382 Chapter 13
basic parameters such as cross talk and distortion are so strongly layout dependent. At the
very least the PCB designer should understand the points that follow.
13.1.1 Cross Talk
All cross talk has a transmitting end (which can be at any impedance) and a receiving
end, usually either at high impedance or at virtual earth. Either way, it is sensitive
to the injection of small currents. When interchannel cross talk is being discussed,
the transmitting and receiving channels are usually called speaking and nonspeaking
channels, respectively.
Cross talk comes in various forms:
● Capacitative cross talk is a consequence of the physical proximity of different
circuits and may be represented by a small notional capacitor joining the two
circuits. It usually increases at the rate of 6 dB/octave, although higher dB/octave
rates are possible. Screening with any conductive material is a complete cure, but
physical distance is usually less expensive.
● Resistive cross talk usually occurs simply because ground tracks have a nonzero
resistance. Copper is not a room-temperature superconductor. Resistive cross talk
is constant with frequency.
● Inductive cross talk is rarely a problem in general audio design; it might occur if
you have to mount two uncanned audio transformers close together, but otherwise
you can usually forget it. The notable exception to this rule is the Class-B audio
power amplifi er, where the rail currents are halfwave sines that seriously degrade
the distortion performance if allowed to couple into the input, feedback, or output
circuitry.In most line-level audio circuitry the primary cause of cross talk is unwanted capacitative
coupling between different parts of a circuit, and in most cases this is defi ned solely
by the PCB layout. Class-B power amplifi ers, in contrast, should suffer very low or
negligible levels of cross talk from capacitative effects, as circuit impedances tend to
be low and the physical separation large; a much greater problem is inductive coupling
between the supply-rail currents and the signal circuitry. If coupling occurs to the same
channel, it manifests itself as distortion and can dominate amplifi er nonlinearity. If it
occurs to the other (nonspeaking) channel it will appear as cross talk of a distorted signal.
In either case it is thoroughly undesirable, and precautions must be taken to prevent it.