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Octave for electric guitar

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fatboy

New Member
Help, my electronics knowledge is rusty (been about 15 years)...

Want to build a device for my son that takes his guitar signal and either doubles the frequency or divides it by 2 (to get upper and lower octaves of the notes played).

I originally thought of using a dual PLL but then I feared losing the dynamics..don't want a synthesizer sound.

Any suggestions on a cheap approach?


Greatly appreciate it!
 

Sceadwian

Banned
You can't just arbitrarily double or half the frequency on a musical instrument, the harmonics involved make it impossible. Sounds like what you're looking for is a 'pitch bender' Which shifts the entire audio spectrum up or down. You need a hardware synth or software for that.
 

fatboy

New Member
Keep in mind, I am an old fart now and am pulling from my memory of probably 18 years ago.


I made a device like this years ago... I used two 4046's and then ran the output to an OTA that referenced the original signal to put the dynamics back in. I think I then used a diff amp to pull out the octave. I remember it being a pain in the rear to get the reference signal to account for processing time of the OTA through each stage.

Can't find my old notes which really sux because I made a pcb layout when I designed the original - which worked pretty good btw.

Thanks,

JZ
 

Mikebits

Well-Known Member
Digitech makes a box that does this function plus much much more for under $200.
 

fatboy

New Member
Thanks for the replies guys! Let's assume I'm a glutton for punishment and want to build one anyway. BTW, I'm aware of what's out there commercially. My 12 yr old and I have been doing projects together (talk box, fuzz box, and overdrive). He is very much into guitar and I've been using the effects projects as a way for us to do something together and teach him a little electronics along the way.

That being said, what if I used a PLL and then converted the square wave output to a sinewave? I understand that I will for sure loose the attack dynamics, but shouldn't I be able to get a clean freq output that is an octave up or down from the source freq?

Maybe this one is over my head now. Been too long and I'm too rusty. Getting old and migrating from EE to account mgt does that .

Thanks for all your replies!

JZ
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Yes, you can get double (or half) the original SINGLE frequency in that way, and even use a VCA to give similar attack/decay characteristics as the original.

But it's complicated, expensive, and doesn't work terribly well.
 

transistance

New Member
That being said, what if I used a PLL and then converted the square wave output to a sinewave? I understand that I will for sure loose the attack dynamics, but shouldn't I be able to get a clean freq output that is an octave up or down from the source freq?
Yeah you can produce 2nd harmonic of your fundamental frequency but you can not reproduce the guitar sound like that. Your produced output will sound more like a siren as all sine waves do.

Are you looking for an octave shifter, wah or phase shifter? I had found a zip of all guitar effect schematics on the web once through some googling. You might come across it, too.
 

Mikebits

Well-Known Member
Thanks for the replies guys! Let's assume I'm a glutton for punishment and want to build one anyway. BTW, I'm aware of what's out there commercially. My 12 yr old and I have been doing projects together (talk box, fuzz box, and overdrive). He is very much into guitar and I've been using the effects projects as a way for us to do something together and teach him a little electronics along the way.

That being said, what if I used a PLL and then converted the square wave output to a sinewave? I understand that I will for sure loose the attack dynamics, but shouldn't I be able to get a clean freq output that is an octave up or down from the source freq?

Maybe this one is over my head now. Been too long and I'm too rusty. Getting old and migrating from EE to account mgt does that .

Thanks for all your replies!

JZ
Getting the PLL LPF setup up for proper attack and decay would be a big problem I think.
 

unclejed613

Well-Known Member
Most Helpful Member
actually, i designed an effects box once that does exactly what you're describing. it's a purely analog method of frequency doubling. you use an analog multiplier and an integrator circuit. the integrator circuit gives you a 90 degree phase shift. raw signal is applied to the X input of the multipler, phase shifted signal to the Y input, the output is a frequency doubled copy of the input.

getting a clean half frequency sound is a bit more difficult. you turn the input into a square wave, run it through a divide by 2 counter, filter the heck out of it to get it back to a somewhat sinusoidal wave shape again, and modulate the output with a VCA and envelope follower that follows the input envelope. or use a PLL that has a half frequency output, but you still need a VCA and envelope follower.

analog multipliers that are basically as easy to use as op amps are made by Analog Devices Analog Devices, Inc.: Converters Amplifiers Processors MEMS A/D Converters Analog to Digital Video Converter Temperature Sensors Analog Device RF Amplifiers Differential Amplifiers Digital Signal Processing Thermal Management D to A Converters Microc
the VCA can also be made with an analog multiplier.
 

