Pre-emphasis & CDs
Although rarely used, there exists the capability for standardized emphasis in Red Book CD mastering. As CDs were intended to work on 14-bit audio, a specification for ‘pre-emphasis’ was included to compensate for quantization noise. After production spec was set at 16 bits, quantization noise became less of a concern, but emphasis remained an option through standards revisions. The pre-emphasis is described as a first-order filter with a gain of 10 dB (at 20 dB/decade) and time constants 50 μs and 15 μs ~ http://en.wikipedia.org/wiki/Emphasis_(telecommunications)#Red_Book_Audio
Emphasis came about because of early converter design. The entire sampling process was new, and A to D converters exhibited low level noise because of bad linearity in the conversion process. This process added some high frequency broadband noise to the digital signal. Manufacturers overcame this byproduct by boosting (emphasis) the high frequencies during the conversion from analog to digital, and then rolling off (de-emphasis) the high frequencies by the same amount after the conversion back from digital to analog. This process was optional and there was a switch to select emphasis on each track during record. A flag was set in the digital bit-stream, which automatically activated de-emphasis during playback. All CD players, DVD players, and DAT machines detect this flag and turn on a high frequency roll-off in the analog domain during playback. If the digital signal contains emphasis and the flag is missing or turned off, then the roll-off does not occur and the audio will be brighter than normal.
This emphasis feature was the biggest reason why different CD players sounded different when playing back the same CD, or DAT machines differed playing back the same DAT tape. The digital part and the conversion to analog were basically the same in all of the machines. The de-emphasis circuit was implemented in the analog domain using the least expensive circuit to perform the operation. There was high-end EQ on the output of every digital playback device, and there was no standard or calibration for how it was performed. If you played back a CD without emphasis, then all of the CD players sounded pretty much the same. If you played a CD with emphasis, then each playback device sounded very different from every other player.
Producers and engineers started turning off the emphasis switches. Converters were getting better so there was less converter noise, and the use of de-emphasis circuits was eliminated. ~ Roger Nicolls
As I understand it, the Redbook CD standard calls for decoding of pre-emphasis flags. So any player bearing the CD format logo should handle pre-emphasis discs properly. ~ http://forums.stevehoffman.tv/threads/whats-pre-emphasis.49396/#post-971902
Working with CDs with pre-emphasis
Foobar plugins to de-emphasis pre-emphasized CDs:
- Postprocessing (foo_deemph): works only with lossless sources; always active during playback and ReplayGain scan.
- * DSP plugin (foo_dsp_deemph): works with any source
Use post-processing if you want to add correct ReplayGain tags to your files.
Use DSP if you have lossy files that you want to de-emphasize.
Otherwise, there’s no difference. ~ http://hydrogenaud.io/index.php/topic,99394.50.html
Lists of CD’s with pre-emphasis: