simple, clean-ish octave down

[oskar]

I use Ubuntu... Java fights me, so I am going off of Paul's screenshot.

I'm running mint and this simulator wouldn't work in Firefox, but it ran in Chromium.

[Earthscum]

A quad SPDT analog switch?  I've not seen one of those... ?
....

In the simulation I used 0-9V for the op-amp supply and the battery is 4.5V to bias the op-amp inputs at the midpoint.

In the schem you have 1 SPDT= half a quad analog switch (ie CD4066). Guess it's called "Quad Bilateral Switch".
...

Aah... I was guessing that, but wasn't quite sure if my assumption was correct.

The kewl thing about this is I was pondering using a Darlington phase splitter and fets (switch). I was going to use the second op amp as an integrator to feed another phase splitter to drive the fets. Similar outcomes, but I hadn't done much planning driving the F-F. This seems like a much simpler approach to what I was drawing up. I couldn't get rid of enough of the switching noise in "blue-box" style circuits without killing anything related to original signal, so was playing around with ideas.

pretty kewl.

[Gurner]

Hmmm... How's this?

Low-pass filter -> Peak Detector -> Attenuator -> Comparator
               |                                      ^
               |                                      |
               |--------------------------------------|

That would probably allow you to ignore most of the junk zero crossings by measuring crossings at a specific percentage of the wave height. You'd have to throw something in to prevent noise, but this is the rough idea. You could probably do this with a dual op amp. Disadvantage: the symmetry of output pulse would be different from the symmetry of the input. This means that it wouldn't work for Alex's idea.

That would work *if* we knew what the incoming fequency was meant to be, but it can be anything between 80Hz & 1500Hz.

Even if we try to slim the expected range down to one pickup per string...there's still a difficulty - the strongest harmonic in any plucked sting is  one octave up from the required fundamental....any such implementation to remove the harmonic by dynamic filtering would hit the headwind of not knowing whether the strong harmonic content of the note being played is from say open E or the E at the 12th fret.

Hi

There's nothing in my schematic to discourage the first harmonic from becoming a false trigger.  I'm not really sure what to do about that

An Auto Gain Control could remove the "fresh plucked" effect (ie compressing the dynamic range to half or quarter (log, dB) really diminishes the undertones and overtones).
cheers

This would have no impact...the pesky octave harmonic is there at source...and it crosses zero...by compressing the plucked string signal , it's still there, still crossing zero ....no winnage.

I've pondered this one for far too long...and with my limited IQ I can't see how the problem can be addressed in the much sought after analogue way ...FFT/MCU maybe, but using an opamp/comparator & a cap ....no.

[slacker]

It's not that hard to get perfectly usable results with filtering or other tricks like the harmonic rejection in the OC-2. If you want it perfect over the whole range especially as notes decay, then yeah it gets hard or even impossible.

Mr Giles will be along in a minute to extol the virtues of the EHX guitar synth

[CynicalMan]

Hmmm... How's this?

Low-pass filter -> Peak Detector -> Attenuator -> Comparator
               |                                      ^
               |                                      |
               |--------------------------------------|

That would probably allow you to ignore most of the junk zero crossings by measuring crossings at a specific percentage of the wave height. You'd have to throw something in to prevent noise, but this is the rough idea. You could probably do this with a dual op amp. Disadvantage: the symmetry of output pulse would be different from the symmetry of the input. This means that it wouldn't work for Alex's idea.

That would work *if* we knew what the incoming fequency was meant to be, but it can be anything between 80Hz & 1500Hz.

Even if we try to slim the expected range down to one pickup per string...there's still a difficulty - the strongest harmonic in any plucked sting is  one octave up from the required fundamental....any such implementation to remove the harmonic by dynamic filtering would hit the headwind of not knowing whether the strong harmonic content of the note being played is from say open E or the E at the 12th fret.

That's exactly the issue that this setup tries to combat. The low-pass filter that reduces the level of harmonics in relation to fundamentals. This reduces the amount of fundamental significantly if its frequency is relatively high, but the peak detector reacts to the varying wave sizes. Then the comparator compares the input with a percentage of the peak voltage, say 90% of the peak. With the filtering, the fundamental will hopefully be the only component of the waveform above 90% of the peak. With the peak detector, this setup is relatively immune to changes in frequency. There would be issues if the harmonics are very high or if the noise level is high enough to trigger the comparator, but these are issues with nearly all fundamental detectors.

[Gurner]

Aaah, I see - but where to set the lpf? (a guitar spans almost two decades in frequency content) ...with the LPF too low, the associated circuit will be in tatters with the guitar's high frequencies, with the LPF too high, the low notes are still gonna have too much harmonic content. Have you tried such an implementation?

[CynicalMan]

No, I haven't. It was just an idea that was floating around in my head. A 1-pole filter set at 160Hz would only divide by 10 at 1.5kHz, which might work, but it definitely should be tested.

[Mark Hammer]

Are any of you familiar with the dbx Boombox Subharmonic synthesizer? http://matrixsynth.blogspot.com/2010/06/dbx-boombox-subharmonic-synthesizer.html  I bought one of these 10 years ago.  And while I have to confess to not having used it, I did take it apart to see what was inside.  Naturally, it ultimately relies on a 4013.  But there is a whole lot more to signal preparation for listenable octave-division; particularly since this is supposed to provide subharmonics for multi-source signals, not just one stinking guitar.

Poke around and see if you can locate a schematic for it.  Maybe there are some pertinent secrets locked away in that unit, or its dbx successors.  Either that, or just buy one for $25 like I did and adapt it for guitar.

[CynicalMan]

OK, I've been spicing my idea, and I've found some interesting stuff:

Good:
Worked under certain circumstances with 2nd harmonics at twice the level of the fundamental.
Worked at 80Hz to 1.5kHz

Bad:
You might need a precision op amp or comparator
You have to switch the polarity of the peak detector based on the phase of the second harmonic
Might take tweaking based on the situation

I might find time to breadboard it soon, but if I don't, I'll put up a tentative schematic.


Here's the dbx schematic: http://i646.photobucket.com/albums/uu186/Justin-G-2009/dbx500.jpg

[slacker]

Sounds interesting Mark, never heard of that before. I found a schematic of a later version here http://www.dbxpro.com/product_downloads/Schematics/120XP%20Schematic.pdf the actual octave divider part looks similar to the OC-2, I'll have to try and figure out what all the pre processing stuff is doing.

Sounds good Alex look forward to seeing where it goes.