Possible new way of clean pitch shifting??

[trad3mark]

Hello all,
One of the things i've always wanted was a really clean pitch shifter/harmonist type pedal. As we all know, sometimes, the likes of the POG or Whammy have issues with tracking the octaves. So i started looking at the theory behind pitch shifting.

Typically, for octave up, it's basically wave rectification, and for octave down, it's square wave > flip flop > LPF, or so i can find on the interwebs. Maybe there's a different way of looking at it.

Ok, so for the moment, lets look at shifting up an octave. Lets forget about the way it's typically done, and consider new options. What is actually happening? The frequency is doubled, tripled, quadrupled, etc etc. I then started looking at some circuits i've used involving doubling and tripling. Most commonly, with voltage. Obviously voltage and frequency are two very different things, HOWEVER, there are ways to convert voltage to frequency, and vice versa. Which leads me nicely onto the theory for a new type of pitch shifting.

What if you take the signal, and convert it into voltage. Then, the voltage could be multiplied by a number. For up one octave, lets say we use a voltage doubler. We then use a voltage to frequency converter, to convert it back into the required output. So, as a block diagram:

Input > Frequency to Voltage > Voltage Doubler > Voltage to Frequency > output

I'd love to hear of anyone elses input on this. I might give it a go tomorrow if i have the parts. I know i'll need a few 555's and i'm still looking about at V>F converters at the moment.

the game is afoot!
tm

[Ice-9]

Wouldn't you lose all the character from the sound of the string this way. A string has many different harmonics other than the fundematal pitch, It does sound an interesting way to approach this but i beleive you may just end up with a early monotone syth type of octave sound. 

[dschwartz]

sounds like a nice idea, but by "clean" you mean the original is unaltered and you add the octave? if so, maybe you have something there..

but the F2V-->Vbias-->V2F will definetly sound sinthy.

and remember that the Whammy is a digital device, based on algorithms..your idea could be neat for single notes, but it will never work as a whammy.

[slacker]

That's basically how analogue guitar synths work, except they often had special pickups that gave you 6 outputs, one for each string so you could play chords. You'd convert the pitch to a voltage then use the voltage to drive a voltage controlled oscillator (VCO) that will then give you a wave form, the pitch of which will trackthe guitar's frequency. If you multiplied the voltage in between these two stages that would give you your octave or any other division you wanted. Like Mick said it won't be clean though, the output will be what ever wave shape your VCO can produce, square, saw, sine or whatever.
The end result would be the same as the traditional way of doing it, only a lot more complicated.

[Hides-His-Eyes]

can you do it with like a pair pt2399 or a BBD or whatever-

have the delay chips set to exactly 2 repeats

one records while the other one is playing back

then the delay time is halved on the chip that was just recording while the other one records

with a flip flop setup so that only one of them is recording at once?

with that flipflop setup you could get anything from an octave down to an octave up depending on the secondary (flipflop) clock speed!

[Fender3D]

you may explore the Boss DF2 schematic, I guess it won't be a nice "clean" sound though...
If you change the input waveform, you'll have a hard time restoring it back

[earthtonesaudio]

This works provided you have a simple (i.e. pure sine or square wave) input waveform, and you do the whole operation separately for each note.

[R.G.]

Lets forget about the way it's typically done, and consider new options. ...Which leads me nicely onto the theory for a new type of pitch shifting.
...
the game is afoot!

Actually, that particular game has been afoot for some decades now. The synth guys used it way back when. 60s, at least, I'd guess.

The comments are correct. It works for single notes made of simple waveforms, and it loses the initial waveforms, subbing in the waveform of the oscillator in the V-F. That works OK for simple waveforms (or just filtered ones) where you don't care about the new holy grail of "transparency".

The need for a simple waveform for midi and guitar synth tracking is the raison d'etre for hex pickups, although they're useful for other things too.

You can make a simple F-V with the archaic, but useful National Semi LM2907/2917, and a simple but useful V-F from the venerable CD4046 and its successors.

Guess how I know these things... 

Don't get discouraged. We all learn best by thinking up anew things which are new to us. That way we not only have the result, but have and fully understand the process to get to the result. At some point you outrun what's already been done and invent something that no one has already done. This is how ALL invention is done. Keep thinking and keep inventing!

Did I ever mention I invented the flying lawnmower? 

[trad3mark]

hahaha! R.G, pics of the flying lawnmower or it didn't happen!

So if i was to go do it by this method, it'd probably be very synthy and odd sounding?

could be cool....

[earthtonesaudio]

You might check out the "Synthbox" thread.  There is heavy lowpass filtering of the guitar signal before it reaches the F-V converter, which could apply toward any similar device.

[R.G.]

hahaha! R.G, pics of the flying lawnmower or it didn't happen!

Ask and you shall receive: http://en.wikipedia.org/wiki/Flymo

Note that this is not MY flying lawn mower. I actually did drawings of this back when I first saw hovercraft in the early 60s. This was exactly what inspired Karl Dahlman to invent it. I was in grade school at the time, and had no possible way to know it was otherwise invented, no way to capitalize on it if I did. I note the date in wikipedia, 1964. I was fifth or sixth grade then, and that is when I remember drawing up the sketches; after seeing some pop magazines showing hovercraft. I can't prove the actual date, but I may or may not have beaten Dahlman to it.

This is completely aside from the flying lawn mower being a truly ugly, silly idea. Think about it. Try mowing on a slope. Or try mowing long grass that bends over under the pressure of the air being forced down. Or let go the handle and begin worrying about where it goes since it has no more friction than an air-hockey puck, as it begins spinning in counter rotation to the blade. As I said, I was very young.  

So if i was to go do it by this method, it'd probably be very synthy and odd sounding?
could be cool....

