kurz:
- tonal spielbar
- ein thru zero osc klingt anders, als ein frequenzmodulierter oszillator, der sich nur im positiven bereich modulieren lässt
aus der bedienungsanleitung für DCAM synth squad (dort mit bild):
"Thru-zero oscillator FM
The term ‘thru-zero FM’ refers to the ability to modulate the frequency of an oscillator beyond zero into negative frequency values. Negative frequencies are vitally important for a stable pitch response with any possible amount of modulation, and is a prominent characteristic of the Yamaha DX-series’ implementation of FM. Cypher’s implementation gives you digital FM-style sounds rendered by a realistic analogue circuit model, with the convenience and polyphony of a software instrument.
Thru-zero FM is not usually possible with analogue oscillators – instead of the frequency travelling beyond zero when modulated, it simply stops at zero until the modulation causes it to rise into positive values again. The result is that, at harmonic ratios, the overall pitch that is heard is irregular at lower frequencies. Thru-zero FM allows for the timbre to remain harmonic at all frequencies even with very large modulation.
The following waveform plots show thru-zero and normal analog FM using a square wave as the modulator, and a sawtooth wave as the signal being modulated. While the square wave is high, the pitch of the saw is increased, and when the square wave is low the pitch is decreased.
0.51.01.52.0TimemS0.40.20.20.4VoltageV0.51.01.52.0TimemS0.40.20.20.4VoltageVThru-zero FM:The decrease in pitch results in a negative frequency, and the saw-up waveform becomes a saw-down waveform. A triangle waveform corner is created at the point of transition.Normal analogue FM:The osc is unable to produce a negative frequency when the pitch decreases. It becomes clamped at zero and stays constant until the modulator increases the pitch. The result is that the resulting pitch is not the same as the original saw wave.
There are currently some thru-zero analogue oscillators available for modular systems, such as the Cyndustries Zeroscillator. However, these tend to be rather expensive (especially if you need a polyphonic system) and there are still inherent problems such as the inability to achieve exact tuning ratios.
Thru-zero FM is very easy to implement on digital synths that use wavetable-lookup techniques, such as the Yamaha DX series. Cypher’s oscillator FM is capable of thru-zero FM using true virtual oscillators generated in real time, without using wavetable-lookup techniques (which cannot offer the same sound quality as a modelled analogue-style oscillator)."
