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From: janzen@pldvax.DEC (Tom J. LMO2-1/E5 279-5421)
Newsgroups: net.music.synth
Subject: Digital delays
Message-ID: <1848@decwrl.DEC.COM>
Date: Mon, 24-Mar-86 09:03:56 EST
Article-I.D.: decwrl.1848
Posted: Mon Mar 24 09:03:56 1986
Date-Received: Wed, 26-Mar-86 02:14:47 EST
Sender: daemon@decwrl.DEC.COM
Organization: Digital Equipment Corporation
Lines: 43

>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.

This "explanation" uses anachronistic terminology incorrectly.
A "bucket-brigade" is an analogue circuit which passes stored voltages
(or charges if you like) from little op-amp circuit to op amp, or from
fet to fet in a Charge-Coupled device.  Digital delays do not use
"bucket-brigades."  Digital delays are digital becuase they use digital computer
memories to store the sound.  Digital delays require 
Analogue-to-Digital convertors (ADCs)and Digital-to-Analogue
Converters(DACs) to format the sound for the memory.  The computer 
memory, a Random-Access-Memory (RAM),
stores the music in samples taken by the ADC and its conditioning circuits.
The reading circuits reads from the RAM and sends one data word at a time to
the DAC and its conditioning circuits to the line level output.
It is possible to build analogue delays with op-amp and switched circuits,
or with monolithic Charge-coupled device delays, which move each charge of
electricity (each of which represents a sample, in which the charge
changes size with the voltage of the input) along a snake-like path
to the output.  CCDs are limited to about 1024 samples on a chip, and
are prone to noise of various types, so chaining chips to accumulate delay.  
is not desirable.  Digital (computer) memory-based
delays are not limited in size, only by money.  A 5-minute delay would
probably cost about $2000 in parts, for example.  A cheap 2 second
delay is now $240.  I know it's not proportional.  

I am designing a harmonizer.  CCDs can be used to make a harmonizer
(instantaneous universal transposer); digital memories can also.

Tom Janzen DEC 111 Locke Marlboro MA 01752