1202 Chapter 32
imperfections. But, as discussed in Section 32.6.4, it is
virtually always superior to an unbalanced interface!
32.5.4 Coupling in Unbalanced Cables
The overwhelming majority of consumer as well as
high-end audiophile equipment still uses an audio inter-
face system introduced over 60 years ago and intended
to carry signals from chassis to chassis inside the
earliest RCA TV receivers! The ubiquitous RCA cable
and connector form an unbalanced interface that is
extremely susceptible to common impedance noise
coupling.
As shown in Fig. 32-38, noise current flow between
the two device grounds or chassis is through the shield
conductor of the cable. This causes a small but signifi-
cant noise voltage to appear across the length of the
cable. Because the interface is unbalanced, this noise
voltage will be directly added to the signal at the
receiver.^33 In this case, the impedance of the shield
conductor is responsible for the common impedance
coupling. This coupling causes hum, buzz, and other
noises in audio systems. It’s also responsible for
slow-moving hum bars in video interfaces and glitches,
lock-ups, or crashes in unbalanced—e.g., RS-232—data
interfaces.
Consider a 25 ft interconnect cable with foil shield
and a #26 AWG drain wire. From standard wire tables
or actual measurement, its shield resistance is found to
be 1.0:. If the 60 Hz leakage current is 300μA, the
hum voltage will be 300μV. Since the consumer audio
reference level is about –10 dBV or 300 mV, the 60 Hz
hum will be only
relative to the signal. For most systems, this is a very
poor signal-to-noise ratio! For equipment with
two-prong plugs, the 60 Hz harmonics and other
high-frequency power-line noise (refer to Fig. 32-20)
will be capacitively coupled and result in a
harmonic-rich buzz.
Because the output impedance of device A and the
input impedance of device B are in series with the inner
conductor of the cable, its impedance has an insignifi-
cant effect on the coupling and is not represented here.
Common-impedance coupling can become extremely
severe between two grounded devices, since the voltage
drop in the safety ground wiring between the two
devices is effectively parallel connected across the
length of the cable shield. This generally results in a
fundamental-rich hum that may actually be larger than
the reference signal!
Coaxial cables, which include the vast majority of
unbalanced audio cables, have an interesting and under-
appreciated quality regarding common-impedance
coupling at high frequencies, Fig. 32-39. Any voltage
appearing across the ends of the shield will divide itself
between shield inductance Ls and resistance Rs
according to frequency. At some frequency, the voltages
across each will be equal (when reactance of Ls equals
Rs). For typical cables, this frequency is in the 2 to
5 kHz range. At frequencies below this transition
frequency, most of the ground noise will appear across
Rs and be coupled into the audio signal as explained
earlier. However, at frequencies above the transition
frequency, most of the ground noise will appear across
Ls. Since Ls is magnetically coupled to the inner
conductor, a replica of the ground noise is induced over
its length. This induced voltage is then subtracted from
the signal on the inner conductor, reducing noise
coupling into the signal. At frequencies ten times the
transition frequency, there is virtually no noise coupling
at all—common-impedance coupling has disappeared.
Therefore, common-impedance coupling in coaxial
cables ceases to be a noise issue at frequencies over
about 50 kHz. Remember this as we discuss claims
made for power line filters that typically remove noise
only above about 50 kHz.
Unbalanced interface cables, regardless of construc-
tion, are also susceptible to magnetically induced noise
caused by nearby low-frequency ac magnetic fields.20 log 300 Ve300 mV –= 60 dBFigure 32-38. Common impedance coupling in an unbalanced audio, video, or data interface.RCP DriverCPCPCPICBCADEVICE A DEVICE B
ReceiverR = Cable shield + contact resistance
I = Circulating interference currentA = Receiver ground = reference point
B = Interference voltage at driver ground, E = I × R
C = Interference voltage + signal receiver input