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From: myers@hpfcdj.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: Frequency divider (audio)
Message-ID: <17660026@hpfcdj.HP.COM>
Date: 6 Oct 89 18:05:00 GMT
References: <3923@blake.acs.washington.edu>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 39

>   Hello, I need some help from those people out there who understand
>analog circuitry (in depth, I mean...)  I'm trying to build a simple
>circuit to divide the frequency of in incoming (audio) signal down
>an octave or several. What I was thinking of doing was to digitize the
>signal with a comparator of some sort, feed that through a flip-flop
>or two, and then put capacitors across and in series with the output
>to re-"analog"-ize it. (ie, chop off the aliasing frequencies with a
>capacitor across the outputs, and remove any dc using a capacitor 

Well, one-bit sampling at a sufficiently high rate is certainly possible, but
I don't think that it will either (a) be as simple as you'd like or (b) 
provide the desired results with acceptable signal quality.

You have asked to be able to "divide" the frequencies of an audio signal down
"by an octave or so"; this, though, sounds like the same thing you'd get
(and very easily, I might add) by simply recording the signal on tape and then
playing it back at half speed.  Presto, everything drops to 1/2 the original
frequency, at the expense of taking twice as long to play!  (An example of the
"There ain't no such things as a free lunch!", or TANSTAAFL, Theorem!)

Doing this sort of thing "on the fly" (real time) could be done digitally,
by sampling the signal at the appropriate rate and number of bits, and then
dumping the results into a memory or onto a tape for later playback (which
is really the same as the analog recording) or performing some mathematical
operations to translate the signals down to the desired frequencies - not an
especially simple task.  (You're basically asking for the complement of the
operation performed by those nifty little high-tech cassette recorders, which
can speed up voice recordings without changing the pitch.  You want to keep
the original SPEED, but LOWER the pitch.  I think.)

Some further information, please - why do you want to do this?  What effect are
yoo really trying to acheive?  How concerned are you over the quality 
(fidelity) of the final result?  What other limitations (power, cost, size of 
the unit, etc.) are you faced with?  What frequencies need to be handled?


Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
myers%hpfcla@hplabs.hp.com               |  sentient life-form on this planet.