View source for Pre-emphasis, de-emphasis

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For context, some signal paths are lossy, and may attenuate some frequencies more than others,
often losing high frequencies more than they do low frequencies.


In audio contexts (and similarly for various other other signals), 
the higher frequencies naturally have lower ampolitude than the lower ones,
meaning that in noisy contexts, noise will ''relatively'' intrude more on the higher-frequency part.



So emphasis just means boosting some frequencies.

Pre-emphases is emphasis doing it ''before'' another functional part

De-emphasis is reducing instead. In various cases it is the mirror of earlier emphasis, but is also sometimes applied alone.



Examples:
* FM modulation is on a noisy medium, and the quieter higher frequencies more easily fall into noise.
:: so regular FM you can emphasize the high frequencies,
:: and receivers are expected to de-emphasize it back.

* Gramophone used pre-emphasis
:: the net effect of which is that highs are boosted and lows relatively reduced;
:: gramophone de-emphasis gets you back the original frequency response.

* early CDs used pre-emphasis because ADCs of the time weren't great; 
:: players would see the emphasis bit in the [[CD subcode bits]] 
:: that basically means '10db of pre-emphasis was applied to high frequencies in mastering, please switch in de-emphasis to get back the frequencies as recorded', 
:: though as ADCs got better this became basically pointless to do.


* [[Dolby noise reduction]] is ''based'' on emphasis and de-emphasis,
:: but is also 

* e.g. Praat applies a soft filter to boost higher frequencies
:: just because there is a natural falloff in human speech, and this at least ''shows'' different frequencies roughly on the same level
:: and calls this pre-emphasis


* A lot of fast data links (PCIe, SATA, USB, ethernet, HDMI) also use pre-emphasis and de-emphasis, mostly to deal with the fact that signal losses are frequency-dependent.
:: Basically anything fast enough might want to consider some form of this - though considered because it's sort of one distortion to balance another

These have somewhat more specific meaning around RF modulation.

https://www.geeksforgeeks.org/electronics-engineering/pre-emphasis-and-de-emphasis/


* [[companding]] as used in...
** vinyl, where pre-emphasis was applied while recording, and de-emphasis on playback - seemingly more because strong bass was a ''physical'' problem{{verify}}
** Dolby noise reduction on tape, is also pre-emphasis on recording, and de-emphasis on playback
:: for the net effect of push the noise floor further below the signal (at higher frequencies since they are rarely strong to start with, so will rarely distort due to the pre-emphasis)

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See also:
* https://en.wikipedia.org/wiki/Emphasis_(telecommunications)
* https://en.wikipedia.org/wiki/Preemphasis_improvement
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