Interfacing and Processing 2558.3.4.2 What Does CMR Achieve?
Common mode rejection action prevents the egress and build-up of extraneous hum,
buzzes, and RFI when analogue signals are conveyed down cables, and between
equipment powered from different locations—all the more so in big or complex systems.
CMR helps make shielding more effective by canceling the attenuative residue , the bit
that any practical shield “ lets through. ”
Sending the signal on a pair of twisted and parallel conductors ensures that this latter
residue and any other stray signals that are picked up en route are literally coincident and
appear “ common mode, ” that is, equal to each other in size and polarity. A tight enough
twist makes the conductors almost experience interfering fi elds as if they occupied the
same space. This is true below high RF (200 MHz, say), when averaged out over a cable’s
length.
In contrast, the wanted, applied signal from both balanced and unbalanced output sockets
is distinguished while being no less equal in size by appearingopposite in polarity on
each input “ leg, ” called hot and cold.
CMR also makes shielding more effective by freeing it from signal conveyance, enabling
it to be connected at one end only, according solely to the dictates of optimum RF
suppression and/or individual system practice. Breaking the shields through connection
also prevents (or at least lessens) the build-up of the mesh of earth loops that causes most
intractable hums and buzzes. CMR is also able to cancel differences between disparate,
physically distant and electrically noisy ground points in a system.
Above 20 kHz, even a modest CMR lessens the immediacy of the need for aggressive RF
fi ltering. RF fi ltering can take place at higher frequencies, and both the explicit and the
component-level effects on the audio may be diminished accordingly.
Figure 8.5 shows the CMV that CMR helps the audio system ignore. Even when
connection to mains safety earth is avoided by ground lifting (ground lift switch open)
or by total isolation (switch open and ground lift R omitted), considerable capacitance
frequently remains, through power transformers and wiring dress.
Overall, the rejection achieved (which is a ratio, not an absolute amount) is described in
minus (–) dB. Often the minus is understood and omitted. In plain English, “ CMR 4 0 d B ”
means “ all extraneous garbage entering this box will be made 100 times smaller. ”