Pre-emphasis
Pre-emphasis is a mastering technique that is intended to help reduce high-frequency noise levels on compact discs. It works by boosting the high-frequency spectrum by up to 10dB in the audio written to the disc ("emphasis"); the effect is reversed in real time when the disc is played back ("de-emphasis"). In a standard CD player audio is de-emphasized by a filter in the analog stage that is activated by a flag bit that appears on the disc. This is a per-track setting, so it's possible for emphasized and non-emphasized tracks to appear on the same disc.
Pre-emphasis is usually only found on early CD pressings, though there are some exceptions. Some common titles were pressed with pre-emphasis well into the 1990s. See Pre-emphasis (release list) for a summary of affected titles.
Because emphasis is part of the actual audio data on a CD, any rip of an emphasized CD will result in an emphasized output file unless the ripping software properly detects the pre-emphasis and re-equalizes the data it saves to disk. In practice, de-emphasis will usually have to be done manually after the disc has been ripped.
Detecting Pre-Emphasis
The "pre-emphasis" control flags that tell a CD player to de-emphasize audio on playback appear in two places on a CD: in the table of contents (TOC) that's read when the disc is first inserted into a player/reader, and in the subcode blocks that appear in each data frame in the disc's program area.
A number of releases have been mastered with the pre-emphasis flag set in the program-area subcode blocks but not in the TOC. The vast majority of standalone CD players handle this properly, but some software and equipment only check the CD's TOC and therefore don't know to de-emphasize these discs correctly.
Detection Issues with Specific Hardware and Software
- Current versions of Exact Audio Copy (EAC) cannot detect control flags in the program-area subcode blocks. Older versions, through v0.95prebeta3, could check the flags with the Action > TOC Alternations > Detect TOC Manually command, but because this feature could also be used to circumvent certain CD copy-protection schemes, it was removed by program author Andreas Weithoff due to legal concerns. The detection process was also known to be unreliable under certain circumstances.
- iTunes will automatically de-emphasize ripped audio before writing it to disk, but only checks for the flag in the TOC. Also, versions 10.5.0 and 10.5.1 of iTunes are known to have problems detecting pre-emphasis; this bug was reported fixed in 10.5.2.
- Some, if not all, Oppo Blu-Ray Disc players will only detect pre-emphasis flags in the TOC.