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From: greg@utcsri.UUCP (Gregory Smith)
Newsgroups: net.music,net.music.synth
Subject: Re: Digital Reverb?
Message-ID: <2413@utcsri.UUCP>
Date: Thu, 27-Mar-86 15:02:45 EST
Article-I.D.: utcsri.2413
Posted: Thu Mar 27 15:02:45 1986
Date-Received: Thu, 27-Mar-86 15:39:44 EST
References: <3095@potomac.UUCP> <2920@sjuvax.UUCP> <858@tellab1.UUCP>
Reply-To: greg@utcsri.UUCP (Gregory Smith)
Organization: CSRI, University of Toronto
Lines: 48
Summary: Digital, Analog, Mechanical Reverb talked about

There are 3 basic types of reverb that I know of (1) mechanical (2) digital
(3) analogue
(1) Is the old spring-reverb that goes 'Pow' when you bump your amp - just
    a bunch of springs stretched between two bars with a tiny 'speaker' thing
    at one end and a mike-thing at the other. Different sized springs are
    used in the same unit to avoid strong resonances. There is also a 'plate'
    reverb - using a large metal plate instead of springs - that I have
    heard of. It apparently sounds much better but its size and cost limit
    its use to large studios.

(2)  Digital reverbs. As has been said, these 'sample' the incoming sound,
     store it in RAM, and haul it out later to be converted back to audio.
     To get a better reverb sound (rather than a clean echo ) some other
     things are done. First, the delayed sound is re-mixed with the incoming
     sound to get regeneration. Second, the delayed sound is actually
     'smeared' to get a less 'wired' sound. Suppose d(10) means 10ms-old
     sound. Then a 10-ms delay might actually be done by:

delayed signal = .1*d(9.5) + .2*d(9.8) + .4*d(10) + .2*d(10.2) + .1*d(10.5)

     I think more than 5 'taps' need to be used in practice, though.
     Thirdly, the regenerated ( fed-back ) sound can be filtered, so
     that the sound becomes 'duller' as it fades out ( or whatever effect
     you want). Of course, the 'smearing' is in effect a digital filter,
     as z-transform buffs will recognize.

(3)  Analogue reverbs use CCD's - charge-coupled devices, also known as
     'Bucket-Brigade' devices. This is an integrated circuit with a long
     line of 'charge buckets'. A clock signal causes each bucket to be
     emptied into the next, so that the charges in the buckets propogate
     along the line. The charges are created at the input end in such a
     way that the amount of charge in each created bucket is proportional
     to the input signal at the time of creation. This charge will later
     show up at the output end, and be transformed into a signal again
     ( sorry if this isn't very clear - I need pictures :-) ).
     Thus the signal is still *sampled*, but the samples are stored as
     charges instead of a memory word. The same things can be done here
     as with the digital delay - the output can be fed back, and the
     delay can be 'smeared' by using several taps near the end of the
     delay line and mixing them. This type of reverb is cheaper than an
     equivalent digital delay, but in general much less versatile. Also,
     charges will 'leak' as they pass along the line, resulting in a
     noise level much higher than that possible with a digital unit.

-- 
"No eternal reward will forgive us now for wasting the dawn" -J. Morrison
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Greg Smith     University of Toronto       ..!decvax!utzoo!utcsri!greg