[sdiy] Boss Octave - was Re: Frequency shifted from BBD?

[brianw]

One side effect of the Boss Octave technique is that it's monophonic. Whether you play a six-note chord or one note, the flip-flop is only going to cut the number of rising edges in half. It has no idea of the frequency, and certainly cannot unmix the various notes that are combined on a single guitar cable. When you do play a chord, it's basically a useless effect.

In contrast, the Roland GR-100 has a connector with each string on a different pin, so that sort of setup could potentially shift all six strings down an octave or two without making a mess of it.

Speaking of alternate products that use a CMOS flip-flop, the Syntaur 0705 is based on the same technique. It can add a sub-oscillator to each of the VCO signals in a two-VCO synth, or it can be wired to add two sub-oscillators to a single-VCO synth. The circuit is simpler than the Boss Octave - and more reliable - because you just hook up the square wave output of the VCO without significant processing.

Brian


On Oct 8, 2024, at 12:20 AM, brianw <brianw at audiobanshee.com> wrote:
> The Boss Octave does not actually shift pitch. The algorithm is completely different.
> 
> 
> The analog signal is boosted to the point of extreme clipping, making it effectively a square wave. There's some EQ ahead of this to favor the fundamental rather than the harmonics, but you've probably noticed that harmonics can still slip through. Once the input signal is clipped to a square wave, it is processed with CMOS gates to cut the frequency in half, and the again to one quarter. These represent one octave down and two octaves down. Each of those are square waves, too, so they're processed with a low-pass filter to remove (redundant) harmonics. Finally, the original input is mixed with the one-octave down and two-octave down signals. Voila!
> 
> The Boss Octave technique cannot produce any interval other than one octave down (and two octaves down by repeating part of the process).
> 
> FYI, the digital gate that can drop a square wave by one octave is very simple and cheap, and often there is more than one in the same chip. Thus, the two octaves down signal is practically free.
> 
> Brian
> 
> p.s. I made a decent recreation of this in software until I realized that Logic Studio Pro already has such a plugin that works better than mine.
> 
> 
> On Oct 7, 2024, at 9:49 AM, Didier Leplae <didierleplae at yahoo.com> wrote:
>> Thanks for all the replies to this question! It’s all very interesting. I used to have a Boss Octave pedal back in the 80s. I wonder if it also worked with BBDs.