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From: gibson@unc.UUCP (Bill Gibson)
Newsgroups: net.music.synth
Subject: Re: Digital Reverb? (& delay)
Message-ID: <1141@unc.unc.UUCP>
Date: Mon, 24-Mar-86 13:35:56 EST
Article-I.D.: unc.1141
Posted: Mon Mar 24 13:35:56 1986
Date-Received: Fri, 28-Mar-86 06:55:34 EST
References: <3095@potomac.UUCP> <2913@sjuvax.UUCP> <2920@sjuvax.UUCP> <943@pucc-j>
Reply-To: gibson@unc.UUCP (Bill Gibson)
Organization: CS Dept, U. of N. Carolina, Chapel Hill
Lines: 44
Keywords: Delay line, shift register, RAM
Summary: 

>>What is *digital* about digital delays??

>
>			 <____feedback path___<_
>			 \		        \
>input -> a/d converter -> bucket-brigade memory -> d/a converter -> output
>				   |
>				control
>
>the input signal is digitized and
>fed into a large chunk of memory.  Each lil' chunk of memory passes the word(s)
>along to the next; 

Is a shift register commonly used in this application? I figured that 
digital delay lines just used RAM and a couple of pointers (queue start
and queue end) to simulate the shift register. This would make the main
memory cheap (RAM is cheap these days) and allow a large number of 
feedback/signal "paths", i.e. as many paths as the control processor
can implement in one sample period. A "tap list" of feedback tap pointers
is a simple data structure to maintain; the list would contain pointers
to feedback source (and maybe sink) points.

Of course, a long hardware shift register should be easier to control 
than the RAM, but I didn't realize that such a register exists (unless
you're referring to an analog bucket-brigade delay line). Is this really
what is used in the commercial digital delays?

Emptying the RAM delay line is a simple matter of resetting the queue-end
pointer.

Each feedback path requires a shift/multiply and addition (per sample period). 

Flanging could be accomplished either by modulating the
system clock or by moving a feedback-pointer back and forth. If the
control processor is fast enough, it should be able to flange using multiple
feedback points along the delay line at the same time - this
might be an interesting effect!

If RAM is used as a delay line, the processor in a sampling synthesizer
could be programmed to use spare memory for chorus, echo, or reverb. Is
this how the Kurzweil 250 generates its chorus effect?

Bill Gibson
gibson@unc                   ...[akgua,decvax,philabs]!mcnc!unc!gibson