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AUDIO CONTROL
16.1.5
What are pre-emphasis and de-emphasis and what are they for?
In an FM system, noise has a random spectral distribution of a triangular type, with the result that noise occurs mainly at
high frequencies within the bandwidth. This may be offset, to a limited extent, by increasing the high frequencies prior to
transmission and reducing those quantities of high frequencies during reception. Consequently, reducing the high
frequencies in the receiver also reduces the high frequency noise. These processes of enhancement prior to
transmission and subsequent reduction of certain frequencies are known as pre-emphasis and de-emphasis.
The amount of pre-emphasis and de-emphasis needed is determined by the time constant of an RC filter circuit. In most
parts of the world the time constant of
50μs
is used.
In North America and South Korea the time constant of
75μs
is used. This applies to both MONO transmissions and
STEREO transmissions. In stereo signals, pre-emphasis is applied to the left-hand and right-hand channels prior to MPX
multiplexing.
The amount of pre-emphasis that can be applied is nevertheless limited by the fact that many forms of contemporary
music contain more high-frequency energy than the musical styles that were fashionable at the time FM broadcasting
was introduced, i.e. around the late 1950's. As a result, pre-emphasis cannot be used too much nowadays as it would
cause an excessive shift of the FM carrier. The more modern FM broadcasting systems tend to use a variable pre-
emphasis system which depends on the program and the audio material inputted into the audio processor.
In electronic audio signal processing, pre-emphasis refers to a process system designed to increase certain frequencies
(usually the high frequencies) within a given bandwidth frequency with respect to the levels of other frequencies (usually
the low frequencies). All this is done to improve the S/N (signal/noise) ratio, and consequently minimise the negative
effects of phenomena, such as attenuation, distortion, or saturation, during processes or parts introduced after those
already in the system. This whole system is called "emphasis"
The frequency tripping curve is determined by special time constants and the cut frequency can be calculated using
mathematical formulas. The reverse process, applied to FM receivers, is called
de-emphasis .
One example which clearly illustrates the effects of pre-
emphasis is the RIAA equalization curve on 33 rpm and 45
rpm vinyl records.
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