The CGS747 is the third in the series of CGS drum simulators. It generates a single drum sound that can be adjusted to sound like a cymbal, hi-hat, snare drum, electronic drum, or numerous other percussive sounds.

It is a complete dedicated synthesizer in its own right, including six oscillators, a noise source, a mixer, an envelope generator, a VCF and a VCA. The CGS747 has 2x Noise sources:- a White Noise Generator and a 6x Oscillator Shimmer. The NOISE-SHIMMER control allows for a mix of both signals. The Shimmer section allows for a variable mix of the 6 oscillators creating a custom noise source for the CGS747.

The ENVELOPE GENERATOR

There are two inputs for controlling the envelope of the generated sound:-

1. [DECAY] - the enevlope attack is preset to the fastest rate possible, the [DECAY] controlling the fall-off rate of the envelope.
2. This decay is also affected by the [DAMPER] control, which operates in parallel with the [DECAY] while [SUSTAIN] is low.
   

A gate input into the [SUSTAIN] jack disables the [DAMPER], thus allowing two different rates of decay to be set depending on the input to the [SUSTAIN] jack. Note: if the damper decay rate is set to minimum, and the [SUSTAIN] input is below about 2V, the envelope will be set at minimum, with the decay control having very little effect, so if you do not wish to use the [SUSTAIN] input, the damper decay rate should be turned to maximum. The [TRIGGER] input needs to be a minimum of 2.5V and would normally peak at around 5V although signals up to 10V will work fine. Varying this signal between 2.5V and 3.5V will provide a small level of 'velocity' control. If a square wave or similar gate waveform is connected to both the [SUSTAIN] and [TRIGGER] inputs, the fall-off of the envelope will be affected by the [DECAY] control while the gate signal remains high, the damper kicking in as soon as the gate falls low. This allows for easy implementation of open/closed hi-hat sounds etc. The generated envelope is also passed through a processor that allows it to be scaled, and inverted, giving a range of positive and negative going envelopes to drive the VCF.

The SOUND SOURCE MIXER

The sound source for the CGS747 is created by a White Noise generator and a 6-oscillator shimmer generator. The [SHIMMER x] knobs control the rate or frequency of six simple square-wave oscillators which are ring-modulated together. This section actually comprises 2x 3-oscillator generators with the 3x [SHIMMER x] controls on the left controlling the first shimmer generator and the 3x [SHIMMER x] controls on the right controlling the second shimmer generator. The [NOISE-SHIMMER] control lets you adjust the blend of the 'sound source' used. Turned fully to the left and you get just the White Noise source while turned fully to the right you get just the SHIMMER noise source. In between these you get an appropriate blend of the 2 signals.A third signal is the [IMPACT] signal. This is simplay a filtered version of the [TRIGGER] pulse and is used to generate an impact sound. This is mixed in with the shimmer/noise section by the [IMPACT] control.

THE FILTER

The output from the mixer is then fed to the VCF-VCA sction of the module. The [VCA-VCF] switch lets the user determine the signal path through these two stages. If you select VCA-VCF, then when the VCF resonance is set just short of self oscillation, the VCF can be used as a drum sound.

Build Guide