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 UUCP: ...!{tikal, sun, uunet, elwood}!microsoft!brianw InterNet: microsoft!brianw@uunet.UU.NET or: microsoft!brianw@Sun.COM Bitnet brianw@microsoft.UUCP