ark@alice.UUCP (Andrew Koenig) (07/20/85)
Quote from a news article about Compact Discs Another item for buyers to ask about is oversampling, which is the ability of the player to ask the disc for information more often than necessary. "What makes the difference in sound at this level is how hard the computer is working to translate," Harvey said. Oversampling allows the microcomputer to cover up damaged information by overplaying the material just before it to cover up. So THAT's how they do it!
doug@prime.UUCP (Douglas Hamilton) (07/23/85)
> Quote from a news article about Compact Discs > > Another item for buyers to ask about is oversampling, > which is the ability of the player to ask the disc for > information more often than necessary. > > "What makes the difference in sound at this level is how > hard the computer is working to translate," Harvey said. > Oversampling allows the microcomputer to cover up damaged > information by overplaying the material just before it > to cover up. > > So THAT's how they do it! No, it's not. Oversampling refers to interpolating between the samples actually recorded on the disk. CD's are recorded with a 44KHz sampling rate which (remembering your sampling theory) gives a 22KHz bandwidth. The problem is that if one feeds the samples directly to the DAC's, the output "staircases" from one sample to the next. It's desirable to remove this high frequency "switching" noise but of course any kind of low-pass filtering causes phase shift near the cutoff frequency (also undesirable.) To minimize this effect, some CD manufacturers interpolate much the way you would with a trig table (remember before calculators?) to find intermediate values. By creating "samples" at an 88 or 176KHz rate to be fed to the DACs, they've moved the noise up to a much higher frequency, more removed from the music, thus making it easier to filter the switching noise out without introducing phase shift. Oversampling has nothing whatsoever to do with error correction, which happens at an earlier stage in the processing. -- Douglas Hamilton Prime Computer,Inc. 617-626-1700 x3956 Video Products Group 492 Old Connecticut Path Framingham, MA 10701 {seismo,ihnp4,allegra,ut-sally}!harvard!prime!doug