Rocktave Divider: how does it work?

[Paul Perry (Frostwave)]

I'm not asking for a schematic (anyway, one day I'll find where my copy of the book is :x ).
But, can anyone say in words what is special about the front end (wiht the squaring). Thanks!

[Paul Perry (Frostwave)]

:oops: never mind, the article is at Mark Hammer's site! (not that my machine here will download more than 29% of it, for some reason... definitely a problem at MY end, though!)

[toneman]

Well, most octave dividors dont use a compressor in the frontend.
this one does.
Supposed to reduce "sputtering" when the picked note starts loosing
volume.  There are mods 2 taylor it 2 the fret range U use most.
including bass guitar 2.
this would B fairly easy to modify 4 octave Up, that is octave
multiplying using quad nor gate.
u'd have 2 add another mixer stage and maybe a tone stack.
this product has been produced for over 20yrs.
PAiA *still* sells the kit, but the fancy painted box is not produced.
U could use T-shirt xfer to Re-Retro this little gem.
Staytuned
tone

[R.G.]

The Roctave does the usual for an octave down box: signal filtering to cut some of the harmonics that could confuse the divider, squaring up the signal and feeding it to a CD4013 dual flipflop where it's divided by two and four to get one and then two octaves down.

The only unusual thing about the Roctave is the use of the NE570 compander chip to get better tracking. The input is compressed for better results in squaring up, then fed to the divider. The second section of the 570 is set up as an expander, but it's fed from the envelope generated from the input compressor. That way, it impresses the input envelope on the fixed-volume output, resulting in a much more natural sound. The envelope fed to the expander is slightly lower than the one from the compressor so the bass signal decays a bit faster and cuts off any signal sputtering from the input signal dropping below the threshold of the squarer.

This is the best octave divider I've ever heard. For the life of me, I can't figure out why there aren't commercial knockoffs.

[Mark Hammer]

Almost as clever as the discrepant time constants of the compressor and expandor sections is the effect switching scheme.  The SPST switch essentially converts the second op-amp from an inverting gain stage with a gain of around 14 to a gain stage of about 0 simply by shunting the feedback resistor in the second stage.  Since the straight signal is tapped between the 1st and 2nd op-amps, all this cancels if the octave/fuzz sounds, leaving the straight signal intact.  In a sense, it is the analog of what happens with a great many commercial phaser, chorusses, delays, and flangers where the switching scheme simply kills or enables the wet signal with a single FET.  No FETs are used here, but the net outcome is the same.

I'm sure there are other effects categories/designs where this could work, although the requirements can be pretty specific.

[Paul Perry (Frostwave)]

The other crucial thing, is that the envelope (available from the first compressor section) is used as an input to the comparator that does the squaring. So that the threshold of the comparator is tracking the signal amplitude, as happened in %^&*rell's EH guitar synth. Pretty neat!

[Tim Escobedo]

This is the best octave divider I've ever heard. For the life of me, I can't figure out why there aren't commercial knockoffs.

I've wondered if the rastopdesigns octave divider was based on the design+mods.

[Paul Perry (Frostwave)]

http://www.rastopdesigns.com/effects.htm
thanks for the tips everyone, and thanks Tim for mentioning the rastopdesigns, plenty of inspiration at that site! pretty interesting reading. I hadn't come across them before.

[Hal]

ahhh logic confuses the crap out of me :-D

does anyone know where i can find a schematic/explanation, not for this effect, but anything based on a similar concept?  I wanna learn more about this stuff...

[Paul Perry (Frostwave)]

does anyone know where i can find a schematic/explanation, not for this effect, but anything based on a similar concept?  I wanna learn more about this stuff...

If you go to http://hammer.ampage.org/ and go on for four or five pages, you will find the original magazine article describing in excruciating detail the way the Rocktave operates. Thanks Mark H! (BTW Mark, if the RSI is only in your right hand, try mousing with the left. That's what I'm doing at the moment.. also, suprisingly, it makes a LOT of difference what chair I use).