Pre-emphasis

Pre-emphasis is the first part of a noise reduction technique in which a signal's weaker, higher frequencies are boosted before they are transmitted or recorded onto a storage medium. Upon playback, a de-emphasis filter is applied to reverse the process. The result is a higher signal-to-noise ratio (SNR); the original frequencies are restored, but noise that was introduced by the storage medium, transmission equipment, or analog/digital conversions is quieter than it would have been if no filtering had been done. Pre- and de-emphasis can be collectively referred to as just emphasis.

Emphasis was sometimes used in digital storage media in the late 1970s through early 1980s, including on a small percentage of audio CDs. Emphasis is akin to Dolby noise reduction for tapes, or the RIAA equalization curve for vinyl records.

Pre-emphasis on audio CD

Some early digital recording & playback equipment, including CD players, used 14-bit converters, even though they were dealing with 16-bit audio. The resulting noise introduced by these converters could be made relatively quiet by using pre-emphasis: boosting the signal (especially the higher frequencies) in the recording before it was put onto CD, and embedding flags in the disc's subcode to tell a CD player to apply de-emphasis on playback. Some CD players even had a de-emphasis button that could be used to manually apply de-emphasis, but now it's just a built-in feature of nearly all dedicated audio CD players. By the late 1980s, pre-emphasis stopped being used because reliable 16-bit DACs with oversampling and other technologies minimized the conversion noise without the need for pre-processing the recording.

Most major-label CDs with pre-emphasis were manufactured in Japan in the early and mid-1980s. Relatively recent forum posts indicates that pre-emphasis is still used on newly manufactured CDs by some indie labels, mainly for classical titles.

A pre-emphasis flag for each track is normally stored in the subcode along with the audio data. It's also supposed to be stored in the table of contents (TOC), but many CDs have TOCs that say there's no pre-emphasis when in fact the subcode says there is.

Pre-emphasis on audio CDs in computers

CD drives in computers aren't like regular CD players. They normally use a digital interface to send the raw audio data straight from the disc to the operating system and whatever program needs it. Pre-emphasis flags in the disc's subcode usually aren't even checked for, and even when they are detected, they're usually ignored. Flags in the TOC are universally ignored.

So when you play a CD through your computer, or use CD ripping software to get the audio content, you're probably getting the pre-emphasized audio data. Since it has not been de-emphasized, it will probably sound too "bright" and/or hissy (although your audio equipment or your damaged ears may keep you from noticing). Therefore you may want to do de-emphasis processing yourself.

According to forum posts, there's one CD ripper which will detect pre-emphasis flags in the subcode and will apply a de-emphasis filter to the data before writing it to disk: recent versions of iTunes. Recent versions of Windows Media Player will detect pre-emphasis and apply a filter for playback, but not for ripping.

So if you're ripping CDs with software other than iTunes, then there are various methods for applying de-emphasis, each with pros & cons. They all basically amount to turning down the treble with a special EQ curve that bottoms out at about -10 dB @ 20+ kHz. This can be done during playback with a simple equalizer setting, or you can modify the audio data itself so it will be as if it never had pre-emphasis.

External links

The following threads & articles contain useful information

Digital de-emphasis tools

WaveEmph (after unzipping, right-click on the .exe and set Win98 compatibiliity)
SOX has a deemphasis function