By Thinkman ยท June 29, 2012
We use mobile phones and digital audio devices every day โ and the audio signals inside them are digitised, transmitted, and converted back to analogue signals that we hear. One of the most important technologies behind reproducing the best possible sound is the concept of pre-emphasis and de-emphasis.
In processing electronic audio signals, pre-emphasis refers to a system process designed to increase โ within a frequency band โ the magnitude of some frequencies (usually the higher ones) with respect to the magnitude of others (usually the lower ones). The purpose is to improve the overall signal-to-noise ratio by minimising the adverse effects of phenomena such as attenuation distortion or saturation of recording media in subsequent parts of the system.
The frequency response curve is determined by special time constants, from which the cutoff frequency can be calculated. Pre-emphasis is the mirror of de-emphasis โ together they form a system called emphasis.
Pre-emphasis is commonly used in:
In high-speed digital transmission, pre-emphasis is used to improve signal quality at the output of a data transmission. At high data rates, the transmission medium introduces distortions โ so pre-emphasis deliberately distorts the transmitted signal in advance to correct for those expected distortions. When done correctly, the received signal more closely resembles the original, allowing higher frequencies to be used or producing fewer bit errors.
In telecommunication, de-emphasis is the complement of pre-emphasis. Where pre-emphasis boosts higher frequencies at the transmission end, de-emphasis attenuates them at the receiving end โ restoring the original frequency balance and in doing so suppressing any noise that was picked up during transmission, since the noise primarily affects the boosted (high) frequencies which are then cut back down.
Special time constants dictate the frequency response curve, from which the cutoff frequency is calculated.
In serial data transmission, de-emphasis takes on a slightly different meaning โ it reduces the level of all bits except the first one following a transition. This causes the high-frequency content from the transition to be emphasised relative to the lower-frequency content. It is a form of transmitter equalisation that compensates for channel losses which are disproportionately larger at higher frequencies. Well-known serial data standards including PCI Express, SATA, and SAS all require transmitted signals to use de-emphasis.
The elegance of the system lies in its symmetry. Think of pre-emphasis and de-emphasis as a matched pair โ a codec for analogue signal quality:
| Stage | Process | Effect |
|---|---|---|
| Transmit / Record | Pre-emphasis โ boost high frequencies | Signal rides above noise floor during transmission |
| Channel / Medium | Noise and attenuation introduced | High frequencies affected โ but they were boosted in advance |
| Receive / Playback | De-emphasis โ cut high frequencies back down | Signal restored to original balance โ noise cut along with it |
The result is a received signal that more faithfully represents the original โ cleaner, clearer, and with a significantly improved signal-to-noise ratio. Every time you enjoy music on an FM radio or a vinyl record, this quiet pair of processes is a part of why it sounds as good as it does.
