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From: rsk@pucc-j (Wombat)
Newsgroups: net.music.synth
Subject: Re: Digital Reverb? (Why it is digital)
Message-ID: <943@pucc-j>
Date: Fri, 21-Mar-86 12:09:43 EST
Article-I.D.: pucc-j.943
Posted: Fri Mar 21 12:09:43 1986
Date-Received: Sat, 22-Mar-86 22:14:38 EST
References: <3095@potomac.UUCP> <2913@sjuvax.UUCP> <2920@sjuvax.UUCP>
Reply-To: rsk@pucc-j.UUCP (Wombat)
Organization: Purdue University
Lines: 50

In article <2920@sjuvax.UUCP> lp102918@sjuvax.UUCP (Larry Palena) writes:
>What is *digital* about digital delays??
>Do they sample the delayed signal,or use microprocessors 
>to control feedback,or both??...

One of the most common ways of building a digital delay line is to construct
a box something like this:

			 <____feedback path___<_
			 \		        \
input -> a/d converter -> bucket-brigade memory -> d/a converter -> output
				   |
				control

The idea is simple; the input signal is digitized (converted from an analog
waveform to a series of binary words of some size, typically 8-14 bits) and
fed into a large chunk of memory.  Each lil' chunk of memory passes the word(s)
along to the next; eventually, after some time has passed, it pops out the
end other end.  It is there converted back to an analog waveform, (optionally)
filtered, and there's your delayed signal.

Now, the nifty effects you can create depend on the sophistication of the
control circuit (which might be a microprocessor), the modes under which
you can access the memory, and the alternate signal paths available.
If the memory allows random-access (or *something* more sophisticated
than FIFO) then some real possibilites open up.

For instance, by feeding back the output of the delay line to the input,
you can create reverberation and echo.  If you can control the amplitude
of the fed-back signal and the time displacement w.r.t. the input, then
you can control the behavior of these effects.  If you can modulate the
time delay about zero, you can create flanging, and so on.

Your basic vanilla time delay (like the one I designed some years ago for ITC)
has two controls: amplitude and delay.  The amplitude control sets the output
level w.r.t. input level; the delay control sets (you guessed it) the delay
time.  [This is the sort of delay that is used in broadcast applications for
screening callers who may happen to say something, er, inappropriate.]
Oh, almost forgot--it usually also has a "thru" button or somesuch that
flushes the contents of the memory and connects the input straight through
to the output.

>..also,why has mechanical (ie. spring-controlled) reverb died??

It's not reliable (in the long term); it's susceptible to shock and vibration
damage; it's not capable of the vast variety of effects possible with digital
equipment; and it can occasionally make a horrendous racket when you don't
want it to.  (Ever bumped your amp when the reverb was on 9?)
-- 
Rich Kulawiec  pucc-j!rsk or rsk@asc.purdue.edu