Emphasis (telecommunications)
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 FM broadcasting (preemphasis improvement) and vinyl (e.g. LP) records. For example, high-frequency signal components may be emphasized to produce a more equal modulation index for a transmitted frequency spectrum, and therefore a better signal-to-noise ratio for the entire frequency range.
In digital transmission[edit]
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 that more closely resembles the original or desired signal, allowing the use of higher frequencies or producing fewer bit errors.
In serial data transmission, de-emphasis has a different meaning, which is to reduce the level of all bits except the first one after a transition. That causes the high-frequency content due to the transition to be emphasized compared to the low-frequency content which is de-emphasized. This is a form of transmitter equalization; it compensates for losses over the channel which are larger at higher frequencies. Well-known serial data standards such as PCI Express, SATA and SAS require transmitted signals to use de-emphasis.
This article incorporates from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22. (in support of MIL-STD-188).
