30 Chapter 1
is attempting to drive what amounts to a short circuit and can only develop very
small voltages. Furthermore, shunt capacitance in the cable has very little effect. The
destination has a somewhat higher impedance (generally a few k Ω to avoid excessive
currents fl owing and to allow several loads to be placed across one driver).
In the absence of fi xed impedance, it is meaningless to consider power. Consequently, only
signal voltages are measured. The reference remains at 0.775 V, but power and impedance
are irrelevant. Voltages measured in this way are expressed in dB(u), the most common unit
of level in modern analog systems. Most installations boost the signals on interface cables
by 4 dB. As the gain of receiving devices is reduced by 4 dB, the result is a useful noise
advantage without risking distortion due to the drivers having to produce high voltages.
1.13 Audio Level Metering ..............................................................................................
There are two main reasons for having level meters in audio equipment: to line up or
adjust the gain of equipment and to assess the amplitude of the program material.
Figure 1.24 : (a) Traditional impedance matched source wastes half the signal
voltage in the potential divider due to the source impedance and the cable.
(b) Modern practice is to use low-output impedance sources with
high-impedance loads.