[sdiy] guitar synths was too quiet...

[David Moylan]

On 7/21/11 8:42 AM, Harry Bissell wrote:

> I have two options I'm using to convert from a filtered guitar fundamental to square (pulse) wave.
> One is a variant of the method used in the Gentle Electric P/V converter, only my method uses two
> opamps as comparators rather than the original method. The gist of this one is to use two peak
> tracking detectors, and compare the input signal against some percentage of the peak (maybe 90%), then
> use this to drive a schmitt trigger latch.  You set it high with (hopefully) the biggest peak in one
> polarity, reset with the biggest peak of the opposite polarity. The original works well as long as the input
> signal is strong, and stops working (rather cleanly btw) as the signal drops.  My variation tries to keep
> working right down into the soup...
>
Is this the same scheme used in the Boss OC-2 and most other Octave pedals?


> The other scheme is to use a single peak detector and a sample/hold (of sorts). I got this from Electronics
> magazine design idea (cir. 1980 or so) and this method is the basis for the Calman Gold patent (New England Digital).
> I've used this one in my present guitar synth.
>
> Opamp count is about the same (maybe 5 - 6 opamps).
>
> The freaky thing is that the simulation outputs for my improved Gentle Electric method and the 1980's method are
> virtually identical in output. The simulation traces lie almost on top of each other.
>
> The peak detecting method has the disadvantage of having some phase jitter as the input signal has multiple peaks
> on one side (or other) of the waveform.  The up-side is that this phase shift distortion is not as obnoxiuos to deal with
> as most other methods, which tend to 'octave-hop'.
>
> The methods can be used over a fairly wide range of input frequencies (unlike the Roland analog method, which can only deal with
> a single string).
>
> H^) harry
>

I've also been working on a guitar synth.  It's polyphonic, analog, and 
based on PLL and a CEM3394 per string.  Phase comparator is an RS flip 
flop.  It uses all the functions of the 3394 and has a suboctave that 
can be mixed in.

I'm building it into a pedal box, which is the largest cast aluminum Bud 
box I could find and it just fits.  Because of the space constraints I 
had to keep the input processing as simple as possible.  I'm using a 2 
pole low pass with the cutoff at the open string fundamental frequency 
and that goes directly into an opamp with trim-able positive feedback. 
It works surprisingly well.  The trimming is a trade off between sustain 
and suppression of those darn lumps in the waveform caused by the harmonics.

The other difference with most guitar synths I've seen is that I'm using 
triggered AD envelopes with the VCA switchable between envelope and 
gate.  The theory here is that the guitar string IS decaying and at some 
point will reach the threshold where it starts to sputter out, so might 
as well be turning down the volume or filter before that happens.  The 
gate also acts as a mute on the envelopes.  The trigger circuit is based 
on a simplified version of the one from Anderton's AMS-100.

Each voice is built on its own card and there are 6 trimmers per voice:

Trigger sensitivity
Sustain / Squelch
Gate off speed
PLL loop filter
VCF offset
VCO center frequency (this would be the first one I'd convert to a fixed 
value).

I've got E and A working and should have D finished tonight.  I'm pretty 
happy with what I have so far but still tweaking.  Some things I may try 
to implement:

simple compressor on the input since I already have a peak envelope
switchable octave up (just have to switch the phase comparator to use 
the suboctave signal)
putting flatwound strings on my guitar to reduce string noise

I'll hopefully get some info up on a web page once I'm happy with the 
results.

Dave