vibrato

[WeAreAs1 at aol.com]

farky at ix.netcom.com wrote:

<< Does anybody know of a simple circuit to produce vibrato on an input
audio signal?  The vibrato I'm speaking of is the pitch-shift kind, not
to be confused with tremolo. >>

You can use an analog delay chip (BBD) and modulate its clock with an LFO.  
The unmodulated delay time needs to be set around a nominal 30ms or so.  
Anything less would sound like chorusing or flanging, anything much more will 
start to sound like a small slapback/doubling delay.  Unlike a chorus or 
flanger, you don't want to mix in any dry signal, just use the delayed 
signal.  This is how it was done in the now-extinct BOSS VB-2 Vibrato pedal.

As you may have read in these parts over the last few days, some of us don't 
really dig the sound quality of BBD chips (well actually, one of us - eh, 
Harry?).  The fidelity and bandwidth of your vibratoed signal will not be as 
good as the straight signal that you're feeding into the BBD.  It would 
probably be acceptable for guitar playing, but horrible for vocals or any 
acoustic sounds.  

The next step up in expense would be to use a digital delay and modulate its 
clock with an LFO (with the same basic delay time settings).  You can buy a 
digital delay on a chip from Holtek (look for the HT8955A "voice echo" at 
www.holtek.com) that will work for this, but you'll have to come up with some 
kind of external clock that can be LFO-modulated (the Holtek delay chips have 
their own internal clock, but it can be bypassed, I think).  Be forewarned, 
though, that the Holtek delay chips don't sound a whole lot better than most 
BBD chips.  The Holtek chips have 10-bit converter resolution, I think, so 
they don't have the sound quality that we have come to expect from most 
modern digital audio equipment.

One more consideration is delay sweep range.  The maximum sweep range of your 
clock-modulation circuit will determine how wide your vibrato can be.  As I 
remember, the BOSS analog vibrato pedal didn't have a huge, 
synth-vibrato-like sweep depth.  However, that was probably intentional on 
the part of Roland's designers (who are always trying to keep people from, 
uh, hurting themselves...) and it probably could have been easily modified to 
have a wider sweep range.  Some digital delays, hoever, have difficulty being 
clock-swept over very wide ranges.

Michael Bacich