio was sent.
Co-incidently even the digital signals are sent as balanced in exactly the
same way as RS485.

Hope that helps

Keith

Keith Doxey
http://www.btinternet.com/~krazy.keith
Krazy Keith's World of DIY Home Automation

-----Original Message-----
From:	Dr John Tankard [SMTP:jtankard@xxxxxxx]
Sent:	Tuesday, July 27, 1999 10:26 AM
To:	ukha_d@xxxxxxx
Subject:	[ukha_d] Re: TV and sound over CAT5

Keith I am a bit of a novice in this department, please explain more about
balanced audio. I want to feed my HiFi from a MP3 source, I dont want to
have a computer in my HiFi stack if I can help it, can I use this to feed a
output from a sound card to my amp ? what cable should be used ? is there a
detectable loss in quality ? over what distance ?

Sorry for all the basic questions

Best regards

John. (still working on the lighting system. Now talking to HomeVision !)
jtankard@xxxxxxx







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