Pitch tracking and octave pedals

[dcjim]

I've been reading up on pitch to voltage conversion and found quite a lot of info but any approach seems pretty complex. Then I got to thinking about my Blue Box clone I built and recalled that the tracking is pretty glitchy but I realised that as it was glitchy then there is at least some kind of tracking going on. Do octave pedals make a pitch to voltage conversion or is something else going on? If they do then what's the theory behind it? Glitchy and monophonic is kind of Ok for my purposes - advantageous even. Looking at the Blue Box schematic it's certainly not as complex as any frequency to voltage convertor schematics I've seen. I do need some kind of frequency to voltage conversion however as I want to drive a VCO from a guitar signal. I'm not looking to get into a thread about pitch to CV as there's plenty out there on the web. I'm just curious what the Blue Box is up to and whether it's a workable solution.

[SISKO]

Octave down is preety easy. A square wave converter, a flip fliop and there you have it. *Clean* octave up is dificult. The tricky part is to keep the picht to cv to pitch part stable and in tune (once you had all the circuit calculated)

[Keppy]

Analogue octave pedals generally use a bistable multivibrator to do the octave down. Every time the input signal goes high (or low, depending on the design), the output signal changes state (low to high OR high to low). Thus, it takes two cycles of the input wave to produce one full cycle of the output wave. There's really no frequency detection, just zero-crossing (or other threshold detection).

EDIT: Yeah, like he said 

[Seljer]

Yep, other slightly cleaner monophonic octave down effects like the Mutron/EHX/Boss use the same principle but have more advanced filtering and edge detection which make the tracking a bit better and the output sound less like an angry robot.

Also the Boss also doesn't directly use the direct output of the flipflop as audio, but instead, it uses it to modulate the amplitude of the input signal so you get a cleaner output instead of big square waves


That said, I find my bluebox tracks suprisingly well on the neck pickup with the tone rolled off.

[Processaurus]

A great article from stompboxology on octave dividers, hosted by moosapotamus:

http://moosapotamus.net/files/stompboxology-freq-div-sub-synth.pdf

I personally would avoid pitch to voltage conversion unless you specifically wanted to control an analog synth.

[dcjim]

A great article from stompboxology on octave dividers, hosted by moosapotamus:

http://moosapotamus.net/files/stompboxology-freq-div-sub-synth.pdf

I personally would avoid pitch to voltage conversion unless you specifically wanted to control an analog synth.

Well it is for driving a synth, not analog but one using CMOS logic chips for tone generation. So ignoring octave pedals as it appears they don't do any pitch conversion I continued to explore and ... I seem to be getting mixed messages. On the one hand pitch to voltage seems to generate all sorts of horrendous complexity whereas frequency to voltage returns lots of, "oh yeah just use this chip or that chip, it's easy." Using a PLL IC seems a common solution.

So what gives? Is pitch to voltage a specifically difficult application of the general case of frequency to voltage rendered harder still by the fact a guitar signal is a complex waveform full of additional harmonics and what not?

[Seljer]

Precisely that. This is the bane of all guitar synth projects that want to use actual oscillators, its tough to do it without DSP. One issue is getting it to first convert the frequency propely, the next issue is getting the tracking to be acceptable so you can actually play it.

[dcjim]

Precisely that. This is the bane of all guitar synth projects that want to use actual oscillators, its tough to do it without DSP. One issue is getting it to first convert the frequency propely, the next issue is getting the tracking to be acceptable so you can actually play it.

Well it's a damn shame I tell you, a goddam shame! 100% accurate instantaneous polyphonic tracking of guitar to analog synth would be more fun than a barrel full of puppies on laughing gas.

Back to the real world

So you filter the guitar signal, turn it into a square wave, wait for the zero-crossing, sample and hold the signal to avoid the impending harmonics and convert that frequency to a voltage. That's about the long and the short of it isn't it? And that's just the pitch, as for gate ... hello envelope follower

[slacker]

In very general terms frequency to voltage is easy, where it gets hard very quickly is trying to do it so the voltage tracks the frequency accurately enough to do something musically useful with. In my experience it's much easier to convert the guitar signal into a relativly clean square wave, you can then divide or multiple this to get different octaves, you can also generate different waveshapes. So basically instead of using your guitar to control the pitch of a VCO you just make the guitar sound like a VCO. You can then apply whatever synth affects you want to it.
Search the forum for slacktave this is a little octave down pedal I "designed" that generates a square wave and tracks pretty well.

[dcjim]

In very general terms frequency to voltage is easy, where it gets hard very quickly is trying to do it so the voltage tracks the frequency accurately enough to do something musically useful with. In my experience it's much easier to convert the guitar signal into a relativly clean square wave, you can then divide or multiple this to get different octaves, you can also generate different waveshapes. So basically instead of using your guitar to control the pitch of a VCO you just make the guitar sound like a VCO. You can then apply whatever synth affects you want to it.
Search the forum for slacktave this is a little octave down pedal I "designed" that generates a square wave and tracks pretty well.

I'll check them out. I'm interested in the sounds you can create using CMOS logic as tone generators. There's a fair few people doing it. It's glitchy, angry robot territory so not everybody's cup of tea but I thought it could be an interesting new direction for guitar synthesis. But you're right of course, when you're talking about a musical instrument there has to be an accurate mapping of pitch between the fretboard and whatever else.

[Seljer]

Someone hear once proposed (made?) a kind of sitar-like sympathic string circuit by sending the audio through very narrow bandpass filters that triggered appropriately tuned oscillators.

[dcjim]

Someone hear once proposed (made?) a kind of sitar-like sympathic string circuit by sending the audio through very narrow bandpass filters that triggered appropriately tuned oscillators.

That sounds like the Jawari project which I think is on Tonepad.

[slacker]

...not everybody's cup of tea

Very much my cup of tea, I spent a lot of time hanging out over here http://electro-music.com/forum/index.php?f=160 a while back. CMOS noise makers are great fun.

[dcjim]

...not everybody's cup of tea

Very much my cup of tea, I spent a lot of time hanging out over here http://electro-music.com/forum/index.php?f=160 a while back. CMOS noise makers are great fun.

Yup there's some very cool stuff out there. Without offending anybody here guitar effects are ... well .. a bit conservative on the whole ... don't get me wrong, I love all the classics as much as anyone else and the classics are classics for good reason, namely because they're musically useful, but ... I still think the sound of an electric guitar could go way further than it has ... but, it's got to be musically meaningful. There's no point using a guitar to play CMOS synths if there's no musical relationship between finger position and pitch heard.

Maybe guitars and synths are just never destined to be together harmoniously. Many people believe the Roland GR300 remains the pinnacle of man's efforts in this regard. That was nearly 30 years ago!

[Jazznoise]

No relationship or no discernibly obvious relationship?

If you've ever played with an FM synth you'll know what I mean. Sometimes musicaly interesting sounds require a new means of interaction. MIDI Pickups are an example that would do the job fine, but just don't appeal to guitar players for some reason!

[Processaurus]

Korg made a semi usable (though always amusing) pitch to volts converter for the external instrument input of the MS-20, but it is made for driving a Hz/volt synth rather than octave/volt synthesizer.  The basic ideas are there, however: gain, a hi and lopass filter, a PLL circuit, something to scale the voltage so the polyphony is in tune (so an octave played is actually an octave on the synth), and something to shift the voltage to tune (maybe not necessary if all of the VCOs have tuning).  Also you need an envelope detector, and as part of that, a gate output with a sensitivity threshold control.