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FALCON · User Manual · ENG
5.3.6 FM PROCESS
Here it can manage the input PreEmphasis, output DeEmphasis, and the Delay.
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
minimize 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.