One of the cleanest explanations I know of for through-zero FM is based on the analogy between an oscillator and a spinning wheel (which can be a very productive analogy — in the case of the gamechanger audio motor synth (https://gamechangeraudio.com/motor-synth-mkii/), it isn't even an analogy!).
An oscillator producing a sine wave at a fixed frequency is like a wheel spinning at a fixed rate. Frequency modulation means changing the speed at which the wheel spins. Send in a positive voltage, the wheel spins faster. Send a negative voltage*, it slows down. Normal (non-through-zero) oscillators are only able to spin in one direction, let's say clockwise. If you send in a large enough negative voltage, the wheel stops spinning. If you send an even larger negative voltage, nothing new happens; the wheel stays stuck. Through-zero FM means the wheel can spin in both directions: spinning backwards (here, counter-clockwise) corresponds to "negative frequencies."
With normal analog FM, the fact that the wheel can't spin backwards means something violent happens when the modulation signal passes through the critical negative value: the wheel gets stuck until the modulation signal goes above that value again. With TZFM, nothing drastic happens: as the modulation signal becomes more and more negative, the wheel smoothly slows down, stops, and starts spinning in the other direction. This is the smooth continuation that Yes Powder described, of modulation in TZFM as the depth is increased.
*All voltages are defined relative to some reference; in a modular synth, negative voltage means a voltage below the 0V reference used in that synth.
An oscillator producing a sine wave at a fixed frequency is like a wheel spinning at a fixed rate. Frequency modulation means changing the speed at which the wheel spins. Send in a positive voltage, the wheel spins faster. Send a negative voltage*, it slows down. Normal (non-through-zero) oscillators are only able to spin in one direction, let's say clockwise. If you send in a large enough negative voltage, the wheel stops spinning. If you send an even larger negative voltage, nothing new happens; the wheel stays stuck. Through-zero FM means the wheel can spin in both directions: spinning backwards (here, counter-clockwise) corresponds to "negative frequencies."
With normal analog FM, the fact that the wheel can't spin backwards means something violent happens when the modulation signal passes through the critical negative value: the wheel gets stuck until the modulation signal goes above that value again. With TZFM, nothing drastic happens: as the modulation signal becomes more and more negative, the wheel smoothly slows down, stops, and starts spinning in the other direction. This is the smooth continuation that Yes Powder described, of modulation in TZFM as the depth is increased.
*All voltages are defined relative to some reference; in a modular synth, negative voltage means a voltage below the 0V reference used in that synth.
Statistics: Posted by ari ellis — Fri Apr 19, 2024 8:04 pm