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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

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-- 
				Shaun Simpkins

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