Handbook for Sound Engineers

Grounding and Interfacing 1185

than 7 ft for 10 MHz video, 8 inch for 100 MHz FM

radio, and 0.8 inch for 1000 MHz CATV signals.

Significantly different instantaneous voltages exist

along the length of a transmission line. For all practical

purposes, its electrical equivalent is a distributed circuit

consisting of a large number of small inductors and

resistors in series and capacitors in parallel. If an elec-

trical impulse were applied to one end of an infinitely

long cable, it would appear to have a purely resistive

impedance. This characteristic impedance of the cable

is a result of its inductance and capacitance per unit

length, which is determined by its physical construction.

Theoretically, the electrical impulse or wave would

ripple down the infinite length of the cable forever. But

actual transmission lines always have a far end. If the

far end is left open or shorted, none of the wave’s

energy can be absorbed and it will reflect back toward

the source. However, if the far end of the line is termi-

nated with a resistor of the same value as the line’s char-

acteristic impedance, the wave energy will be

completely absorbed. To the wave, the termination

appears to be simply more cable. A properly terminated

transmission line is often said to be matched. Generally,

impedances of both the driving source and the receiving

load are matched to the characteristic impedance of the

line. In a mismatched line, the interaction between

outgoing and reflected waves causes a phenomenon

called standing waves. A measurement called

standing-wave ratio (SWR) indicates mismatch, with an

SWR of 1.00 meaning a perfect match.

32.3 Electronics of Interfaces

32.3.1 Balanced and Unbalanced Interfaces

An interface is a signal transport subsystem consisting
of three components: a driver (one device’s output), a
line (interconnecting cable), and a receiver (another
device’s input). These components are connected to
form a complete circuit for signal current, which
requires a line having two signal conductors. The
impedances of the signal conductors, usually with
respect to ground, are what determine whether an inter-
face is balanced or unbalanced. A concise definition of
a balanced circuit is:

A balanced circuit is a two-conductor circuit in

which both conductors and all circuits

connected to them have the same impedance

with respect to ground and to all other conduc-

tors. The purpose of balancing is to make the

noise pickup equal in both conductors, in which

case it will be a common-mode signal that can

be made to cancel out in the load.^2

The use of balanced interfaces is an extremely potent

technique to prevent noise coupling into signal circuits.

It is so powerful that many systems, including telephone

systems, use it in place of shielding as the main noise

reduction technique!^3

Theoretically, a balanced interface can reject any

interference, whether due to ground voltage differ-

ences, magnetic fields, or capacitive fields, as long as it

produces identical voltages on each of the signal lines

and the resulting peak voltages don’t exceed the capa-

bilities of the receiver.

A simplified balanced interface is shown in

Fig. 32-8. Any voltage that appears on both inputs,

since it is common to the inputs, is called a common-

mode voltage. A balanced receiver uses a differential

device, either a specialized amplifier or a transformer,

that inherently responds only to the difference in

voltage between its inputs. By definition, such a device

will reject—i.e., have no response to—common-mode

voltages. The ratio of differential gain to common-mode

gain of this device is its common-mode rejection ratio,

or CMRR. It’s usually expressed in dB, and higher

numbers mean more rejection. Section 32.5.1 will

describe how CMRR often degrades in real-world

systems and how it has traditionally been measured in

ways that have no relevance to real-world system

performance.

Two signal voltages have symmetry when they have

equal magnitudes but opposite polarities. Symmetry of

the desired signal has advantages, but they concern head

room and crosstalk, not noise or interference rejection.

The noise or interference rejection property is indepen-

dent of the presence of a desired differential signal.

Therefore, it can make no difference whether the

desired signal exists entirely on one line, as a greater

voltage on one line than the other, or as equal voltages

on both of them. However, the symmetry myth is wide-

spread. A typical example is: Each conductor is always

equal in voltage but opposite in polarity to the other.

The circuit that receives this signal in the mixer is called

a differential amplifier and this opposing polarity of the

Figure 32-8. Basic balanced interface.

Device A Device B

Signal Out In Signal

Zo /2

Zi

Zo /2