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From: kunz@hplsle.UUCP (kunz)
Newsgroups: net.audio
Subject: Re: Carver CD Player & Technology
Message-ID: <5000001@hplsle.UUCP>
Date: Tue, 12-Mar-85 13:47:00 EST
Article-I.D.: hplsle.5000001
Posted: Tue Mar 12 13:47:00 1985
Date-Received: Thu, 14-Mar-85 06:18:49 EST
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Nf-From: hplsle!kunz    Mar 12 11:47:00 1985


(Re: why do 4x oversampling?)

vice!shauns says....
> ... DAC sharing (i.e.,
> channel after the DAC) effectively doubles the data rate.  Solution? Go to
> 14 bit DACs that settle faster if you want to use only one, or make do with
> 2x oversampling if you still want 16 bit conversion.  This is what Yamaha did.
> I suspect that Philips made a silk purse out of a sow's ear with their 14bit
> 4x scheme \- at the time that chip set came out cheap 5us 16bit DACs weren't
> even on the drawing board but 14 bits was within the reach of existing IC
> processes.  ...
> 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
> 

The real reason to do 4x oversampling is not to accomodate the lack of
technology in DACs, but rather, to ease the design requirements on the
analog filter that follows the DAC.  If one were to use 1x sampling (44.1 KHz),
the analog filter would need to have a very sharp cutoff around 20 KHz which
would result in tremendous phase shift (but can we hear it!) and potential
ripple in the pass band (say... over 0.1 dB, but can we hear it?).  With the
cutoff for the filter out at 88 KHz (approx), the phase shift at 20 KHz can
be substantially reduced and the passband ripple likewise reduced.  Components
in the frequency domain between 20 Khz and 88 KHz are reduced by the digital
filter before the DACs.

There is also the whole topic of aliasing, which 4x oversampling makes easier
to accomodate for. (I.E. eleminate)

4x oversampling is accomplished by padding the one real sample with 3 zero
samples (BTW - done in the digital filter) and producing a sample stream at
the output of the digital filter at a rate of 176.4 KHz.  This sampled data
only needs to be converted with 14 bit precision because the 'extra' bits
come from the oversampled samples.  (It's a difficult concept to write
about but can be shown graphically very easily)  The resultant output still
has 96 dB dynamic range even though the DAC is a 14 bit one.

Using 4x oversampling and 16 bit DACs is overkill since there is no
information available in the LSB's (unless one wants to manufacture it as
noise somewhere in the system).

I own some of Carver's gizmos and they do seem to work as claimed.  The
holographic pre-amp is a real winner, especially with small satellite
speakers (I have M&Ks with a subwoofer).

Good luck in your quest for the 'perfect' CD!

Bob Kunz
{hplabs!hp-pcd, fluke, teltone}!hplsla!kunz
Hewlett-Packard Lake Stevens Instrument Division
(206) 335-2135