Emphasis (telecommunications)

vinyl records

In signal processing, pre-emphasis is a technique to protect against anticipated noise. The idea is to boost (and hence distort) the frequency range that is most susceptible to noise beforehand, so that after a noisy process (transmission over cable, tape recording...) more information can be recovered from that frequency range. Removal of the distortion caused by pre-emphasis is called de-emphasis, making the output accurately reproduce the original input.

Emphasis is commonly used in

frequency range

.

In waveform signals

In processing electronic

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

vinyl records. Another is the Dolby noise-reduction system

as used with magnetic tape.

Pre-emphasis is employed in

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 telecommunication, 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

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.^[1]

In digital transmission

In high-speed digital transmission, pre-emphasis is used to improve

data transmission

. In transmitting signals at high data rates, the transmission medium may introduce distortions, so pre-emphasis is used to distort the transmitted signal to correct for this distortion. When done properly this produces a received signal that more closely resembles the original or desired signal, allowing the use of higher frequencies or producing fewer bit errors.

In

SAS

require transmitted signals to use de-emphasis.

References

^ IEC 60908:1999: Audio recording – Compact disc digital audio system. Geneva: International Electrotechnical Commission. 1999. pp. 29, 131.

This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22. (in support of MIL-STD-188).

External links

[1] IEC 60908:1999: Audio recording – Compact disc digital audio system. Geneva: International Electrotechnical Commission. 1999. pp. 29, 131.

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