It could indeed. It's not bad or wrong, just different from what you thought. Good inventions are often like that.

Be aware that accuracy is going to be a problem. The frequency ratio between two semi-tones is the twelfth root of two: 1.059463094... and people with good hearing can hear differences of 1/100 to 2/100 of that in mid frequencies like a guitar. This interval is referred to as a "cent". This is a factor of 1.00057779... and is quite a small difference indeed, about 0.06%.

So you need to convert to voltage, then convert back to frequency with inaccuracies of no more than perhaps five cents (after all, it's only rock'n'roll!) or about 0.3% through the whole process. Get ready to do some majorly accurate analog work!

[Brymus]

Sorry to be OT here but my sophmore year of high school this idea came to me at a 4 way stop.
Magnetic propulsion and and magnetic hovering streets and freeways instead of using petrol and wheels.
The idea actually came in response to a question in science class, " How would you re-invent the wheel ? "
I toyed with the idea for years and even drew up plans for freeways that were banked like a U to ease congestion and allow faster travel speeds.
Then I discovered that in Europe or Japan ,I forget now ,they actually had magnetic floating trains that travelled on the reverse polarity magnetic track.Just like I had envisioned years earlier,so I dropped the idea as already done and in use.
I then moved on to my "Living Furniture" line of furniture incorporating fish tanks, terreriums and grow chambers for plants,these were designed to create small eco systems within are own living spaces,that could eventually be designed to sustain the dweller through fish and vegetable production.
I went so far as to draw up plans for an entire first line of furniture,including the elctro mechanical requirements,this some 20 years ago.
Yes I was a hippy/eco/activist in my younger years 

[Rectangular]

this reminded me that I still need to clone the pitch-to-voltage section of the korg MS-20.  still not sure how to handle the 5-gang pot problem, however.

[trad3mark]

hahaha r.g. i thought you were only messing when you said you invented the flying lawnmower. Seriously, wow!

back to the pitch shifter. The main reason i was doing this was a random pursuit of making a sort of home-made micropog. In terms of the octave down, there's a fair few decent ways of doing it, like a cross between the bluebox and the MXR Bass octave. However, the octave up.... i never came across a good way to do it. I tried things like that Octup! but i could never get it to shift. and as for things that use transformers, they're a bit too big, bulky and awkward for my liking.

HOWEVER, just 2 seconds ago, i came across another idea. probably overly optimistic, and totally whacky, but sometimes the best ideas need a little pinch of madness. I came across a schematic for a 555 based siren. A simple up/down siren. I wonder could it be adapted to have a guitar input, and make a sort of vibrato out of it?

seriously, sometimes i look at my ideas and think "Neil seriously. WTF?"

[R.G.]

this reminded me that I still need to clone the pitch-to-voltage section of the korg MS-20.  still not sure how to handle the 5-gang pot problem, however.

Digitally controlled resistors, possibly a switched capacitor or switched resistor version.

[trad3mark]

R.G. / others, do you think it's possible to adapt the below circuit to take guitar input and make a sort of "90-degree-vibrato" out of it?

[merlinb]

The main reason i was doing this was a random pursuit of making a sort of home-made micropog. In terms of the octave down, there's a fair few decent ways of doing it, like a cross between the bluebox and the MXR Bass octave.
However, the octave up.... i never came across a good way to do it. I tried things like that Octup! but i could never get it to shift. and as for things that use transformers, they're a bit too big, bulky and awkward for my liking.

I too am fascinated by pitch shifting, although I would say octave-up is easier to do than octave down. The best method I know of is to phase shift the audio signal by 90 degrees, then mix it back with the original. This gives you a new signal of twice the frequency, half the amplitude, from the trigonometric identity:
2sin(f)cos(f) = sin(2f)

I have a schem from an 80s electronics magazine (not handy right now) for a very effective clean octave up pedal, although it uses quite a few opamps to do the 90 degree phase shifting and also some compression to keep the signals of similar amplitude. I expect there is a simpler way to do the same thing, with a little inventiveness.

Recently I have become interested in octave down, which is much harder to do without tracking problems. I am currently building Anderton's Rocktave as an experiment, since I could find no sound clips of it. I also have some ideas for my own octave down circuit, which will have to wait until I've finished the Rocktave. However, from experimentation I have found that it is virtually impossible to get perfect, clean tracking using simple analog circuitry, because the second harmonic from the string changes phase relative to the fundamental, after you pluck the string, which always causes a momentary false track. Avoiding this requires some overly-complex circuitry, like frequency dependant filters.

(I must say I think the Bluebox is an awful sounding pedal. I mean, if you're willing to go with octave down fuzz instead of clean, then you should be able to get very good tracking without trying very hard, but the bluebox just sounds like random spluttering!)

Has anyone built the original Shocktave? http://www.diystompboxes.com/analogalchemy/sch/shocktave.html
I couldn't find any soundclips.

[trad3mark]

merlin if you get a chance at some stage, do you think you might be able to post that 80s octave up?

as for octave down, i keep meaning to try something using a flip flop, and then the LPF from the MXR Bass Octave. The schematic for the BO even points out where it softens up the wave!

[merlinb]

merlin if you get a chance at some stage, do you think you might be able to post that 80s octave up?

Yes next time I got home (where my magazines are stored) I will copy it.

as for octave down, i keep meaning to try something using a flip flop, and then the LPF from the MXR Bass Octave.

That's cheating! You're not supposed to turn the audio into a square wave then just filter it, I mean, anyone can do that!  

[~arph]

Well, the rocktave also makes a squarewave signal, the transistor arrangement at the end is basically a flipflop.
I"m sure you saw the synthbox thread.