sjc@mordor.UUCP (Steve Correll) (02/07/84)
CDs contain 44.1k samples per second per channel. Each sample contains 16 bits. "Oversampling" consists of feeding samples into a digital filter at 44.1kHz and reading out results from the filter at a faster rate (4 * 44.1kHz in the Philips/Magnavox players). Philips gets away with 14 bit D/A converters since (a) oversampling spreads the noise over 4 times the bandwidth, so that the noise within the audio spectrum is 1/4th what it otherwise would be and (b) they keep track of the direction of roundoff error and endeavor to make it average out. In addition, that digital filter reduces the amplitude of the unwanted spectral images above 20kHz. Then a sample-and-hold step filters the signal with a (sin x)/x characteristic, further reducing the images. The turnover point of the analog lowpass filter can therefore be 30kHz rather than 20kHz. The Philips engineers have apparently taken some pains to reduce the much-discussed phase shift problem. The laser does, of course, read many more than 2 x 44.1k x 16 bits per second from the disk: track/index information, error correction, synchronization, and modulation roughly triple the bit rate. In 1982 the Philips Technical Review devoted an entire issue (Volume 40 #6) to very readable papers on the CD system. --Steve Correll (S-1 Project, Lawrence Livermore National Laboratory) MILNET: sjc@s1-c UUCP: ...!decvax!decwrl!mordor!sjc