Path: utzoo!attcan!uunet!microsoft!brianw
From: brianw@microsoft.UUCP (Brian Willoughby)
Newsgroups: comp.dsp
Subject: Re: More digital mixer stuff
Message-ID: <7928@microsoft.UUCP>
Date: 2 Oct 89 03:12:23 GMT
References: <9238@pyr.gatech.EDU> <7905@microsoft.UUCP> <9247@pyr.gatech.EDU>
Reply-To: brianw@microsoft.UUCP (Brian Willoughby)
Organization: Microsoft Corp., Redmond WA
Lines: 80

In article <9247@pyr.gatech.EDU> byron@pyr.UUCP (Byron A Jeff) writes:
>In article <7905@microsoft.UUCP> brianw@microsoft.UUCP (Brian Willoughby) writes:
>-In article <9238@pyr.gatech.EDU> byron@pyr.gatech.edu.gatech.edu (Byron A Jeff) writes:
>-Sure, why overwork the DSP?  Question: how would you know which 16 bits
>-make up a sample word?  I.e. are there some kind of start bits which
>-could be used to signal an interrupt.  A good setup would involve
>-clearing all the shift register bits to one state, and then shifting in
>-until the start bit appears in the MSBit.  Whatever polarity the start
>-bit is, just clear the shift register to the opposite value and interrupt
>-the DSP when a full word is ready.

>I think I'd just attach a 4 bit counter that is clocked and cleared along
>with the shift registers. Then when the counters overflows I generate
>an interrupt. When I read in the parallel value in the shift register
>I clear the counter. Not completely thought out but that's the 
>general idea.

Still suffers from the same synchronization problems.  What if you
accidently clear the counter at bit 14?  Your hardware will happily chug
along and have you the next 16 bits from the continuous bit stream - only
they will be the wrong bits, not word aligned.

>-Check out my posting on digital mixing using time-multiplexing.
 [etc...]

>The only question is what's the sample rate of the mixed signal? It
>would seem to be 8x the original sample rate yes? So for 8 48Khz channels
>I'd need a 384 Khz DAC. Ouch! 

I might be wrong, but due to the simplicity of DACs, you'll probably find
that a DAC capable of 384 kHz is not nearly as prohibitively expensive as
an equally rated ADC.  Most DACs don't really quote conversion rates,
they merely state the settling time of the outputs, which affects how
fast you can clock your S/H on the output, and usually ends up being VERY
fast, even for $1.49 DACs (of course, thats an 8 bit price).

[I've deleted my own overview of Sample and Hold]
>
>This is the same explanation I got in Hal Chainberlin (sp) "Musical
>Applications for Microprocessors". I understand I'll need S/H and filters
>before the ADC and after the DAC. I was figuring on only one or two
>pairs of stereo outputs from the mixer so I don't think that many S/H
>are necessary on output.

Do you own that book, or borrow it from a library?  I tried to order it
from my university bookstore and could not find it listed.  BTW, Hal
Chamberlin (closer spelling, but probably not perfect :-) graduated from
NCSU, where I found his 1973 thesis for a digital waveform synthesizer.
This guy was/is really ahead of his time, and now works for Kurzweil.  He
started out with an Apple II, and did quite a bit with it (which keeps me
encouraged with my own projects).  He started a company in Raleigh, NC,
near the university. which built a dual 5.25 floppy box that would record
CD quality audio directly to disk, and also play back.  Of course, the
operator had to continually keep the drive fed with new disks, but the
amasing thing was that the box could handle managing the buffer/queue of
64K, reading from the disk and playing the sound all at once in
real-time.  No DSP used or needed.

I would appreciate any ordering information, or even just the library of
Congress number from that book.

>Someone suggested that the S/H is built into the ADC so that I can
>hook up the output from the CMOS multiplexor directly into the ADC.
>So only 2 S/H (or one if I decide to go with a single DAC) will be
>necessary (I hope).

One DAC can become two outputs with 2 S/H stages if you design separate
control for each S/H.  3 stages gets you two phase-aligned audio outputs.

>-- 
>Another random extraction from the mental bit stream of...
>Byron A. Jeff
>Georgia Tech, Atlanta GA 30332
>Internet:	byron@pyr.gatech.edu  uucp:	...!gatech!pyr!byron

Brian Willoughby
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