Monkey

New Member
The esiaest way to double the frequency is to use...
a GRAETZ Rectifier without filtering capacitor!

Have fun!
 

unclejed613

Well-Known Member
Most Helpful Member
full wave rectification introduces a DC offset as well as distortion. using an analog multiplier without the 90 degree phase shift acts exactly like full wave rectification. (feeding both X and Y inputs acts as a squaring (mathematically, not analog clipping) function. when the wave is positive, the result (X*Y) is positive, and when the input goes negative, the output is also positive, since the square of a negative number is positive. with the 90 degree phase shift, you get frequency doubling without distortion or DC offset because twice per cycle, you have a positive number multiplied by a negative number, giving (with a sine wave, but works for complex waveforms as well) a distortion free frequency doubling. the box i built was so clean i actually had to switch it in and out of circuit while demonstrating it for anybody to know for sure it was working. the output of the multiplier should be followed by a differentiator to restore a flat frequency response (take an integrator circuit and swap the resistor and the cap), or if no lower frequencies are expected to be used, replace the integrator with a differentiator and you still get a 90 degree phase shift between the multiplier inputs, but the frequency response slopes up 3 db per octave instead of down.

the other low distortion method of shifting up or down an octave "back in the day" was the use of serial analog "bucket brigade" delay chips. instead of keeping a constant clock frequency, the clock rate was ramped up or down at a rate of change that either doubled or halved the number of samples exiting the delay device as went in. this is a complicated way of doing it, and actually requires two systems in tandem, with the control voltages to the two systems out of phase to eliminate the clicks and pops when the sawtooth control voltage retraces. the only noticeable noise and distortion in such a system are the delay artifacts that sound like springs going "sproinggg" in the background noise
 
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mfratus

New Member
Using a PLL to double frequency

if you run your square-waved guitar (clipped and amplified, or run through a schmidt trigger) to the INPUT of a 4046 PLL, then take the phase detector output to the VCO input via a loop filter, then take the VCO output to a divide-by-two IC (flip-flop), then to the REF input (back at the other input to the phase detector), the VCO output will be a square wave of exactly DOUBLE the frequency of you input.
Trouble is, it will sound like a synth or a fuzz box.
The 4046 has two phase detectors, one of which works better than the others.
The LOOP Filter is very important for locking onto your INPUT signal quickly enough to be useful.
You can use a divide-by-ten chip and get a 10x higher frequency if you want. The IC's VCO max frequency is the limit.
I made one of these, also long ago. Lots of fun, hard to adjust just right for any useful purpose, but not impossible.

Mike.
 

unclejed613

Well-Known Member
Most Helpful Member
you could also add an envelope follower so the box output is proportional to the guitar's amplitude envelope. and add enough filtering to get the square wave closer to sounding like a sine wave. that way you end up with a box output that isn't a constant amplitude square wave. the only problem with the PLL is, when you play more than one note at a time, it won't lock, or it will lock to a frequency at a sum or difference frequency, and sound completely off-key. that's why i came up with the multiplier with sine and cosine inputs. it completely follows the guitar's envelope, doesn't have a DC component at it's output, it's output is not distorted, and it's output has the same basic harmonic content as the input, but it's all shifted up an octave.
 

unclejed613

Well-Known Member
Most Helpful Member
if you want a really good way to do it, you could follow the route of most of the guitar amp manufacturers, and some of the effects manufacturers, and use a DSP chip. doubling the frequency would be easy. you sample the input, remove every other sample, and output at the same sample rate. to go down an octave you do the reverse, you sample the input, create duplicate copies of the samples and insert them into the output stream at the same sample rate. or just use the FFT (fast fourier transform) abilities of the chip to shift the frequency up or down an octave (or any amount you want, a fifth, a fourth, a minor third...). of course this is a complicated way of doing it, but DSP chips aren't expensive anymore (about $10-$20) and a DSP chip is basically an audio lab on a chip. you just have to learn how to program it. you could have one small box replace and mimic ANY effects box ever built. amp manufacturers use DSP to mimic classic amplifier sound signatures, and it works quite well (not perfect, but pretty darn close).
 
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