If the interference source is a long way from the cable, then the interference is fairly
consistent in level along a long length of the cable, and the twist of the two wires
inside will ensure that both wires are exposed to an equal amount of interference.
However, this theory falls down if the interference source is very close to the
cable (ie, right alongside it) because by the time the twist has come around the now
closer wire is actually further away from the interference source and so picks up less
interference anyway. So a lazy twist isn't effective in cancelling out interference for
very close sources. A tighter, shorter twist would be better, but its effectiveness is
still restricted, depending on the proximity of the interference source.
Imagine a two core cable, laid alongside a point source of electromagnetic radiation,
with the two cores twisting relatively slowly along the length of the cable. The EM
interference is strongest where the cable is closest to the interference source and if the
two wires happen to be in a part of their twist where they are equidistant from that close
point source of interference, they will both receive exactly the same level of
interference. All will be well because we have a true common-mode signal.
However, if the state of the twist is such that one wire is closer, it will pick up
slightly more interference than in the previous example. Meanwhile the other wire must be
more distant than previously and so will pick up slightly less interference.
These unbalanced levels of interference in the two wires defeat the differential
receiver because it is not a common-mode signal any more, and hence the intereference will
be heard. Not good!
In a starquad cable there are four core wires, connected
in opposite pairs, and the twist length is much shorter than in a normal cable. The
tighter twist length helps to ensure interference cancellation over short cable lengths,
which helps, but the use of four wires connected in opposite pairs is the really clever
Again, let's assume that one pair of connected wires happen to be
exactly equidistant from the interference source (the red and blue wires in diagram 3 in
that article), while the other two (green and white) are closer and further,
The hope is that the sum of the interference signals that break
into the two wires that are equidistant (red and blue) in the middle will be the same as
sum of the interference in the closer (green and slightly more) and further (white and
slightly less) wires.
And generally, that is the case.
Technical Editor, Sound On Sound