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|9/30/1997 12:51 PM|
|R.G.||Re: Revised BlueBox schem|
It's not really something that gets calculated. The way a blue box works is that the first opamp amplifies and filters the input, and has one output feeding the second opamp, and a second output feeding a rectifier/filter combination to make a replica of the loudness envelope of the note. The second opamp is set up as a schmit trigger to convert the signal into square waves. This square wave is in turn sent two places - first to a pair of transistors to provide the "dry" signal (more about this later) and through two flipflop dividers, the A and B side of the 4013 CMOS chip. The final flipflop output from pin 1 drives the base of a transistor. This transistor and the pair of transistors I mentioned earlier each have a resistor to the envelope waveform, and a couple of resistors that join up at the blend control. The effect is that both the square wave generated from the "dry" signal and the divided signal "chop" the envelope waveform, and these are combined in the blend control, which chooses the side derived from the dry signal or the divided signal, or a blend.
The note retains the loudness envelope of the original note because it is made by chopping up the original loudness envelope - neat trick! - and contains a selectable mix of distorted dry signal and twice divided signal.
Phew. This is getting long winded. Sorry. Just a bit more. My point was that in playing a blue box, I didn't like that the signal was twice divided for a two-octaves down signal. The opamps don't have much to do with the octave division, just the digital division by two in the flipflops. If you break the line from pin 1 of the 4013 that goes to the 1M driving a transistor base, and instead connect the resistor to pin 13, you get a one-octave-down bass note instead of a two octave down note.
I have a schematic of a project from a UK magazine that does a phase lock upwards harmony generator that operates on a similar principle.
|eth A phase lock upwards harmony genera... -- 9/30/1997 2:00 PM|