How an Octave Works

[KerryF]

Hey.  I understand what it takes to build a distortion/overdrive (basically- input buffers/filters, clipping stages, output buffers).  I have designed a few (still need to test some), and I am now in the mood to design a Octave pedal.  Octave up, octave down, doesnt matter.

When I was learning about distortions/overdrives, I would look at a few schematics and circle similar parts and figure out what they were doing in the circuit.  Its a bit more confusing with Octaves though and I cant seem to figure out how they work.

If someone would post maybe a schematic and label the areas and what they are doing, that will help alot!  Also, if someone just posts a few links to the scheamtics for octaves, that will also help a lot.

Thanks,
Kerry

[R.G.]

How octaves work:

1. Octave Up: Full wave rectifiers. The signal is made into inverted and non-inverted versions, then rectified by either diodes or active diodes to fold both positive and negative halves to a signal direction. This is just like a power supply rectifier works, except that the signals are so small that the diode drops are significant, so you have to do something to keep the diode drops from eating all of the signal. The main ways of doing this are (a) prebiased diodes - Fox Tone Machine (b) opamp precision rectifier - John Hollis' omnidrive (c) diffamp and cancellation - Univox SuperFuzz (d) amplify until diodes don't matter as much - Tychobrahe Octavia. All of the commercial octave up pedals are done this way, except the DSP versions.
2. Analog Squaring. You can set up an analog multiplier to do the square function, multliplying the signal by itself. Since the square of a negative signal is a positive signal, all output signals are flipped to be positive. This one is tricky to get set up correctly. I only know of one commercial pedal that does this, and it's not a wide distribution pedal.
2. Octaves up and Down: Phase locked loops. The signal is cleaned up, removing harmonics that would interfere with lock, then run into a PLL with a divider network in the feedback. This forces the oscillator to run at a higher frequency to lock, so the VCO output is multiplied by the amount that the divider divides down. A divide by two in the feedback produce an octave up in the VCO output. The output can also be divided down to lower octaves. This technique can also multiply/divide to make harmony generation, of which octaves are a special case.
3. Octaves down: The signal is cleaned up a bit, made into a square wave, and then fed to the clock of a digital flipflop. This divides the frequency by two, giving an octave down. Examples are MXR Blue Box and Anderton's Roctave Divider.

[gez]

Another couple of methods:

Square off your signal, shift it 90 degrees and combine this with the original square wave using an EXOR gate.  You can use a simple RC combo to create the phase shift (one of Tim's circuits does this) but duty cycle will vary with frequency.

Once method I came up with (though I'm sure others have too - very little is new!) is to square off your signal (comparator), invert it, then use each output to charge/discharge two caps fed by constant current sources.  You end up with a parallel chain: tooth-space-tooth and space-tooth-space.  These can be mixed to give you a sawtooth at double frequency.  Amplitude varies with frequency.  If you use a sample and hold frequency to voltage converter, its output can be divided down and used as a reference for a comparator.  The other input of the comparator is fed with the sawtooth giving a square one octave higher than the original.  No latency, no tracking problem and 50:50 duty cycle.

I ballsed up the circuit and ended up with only enough range for a bass.   Had I used higher supply it would have worked fine for a guitar though...need to revisit that circuit one day (only breadboarded it).

[gez]

One more:

convert a square into a triangle and fullwave rectify to give you a triangle at double freqency.  Again, amplitude varies with frequency (you'll need to find a way of keeping it stable or convert the wave form into a square using a method similar to the one in my last post)

[Seljer]

could you do the squaring of the signal thing and get an octave up with a regular passive ring modulator setup (with transformers and diodes)?

[g3rmanium]

2. Analog Squaring. You can set up an analog multiplier to do the square function, multliplying the signal by itself. Since the square of a negative signal is a positive signal, all output signals are flipped to be positive. This one is tricky to get set up correctly. I only know of one commercial pedal that does this, and it's not a wide distribution pedal.

Which one is that?

[KerryF]

Im not talking about what its doing, but what the parts shown in the schematics are doing.  Theres a filter at the beginning.  Then theres.... what?  I want to understand how the schematic is made up, not what an Octave does.  Thanks.

[Seljer]

looking at the green ringer: http://www.generalguitargadgets.com/diagrams/gringer_sc.gif

going from left to right:
C1 - input coupling capacitor
R1 and R2 - bias for Q1
Q1 - amplifys the signal a bit (as it gets lost a bit in the next sections)
Q2 - phase splitter, splits the signals into 2 versions, one thats upside down (180° out of phase), this and the next part are where the magic happens
D1 and D2 - half wave rectifiers for each half of the signals (chop of the bottom half of the signal)....so I guess its a fullwave rectifier of sorts all together
Q3 - mixes that 2 parts of the signal back together (so in essence, the bottom half of the wave, gets flipped over the the top part)
i've got no idea what those resistors around the diodes are doing but thats the base of it

for a visual of that: http://www.geocities.com/gfr.geo/octave.html

the Octavia uses a transformer to do the flipping part of Q2 and then does same deal with the diodes



some more examples of octaves with that transformer:
http://www.geofex.com/Article_Folders/Screamer%20Octave/tsoctave.htm
http://www.home-wrecker.com/neoctavia.html

[KerryF]

Thanks Seljer.  Thats what I was looking for.  That helps a lot.  I was checking out this Octup schematic.  http://www.tonepad.com/getFile.asp?id=109 .  Can someone also describe how this one is working.  It looks a bit simpler so I will be able to get an idea of how the basic Octave Up pedal goes.

[Seljer]

Same deal with the phase splitter, only then into something with jfets which I can't be bothered to attempt to understand at the moment (as its 3am) 

[KerryF]

haha ok then get a good sleep Seljer.  what are you doing up this late?  Its only 9:20pm here when i made this post.