This post covers New Systems Instruments Inertia in High range, i.e. for use as an oscillator or filter. For audio oscillation, mode is usually set to Skew, which allows frequency and waveshape to be controlled separately. But I’ll show one example of using Rise/Fall mode for the VCO that let’s you modulate only the rise time while keeping the fall time at a constant, fast setting. (This technique is often used with slope generators operating as oscillators.)

In High range mode and Skew mode, set all pots to 12 o’clock except for Momentum. Turn Momentum fully clockwise to achieve self oscillation. The V/O input and the CV Frequency input can both be used for Frequency Modulation (FM). Without any external FM CV, centering the Frequency knob lands you around 500 Hz. I measured Order 1 out swinging -7.0V to +6.0V and Order 2 Out at -5.4V to +5V, lagging 45 degrees behind O-1. Both are approximating a good sine wave. Rotating Frequency fully CCW to full CW gives a sweep range from ~1Hz to ~25Khz. Leaving Frequency centered, a +5V into the Freq CV input, will sweep the same range using the attenuverter pot.

Standard Frequency Modulation

Wave Shape Modulation with Sine Wave

See how the rise time is fastest when modulating signal is lowest and vice-versa for fall time.

Momentum Modulation

We can modulate Momentum, even though it’s turned all the way up. This results in the oscillation actually stopping when the modulating CV is low enough.

Modulating Momentum Skew

Modulating Rise Time Only in Rise/Fall Mode

If we hold Fall time constant, we can do FM on the Rise Time only.

Syncing to a short pulse on the Input jack

Syncing to a short pulse on the Trig jack

This sounds much like normal sync.

Resonant Low Pass Filter

Filtering a ramp wave sent to Input, as described in Quick Start.

Low Pass Filtering in Rise/Fall Mode

We set Fall time high and manually sweep Rise time.