Editing Emphasis (telecommunications) (section)

== In audio signals ==
In processing electronic [[audio signal]]s, 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 [[Clipping (audio)|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 (signal processing)|filter]]. The [[frequency response]] is decided by special [[time constant]]s. The [[cutoff frequency]] can be calculated from that value.

Pre-emphasis is commonly used in [[telecommunications]], [[digital audio]] recording, [[phonograph|record]] cutting, in [[FM broadcasting]] [[transmission (telecommunications)|transmission]]s, and in displaying the [[spectrogram]]s of speech signals. One example of this is the [[RIAA equalization]] curve on 33&nbsp;rpm and 45&nbsp;rpm [[vinyl record]]s. 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 (physics)|power]] in terms of [[modulation index|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 constant]]s 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 standard)|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 error|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&nbsp;dB (at 20&nbsp;dB/decade) and time constants 50&nbsp;μs and 15&nbsp;μs.<ref>{{Cite book|title = IEC 60908:1999: Audio recording{{snd}}Compact disc digital audio system|publisher = International Electrotechnical Commission|year = 1999|location = Geneva|pages = 29, 131}}</ref>