In processing electronic
audio signals, pre-emphasis refers to a system process designed to increase (within a
frequency band) the magnitude of some (usually higher) frequencies with respect to the magnitude of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as
attenuation distortion or
saturation of recording media in subsequent parts of the system. The mirror operation is called de-emphasis, and the system as a whole is called emphasis. Pre-emphasis is achieved with a
pre-emphasis network which is essentially a calibrated
filter. The
frequency response is decided by special
time constants. The
cutoff frequency can be calculated from that value. Pre-emphasis is commonly used in
telecommunications,
digital audio recording,
record cutting, in
FM broadcasting transmissions, and in displaying the
spectrograms of speech signals. One example of this is the
RIAA equalization curve on 33 rpm and 45 rpm
vinyl records. Another is the
Dolby noise-reduction system as used with magnetic tape. Pre-emphasis is employed in
frequency modulation or
phase modulation transmitters to equalize the modulating
signal drive
power in terms of
deviation ratio. The receiver
demodulation process includes a reciprocal network, called a de-emphasis network, to restore the original signal power distribution.
De-emphasis In telecommunications,
de-emphasis is the complement of pre-emphasis, in the antinoise system called emphasis. De-emphasis is a system process designed to decrease, (within a band of frequencies), the magnitude of some (usually higher) frequencies with respect to the magnitude of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as attenuation distortion or saturation of recording media in subsequent parts of the system. Special
time constants dictate the
frequency response curve, from which one can calculate the
cutoff frequency.
Red Book audio Although rarely used, there exists the capability for standardized emphasis in
Red Book CD mastering. As CD players were originally implemented with affordable 14-bit converters, a specification for pre-emphasis was included to compensate for
quantization noise. After economies of scale eventually allowed full 16 bits, quantization noise became less of a concern, but emphasis remained an option. The pre-emphasis is described as a first-order filter with a gain of 10 dB (at 20 dB/decade) and time constants 50 μs and 15 μs. == In digital transmission ==