How do you use TZFM

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Foghorn
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How do you use TZFM

Post by Foghorn » Wed Sep 11, 2019 11:20 am

I have a Stillson Hammer and Du-Seq sequencers but they will not output a negative voltage.
Nor will Voltage Block.
When I use them with my Rubicon 1, I can not get down to 0 Hz to cross over zero.
I guess a negative voltage into my T43 or Doepfer Precision adder would do it.
Otherwise I am not sure how to cross 0 Hz.
I can use a bipolar LFO.
Or I think I can use the Keystep sequencer set on - X octaves.

How do you utilize TZFM ?

Foghorn
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.......Not really a musician.......

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starthief
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Post by starthief » Wed Sep 11, 2019 12:14 pm

TZFM isn't about negative voltages.

Let's say you have an oscillator running at 300 Hz. You feed its FM input a sine modulation signal -- frequency doesn't matter, but the amplitude affects how far the carrier is going to sweep.

For instance: let's say the amplitude is set so that the carrier sweeps from 200Hz to 400Hz. The average is still 300Hz, and all is well.

Increase the amplitude, and your carrier sweeps from 100 to 500 Hz.

Increase it again and it sweeps from 0Hz to 600Hz, still good.

Increase it again? Without TZFM, the carrier will "stall" at 0Hz when it's supposed to go down to -100, but it does get up to 700Hz. So the average is now 350Hz, and it's out of tune. There are also weird dead spots in your waveform which affect the timbre.

With TZFM, the carrier can run at -100Hz by reversing the phase, so it stays clean and in tune.


Oscillators without TZFM will often limit the amplitude to prevent stalling or going out of tune. But you're not going to get strong modulation out of them. Others, like the DPO, don't limit you, but they do freak out if you modulate them too hard.

TZFM isn't much of an issue with expo FM since amplitude changes already mess with your tuning. It also wasn't an issue for John Chowning or Yamaha with purely digital oscillators that used a ramp-shaped phasor into a sine lookup table.

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