Preemphasis
Encyclopedia
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. That is the mirror of the deemphasis
. The whole system is called emphasis
.
The frequency curve is decided by special time constant
s. The cutoff frequency
can be calculated from that value.
Pre-emphasis is commonly used in telecommunication
s, digital audio
recording, record
cutting, in FM broadcasting
transmission
s, and in displaying the spectrogram
s of speech signals.
An example of this is the RIAA equalization
curve on 33 rpm and 45 rpm vinyl records
.
In high speed digital transmission, pre-emphasis is used to improve signal quality
at the output of a 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 which more closely resembles the original or desired signal, allowing the use of higher frequencies or producing fewer bit errors.
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
by minimizing the adverse effects of such phenomena as attenuation distortion
Attenuation Distortion
Attenuation distortion is the distortion of an analog signal that occurs during transmission when the transmission medium does not have a flat frequency response across the bandwidth of the medium or the frequency spectrum of the signal....
or saturation
Saturation (telecommunications)
In telecommunications, the term saturation has the following meanings:*In a communications system, the condition at which a component of the system has reached its maximum traffic-handling capacity...
of recording media in subsequent parts of the system. That is the mirror of the deemphasis
Deemphasis
In telecommunication, de-emphasis is the complement of pre-emphasis, in the antinoise system called emphasis. Emphasis is a system process designed to decrease, , the magnitude of some frequencies with respect to the magnitude of other frequencies in order to improve the overall signal-to-noise...
. The whole system is called emphasis
Emphasis (telecommunications)
In FM transmission, emphasis is the intentional alteration of the amplitude-vs.-frequency characteristics of the signal to reduce adverse effects of noise in a communication system....
.
The frequency curve is decided by special time constant
Time constant
In physics and engineering, the time constant, usually denoted by the Greek letter \tau , is the risetime characterizing the response to a time-varying input of a first-order, linear time-invariant system.Concretely, a first-order LTI system is a system that can be modeled by a single first order...
s. The cutoff frequency
Cutoff frequency
In physics and electrical engineering, a cutoff frequency, corner frequency, or break frequency is a boundary in a system's frequency response at which energy flowing through the system begins to be reduced rather than passing through.Typically in electronic systems such as filters and...
can be calculated from that value.
Pre-emphasis is commonly used in telecommunication
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
s, digital audio
Digital audio
Digital audio is sound reproduction using pulse-code modulation and digital signals. Digital audio systems include analog-to-digital conversion , digital-to-analog conversion , digital storage, processing and transmission components...
recording, record
Phonograph
The phonograph record player, or gramophone is a device introduced in 1877 that has had continued common use for reproducing sound recordings, although when first developed, the phonograph was used to both record and reproduce sounds...
cutting, in FM broadcasting
FM broadcasting
FM broadcasting is a broadcasting technology pioneered by Edwin Howard Armstrong which uses frequency modulation to provide high-fidelity sound over broadcast radio. The term "FM band" describes the "frequency band in which FM is used for broadcasting"...
transmission
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
s, and in displaying the spectrogram
Spectrogram
A spectrogram is a time-varying spectral representation that shows how the spectral density of a signal varies with time. Also known as spectral waterfalls, sonograms, voiceprints, or voicegrams, spectrograms are used to identify phonetic sounds, to analyse the cries of animals; they were also...
s of speech signals.
An example of this is the RIAA equalization
RIAA equalization
RIAA equalization is a specification for the correct recording of gramophone records, established by the Recording Industry Association of America...
curve on 33 rpm and 45 rpm vinyl records
Gramophone record
A gramophone record, commonly known as a phonograph record , vinyl record , or colloquially, a record, is an analog sound storage medium consisting of a flat disc with an inscribed, modulated spiral groove...
.
In high speed digital transmission, pre-emphasis is used to improve signal quality
Signal integrity
Signal integrity or SI is a set of measures of the quality of an electrical signal. In digital electronics, a stream of binary values is represented by a voltage waveform. However, digital signals are fundamentally analog in nature, and all signals are subject to effects such as noise,...
at the output of a data transmission
Data transmission
Data transmission, digital transmission, or digital communications is the physical transfer of data over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical fibres, wireless communication channels, and storage media...
. 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 which more closely resembles the original or desired signal, allowing the use of higher frequencies or producing fewer bit errors.