Consoles 851tame the spiky transients, the circuit becomes very
complicated.
Certain routing applications do implicitly require
relays and their lack of concern about the amount of dc
and either common-mode or differential signals of
absurd quantities that may accompany the audio in
balanced networks. Such circumstances are to be found
anywhere a telephone line is used.
This is almost specifically a broadcaster’s concern,
where many external high-quality sources appear down
phone lines and need to be routed before hitting either
the internal distribution amplifier system of the station
or perhaps even a console line input directly. Outside
source selection, as it’s called, does not fortunately have
the same splat-elimination constraints as intraconsole
switching, since the signal is nearly always of high
level, balanced, and riding with at least a little dc
(which will unavoidably click upon switching); most
importantly the selector is very unlikely to be switched
while actually live on air.
25.9.2 Electronic Switching
The wish list for an audio switch is simple:
- It has an infinite off impedance.
- It has a zero on impedance.
- It has a control signal that is isolated from and does
not impinge on the through signal path. - It costs nothing.
In the real world, of course, some leeway has to be
given, but, fortunately, the tradeoffs are more in subtle-
ties than in these basics.
Transistors are out of the picture right away despite
their high on-off impedance ratios, because they are
essentially unidirectional in current flow, and the
control port (the base) is actually half of the signal path
as well. In certain circumstances they have been used in
the place of relays as a soft output muting clamp as in
Fig. 25-30A.
Diodes, Fig. 25-30B, are used extensively for signal
routing in RF equipment, with the required signal riding
on a relatively large dc bias that overcomes the diode
forward voltage drop making it a low-impedance path,
and correspondingly turned off by a large back bias.
Considering that in some audio design cases getting rid
of a handful of microvolts dc can be an ordeal,
somehow a couple of dozen volts hurling about lacks a
certain appeal. Typically, switching diodes for RF with
a PIN structure are chosen; their very small capaci-
tances are considerably less parametric with respect to
varying reverse bias, so minimizing automodulation and
consequent distortions.
FETs have been and still are used extensively for
signal switching. They again have a high on-off ratio,
and the control port (the gate) is of extremely high
impedance and well isolated from the signal path, but
the gate on-off voltage levels are a bit awkward for
interfacing with logic control signals. They also define
signal head room through the switch, based on the gate
on-off biasing voltage range. It is bidirectional, its
channel path being essentially just a voltage-controlled
resistor, but the on resistance tends to vary with the
varying audio voltage across it (auto modulation);
distortion in the more basic FET switching configura-
tions can be a problem. However, they are workable,
Fig. 25-30C.25.9.3 MOSFETs and CMOSClosely related to FETs are metal-oxide-semiconductor
field effect transistors (MOSFETs). They have a
different chemical structure and physical construction
but have essentially similar characteristics with the
exceptions that the gate is of even higher impedance,
and the control voltage swing required is easier to deal
with. Complementary MOSFET (CMOS) elements,Figure 25-30. Solid state devices as simple switches.Vin VoControl, ground
to unmuteA. Transistor as a signal switch.
Dc turn on biasB. Diode as a signal switch.Vin VoVin Vo
Control, higher than
read signal for on,
more than 2 V below
lowest signal for offC. Field effect transistor as a signal switch.