Zero Point Oscillator (ZPO)
The Zero Point Oscillator is not your typical analog VCO or even your standard Thru-Zero FM VCO for that matter. The core of the ZPO was developed through countless hours of experimentation involving Through-Zero modulation techniques.
Through-Zero FM is a form of frequency modulation that produces much deeper and useful FM tones than a typical VCO can accomplish. New techniques have been discovered that not only improve the performance of TZFM, but also grant additional benefits as will be explained.
The ZPO's modulation section is a hybrid involving both TZFM (frequency modulation) and TZAM (amplitude modulation). Coupling these two techniques substantially expands the performance and tonal shaping possibilities, beyond traditional TZFM. The results are more stable and balanced across the modulation bandwidth and both TZFM and TZAM can function together or independently, and anywhere in between.
How Thru-Zero (TZ) FM works on the ZPO:
While typical VCO frequency modulation works by increasing the oscillator frequency as the modulation source goes positive and decreasing the frequency while going negative, TZFM works by rectifying (ie flipping) the negative polarity portion of the modulation source into the positive region. You can read up on “Full Wave Rectification” to help visualize this concept. This keeps the modulation source voltage 100% positive and effectively doubles the rate of modulation.
To fully realize the TZFM sound, the carrier VCO must also reverse direction of oscillation to simulate the requirement of going 'backwards in time' – this is merely a mathematical conundrum as it is quite analogous to simply reversing the wave-form’s polarity.
There have been many techniques for accomplishing this aspect of TZFM, including various VCO core types, and direction-switching techniques.
The ZPO utilizes a new technique involving a tuned TZ Amplifier to accomplish this task. When applying audio rate modulation, the effect is indistinguishable from traditional TZFM with a number of added benefits, including but not limited to, more increased modulation stability, wider range of usable modulation rates, and a much broader range of tonal possibilities using the Zero Point Control.
When the TZ-MOD switch is in the active, LIN, EXP or ALL position, The applied modulation is routed to both the signal rectifier into the VCO core and the Zero Point TZ Amplifier.
The Zero Point control will then offset the balance of polarity reversal, producing an auditory change in the tonal balance of the TZ modulated waveform:
TZFM will be fully balanced when Zero Point Control is centered (ie Zero Point = 0 Volts) and will be offset as the ZP control position is altered. , The TZ Amplitude modulation depth will decrease in intensity as the control approaches the +/- extremes and cease altogether at those extremes.
The effect is dependent upon choice of modulation waveform, output waveform and depth (attenuation) of modulation, but overall similar to a combination of Phase Modulation and PWM and is essentially a built in Dynamic Depth Control.