Radio Modulation Octave pedal/ Detuner

[Astronaurt]

So I've heard about a technique for Frequency shifting an audio signal by means of AM modulation. I'd like to design some kind of Pedal that'll achieve something like the Morpheus Droptune, but I don't really know much about RF circuitry or theory at all. From what I've read the process is something like this:

1) modulate carrier + audio => IF (intermediate freq)
2) filter one IF sideband out
3) modulate that in a down conversion with a different carrier freq
4) from that, filter out one sideband for the new octave lower audio

SO my question is, what would the circuit for all of this look like? And what could the possibilities for this sort of system be? You could use a switch to toggle between different Down-Conversion carrier frequencies (or maybe even up-conversions ) in order to select how to down/up tune. Halving the frequency for an octave down would be the most straightforward, but choosing frequencies to detune in half-step increments would require only slightly more work. Before reading about this, I was convinced that a %100 analog Downtuning pedal would be completely impossible. But if this concept has some merit, maybe we'll finally be able to electronically downtune our guitars without the dreaded Tracking issues of Digital pedals hanging over our heads. Plus, building something like this would be awesome as all hell. 

I'm thinking the schematic would go something like this:

Input > Amplifier Stage > Amplitude modulation stage > Bandpass Filter > Switching between 2nd Carrier Frequency > 2nd Modulation stage > Bandpass Filter > Demodulation > Amplifier/Buffer > Output

I understand Amplifying and Filtering well enough, but RF modulation is where I'm gonna have trouble. Does anyone know much about this stuff? Everyone's Feedback and ideas are totally awesome!

[Taylor]

This type of modulation will not get you anything like a pitch shifter. The reason is that it will shift all frequencies by a static number of hz.

Why's that bad? Don't we want all notes shifted 3 semitones or whatever? Yes, but human hearing perceives pitch on a logarithmic rather than linear scale. So an octave difference in the lowest octave of guitar is 82hz, whereas an octave difference a few octaves up will be 656hz. If you shift all your audio by 82hz, you'll have one note that's really an octave, but everything else will be somewhere between slightly out of tune, and completely nonsensical new notes.

All this is simplified to ignore harmonics, but let's bring them into the discussion - let's take that one note that was truly shifted an octave. That note (your low E in this example) also has harmonics a fifth, octave, octave plus fifth, etc. up from 82hz. Since we recall that everything but that one notes will be shifted by an interval other than an octave, all of your harmonics are shifted by different intervals, which makes your note sound completely unlike a guitar. That can be cool, see ring modulation, but it's not what you're after.

In short, frequency modulation is a massively complex circuit which will not at all achieve what you're after. It can indeed be a cool effect, producing sounds like phasing and into ring modulation-ish territory. But it will absolutely not work like a digital pitch shifter.

Now, you can, at least theoretically, do polyphonic analog pitch shifting. But it's massively complicated. If you use advanced search to search my posts for "pitch" or similar, you'll find a few explanations of how I'd approach it. But to put it simply, this is about the most advanced effect you could possibly want to design - if that sounds like a fun challenge, then good luck to you, I'd love to contribute if I can, but keep in mind that this is not a weekend undertaking.  

[Astronaurt]

Oh dang! The way I understood what I read seemed like it would DIVIDE the frequency, overtones and all, by the ratio of the first carrier to the second. Oh wellll. I can still dream!  I've got three projects underway right now anyway, so I should probably wait before trying to tackle this concept anyway, hahaha! one of these days I'll do it!

[Taylor]

You can hear what frequency shifting sounds like on (bass) guitar here:

http://hallgeir.no/hmt/fs-1/