lp102918@sjuvax.UUCP (03/19/86)
...I know this is gonna sound like a stupid question,but hey!!!,
stupidity is a virtue.What is *digital* about digital delays??
Do they sample the delayed signal,or use microprocessors
to control feedback,or both??...
...also,why has mechanical (ie. spring-controlled) reverb died??
Was it particularly prone to failure? I have a Fender-Twin (circa.
'69) with mechanical that doesn't work,but when it did I thought
it was *fantastic*,super-genuine!!...
Larry Palena
PS- ...and what exactly *was* the story behind net.music.guitar??rsk@pucc-j (Wombat) (03/21/86)
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
kirsch@sjuvax.UUCP (03/21/86)
In article <2920@sjuvax.UUCP> lp102918@sjuvax.UUCP (Larry Palena) writes: > > ...I know this is gonna sound like a stupid question,but hey!!!, > stupidity is a virtue.What is *digital* about digital delays?? > Do they sample the delayed signal,or use microprocessors > to control feedback,or both??... > > ...also,why has mechanical (ie. spring-controlled) reverb died?? > Was it particularly prone to failure? I have a Fender-Twin (circa. > '69) with mechanical that doesn't work,but when it did I thought > it was *fantastic*,super-genuine!!... > > Larry Palena > > PS- ...and what exactly *was* the story behind net.music.guitar?? > Larry, I think the big advantage to the digital reverb is the cleaner sound. Some people don't like the extra bit of treble/tinniness that you get from the spring reverb. I guess it's the same as all other analog vs. digital arguments--with the digital you can get a cleaner sound--which really gets some people off-though others still like the analog sound. -- "Honesty Ain't All that Hard, Just Put Rambo Back Inside your Pants" -Jello Biafra Paul Kirsch St. Joseph's University Philadelphia, Pa { astrovax | allegra | bpa | burdvax } !sjuvax!kirsch
etan@tellab1.UUCP (Nate Stelton) (03/24/86)
In article <2920@sjuvax.UUCP> lp102918@sjuvax.UUCP (Larry Palena) writes: > ...I know this is gonna sound like a stupid question,but hey!!!, > stupidity is a virtue.What is *digital* about digital delays?? > Do they sample the delayed signal,or use microprocessors > to control feedback,or both??... I'm no expert, but my understanding of DDL's is that the sound is sampled much the same way that digital recorders, digital phone lines, and sampling keyboards perform this task. The information is stored in RAM, and released at the end of it's sample duration (set by the 'delay' knob). DDL's are functionally the same as an echoplex. I believe microprocessors are used to control the management of the memory in terms of how long to sample, when to write over what parts of memory, and more recently, extensive MIDI and user interface. > ...also,why has mechanical (ie. spring-controlled) reverb died?? > Was it particularly prone to failure? I have a Fender-Twin (circa. > '69) with mechanical that doesn't work,but when it did I thought > it was *fantastic*,super-genuine!!... I wouldn't say it's died yet, but eventually... The main problems with spring reverbs are the "boing" (try mixing drum tracks through one), and the physical sensitivity (don't bump it with the amp cranked, and don't drop it while moving the equipment). As far as mechanical reverbs go, plate reverbs out-perform the springs for sound quality, but they haven't figured out how to put them in guitar amps yet. I am curious about how digital reverbs work. how do they produce the smear? How many samples are taken, played back, and heard at a given point in time? Anyone care to expound? -etan
gibson@unc.UUCP (Bill Gibson) (03/24/86)
>>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
jer@peora.UUCP (J. Eric Roskos) (03/26/86)
> 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? Yes... there is a 1024-bit shift register, the SAD-1024, made specifically for this purpose. I remember it was used in two of the (much-maligned) PAIA analog synthesizer kits I built a few years back, before digital synthesizers became affordable. If I recall correctly, it's really two 512-bit shift registers in the same package, which you can chain together; this gives you access to the middle of the shift register. Some chorusing circuits I've seen work by slightly varying the rate at which the register is clocked, which cause slight phase and frequency shifts. I think Radio Shack used to carry the SAD-1024 when it first came out, but I think they quit carrying it after awhile. -- E. Roskos "It's Halley's comet!"
browne@savax.UUCP (Duff Browne) (03/27/86)
> > 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? > > Yes... there is a 1024-bit shift register, the SAD-1024, made specifically > for this purpose. I remember it was used in two of the (much-maligned) Reticon has stopped manufacture of the SAD series of bucket brigade lines. The latest issue of Electronic Musician Magazine has an article on this, and tells what the replacement parts are. (They are improved from the SAD-1024)
greg@utcsri.UUCP (Gregory Smith) (03/27/86)
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