"Mike Rivers".

Common mode rejection is the ability to reject noise that's common to
the two wires of a balanced input. An example is RF interference
that's picked up by both wires equally.


** That not a correct example. RF interference is defeated firstly by
the use of shielded cable and secondly by the use of filters to reduce such
signals at the inputs of the balanced pre-amp.

Common mode rejection operates across the audio band and maybe a little
beyond but is usually most effective at the lower frequencies since the main
aim is to eliminate ground hum from audio systems. A ground hum voltage will
appear equally on the two wires and so be rejected.


Since a balanced input works
on the voltage difference between the two wires, if the same stray
signal is picked up equally by both its difference will be zero and it
will be rejected.


** A voltage injected into a balanced audio line by external magnetic
fields ( like nearby high AC current cables and transformers) creates a hum
signal in differential mode that the pre-amp *will* amplify - its CMRR has
no effect.

This sort of interference is reduced by the fact the two wires are
*twisted* inside the cable which reverses the phase of any hum signal picked
up every inch or so along the line and hence cancels it out. Where multiple
twisted pairs are used in the same cable the twisting reduces crosstalk in
the same way as above.

"Star Quad" cable uses four twisted wires instead of two to enhance the
effect of the twisting and virtually eliminates induced hum problems even
when used near to high current AC cabling.




............ Phil