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From: brianw@microsoft.UUCP (Brian Willoughby)
Newsgroups: comp.dsp
Subject: Re: More digital mixer stuff
Message-ID: <7973@microsoft.UUCP>
Date: 6 Oct 89 04:24:03 GMT
References: <9238@pyr.gatech.EDU> <7905@microsoft.UUCP> <9247@pyr.gatech.EDU> <1845@draken.nada.kth.se>
Reply-To: brianw@microsoft.UUCP (Brian Willoughby)
Organization: Microsoft Corp., Redmond WA
Lines: 41

In article <1845@draken.nada.kth.se> d88-jwa@nada.kth.se (Jon W{tte) writes:
>>-In any event, those manufacturers are merely trying to sell you two DACs
>>-instead of one, when a single converter and a few sample and hold stages
>>-would realign the reconstructed signals.  One S/H could be used to
>
>But using one DAC also introduces some distortion in switching between
>channels !

How? when the digital signal 'switches' discretely from one sample to the
next?  There is no more 'switching' in a single DAC circuit than in a
multiple DAC circuit.  This agrees with the definition of PCM.

>>>transitions take longer than 1 to 0 transistions.  Then, for a digital
>>>sample changing from $8000 to $7FFF, you would first get $0000 on the
>>>DAC output before all the other 15 bits changed to $7FFF.  Ugly and BIG
>>>spike!  Analog sample and hold allows the outputs to be disconnented
>
>Heard about (a) a latch and (b) an analog filter after the DAC ?
>Wouldn't this take care of such things ? ARE there S/H stages after
>the DAC in good CDs ? I don't think so, but please correct me, if you
>know for * SURE *.

(a) a latch is no more instantaneous than any other electronic circuit.
It would still suffer from the same variations in transition time.
(b) an analog filter is nothing more than a circuit which reacts in a
defined to its input.  If you give a filter a different input (i.e. an
input with extra spikes it in), then it will produce a different output.
The spikes would be reduced, but your signal would not be as clean as it
would if the spikes were not produced and the filter merely smoothed the
stepping that is normal with PCM.

I can't comment on *all* good CD players, but S/H is considered optimum
(followed by an analog filter at or above the Nyquist point).

>		h+@nada.kth.se

Brian Willoughby
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