Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 (Tek) 9/28/84 based on 9/17/84; site vice.UUCP Path: utzoo!watmath!clyde!burl!ulysses!allegra!bellcore!decvax!tektronix!tekcrl!vice!shauns From: shauns@vice.UUCP (Shaun Simpkins) Newsgroups: net.audio Subject: Re: CD Reflections Message-ID: <77@vice.UUCP> Date: Wed, 16-Jan-85 14:42:39 EST Article-I.D.: vice.77 Posted: Wed Jan 16 14:42:39 1985 Date-Received: Sat, 19-Jan-85 00:23:24 EST References: <15100001@hpfcmp.UUCP> <3411@mit-eddie.UUCP> <1420@hplabs.UUCP> Organization: Tektronix, Beaverton OR Lines: 57 > > The use of a higher sampling rate (oversampling) is used > > so that the low pass filter used to reconstruct the original > > waveform need not have a severe slope in the transition band. > > The claim is that a gentler slope reduces phase and group delay > > effects. For those that care, the CD-2 using 2x oversampling > > requires a 7th order filter to reconstruct the signal. That's > > still a bit steep for some. Several manufacturers offer CD > > players that use 4x oversampling. I haven't heard of any that > > use anything higher. I don't know if anyone has anything in > > the works or even what the theoretical limit is. If anyone > > has some information on this, I'd like to hear it. > > > > Gordon Strong > > {decvax!genrad, ihnp4}!mit-eddie!gs > > GS@MIT-XX > > It seems to me that the disc itself was created with a given sampling rate. > How can a player change this? > > Bob Comment on higher than 4x oversampling: 4x oversampling means that a sample has to be delivered to the D/A every 5.6uS if separate DACs are used or every 2.8us if only one is. Today's consumer-grade 16 bit DACs settle in 4uS, preventing faster than 2x oversampling w/DAC sharing @ 16 bits (YAMAHA) or 4x oversampling with dedicated DACs @ 16 bits (KYOCERA, NAK, etc.). Philips uses only 14 bits - hence, their D/A settles faster and can be time-shared at a 4x rate. If you're willing to perhaps quintuple the price of the DAC and add another $200 to the retail cost of your player, 16 bit D/A converters are available with 350ns settling time, permitting 32x oversampling if you are crazy enough. At this rate, harmonics would be centered at multiples of 1.4MHz and a 1-pole filter could be used for reconstruction. However, another problem would arise - and probably does now, too: the cycle time of the uP doing all the error correcting and filtering would be unbelievablly small. If my memory serves me correctly, most machines use a master clock of 3.58MHz (TV color burstfrequency and 81 times the base data rate on the CD) with instruction processing rates of perhaps 1/5th that. Each sample delivered to the DAC represents at least 4 processing cycles (I would assume); result - you can't do more than 4x oversampling with 3.58MHz clock rates. Finally, cheap uPs max out at around a 4MHz clock. As far as the second comment above, it's been answered already: input data rate doesn't change but `interpolated' data exits the processor at 4x. The wandering squash, Shaun Simpkins uucp: {ucbvax,decvax,chico,pur-ee,cbosg,ihnss}!teklabs!tekcad!vice!shauns CSnet: shauns@tek ARPAnet:shauns.tek@rand-relay -- Shaun Simpkins uucp: {ucbvax,decvax,chico,pur-ee,cbosg,ihnss}!teklabs!tekcad!vice!shauns CSnet: shauns@tek ARPAnet:shauns.tek@rand-relay