Tuesday, March 24, 2009

Dual Edge CV Gate

Allow me to introduce a rather unusual device. I'm not sure if a proper name exists for this, but I have decided to call it a "Dual Edge CV Gate".


RNS File

Actually it's not really a gate - it's more like a switch. The device is designed to switch on or off depending on the input CV signal. When the switch is on, it will output a high CV signal (127); when off it will output a low CV signal (0) . There are two thresholds - lower and upper, such that if the input signal is between these thresholds, then the switch is on. If it is below the lower threshold, or above the upper threshold, then the switch is off. Figure 2 illustrates this relationship.

Figure 2

It would be fairly straightforward to extend this switch to control a gate which passes or inhibits the original input CV signal. This way you would get a 'piece' of the input CV signal for a particular domain, instead of a straight on/off value.

So how does it work?

Figure 3

Well, once again the Thor synthesiser is more than just an audio synth. Inside a Combinator and with some careful routing rules, Thor is able to implement two step functions. One of these step functions is flipped over. When combined with a logical "AND" operation, the result is a pulse, as illustrated by figure 3.

Figure 4

Figure 5

Figure 4 shows a simplified view of the cabling connections on the rear of the device and figure 5 shows the Thor configuration. The input CV signal is connected to the "CV Input" Spider CV Merger, which drives the Combi's Modulation Wheel input and in turn the Thor modulator wheel. Routing rules 1 to 4 combine this value (sum) with DC signals (via CV In1). However these DC values are scaled by the two Thor Rotaries, labelled "Lower Trigger" and "Upper Trigger". Combi knobs 1 & 2 adjust these rotaries, thereby allowing the user to adjust the DC level added to the Mod Wheel value. The output from these rules (CV Out1 & Out2) are connected to the Combi knobs 3 & 4. These are programmed to modulate the Thor Main Buttons. In a similar manner to my Noise Gate, these buttons only activate when the controlling knob reaches it's maximum value (127). Knob 3 activates Button 1 when the sum reaches 127, and knob 4 activates Button 2 when it reaches 0.

This provides two configurable thresholds - upper and lower. The difference between them is that one will activate when the input signal is above the lower threshold, the other activates when the signal is below the upper threshold. You can see the Thor Button1 & Button2 lights activate when the respective thresholds are crossed. You can adjust the thresholds by setting Combi knobs 1 & 2. Typically, you would set knob 1 to be less than knob 2, otherwise the gate will never open.

The long routing rule (CV In1 > CV Out3 / Button1 / Button2) implements an "AND" operation. CV In1 is a DC signal (value 127). This value is passed to CV Out3 when both buttons are on. If either or both are off, CV Out3 is driven to zero.

There's a bit of logic to implement the "invert" function when Button1 on the Combi is selected.

The CV outputs are taken from the "Gate Output" Spider Splitter. The SplitA output is the normal or inverted output, while the SplitB output is always the inverted output.

Figure 6

Figure 6 shows the gate activating when the Modulator Wheel on the Combi is set within the two thresholds. Both Thor Buttons are lit. Note the two DDL Delays in the CV Monitor Combi - as the Modulator Wheel is moved, the gate will open or close and the DDL displays will indicate this. A value of 127 on the left DDL shows that the gate is open (i.e. the switch is on). The right DDL shows the inverted output (switch off).

Figure 7

Figure 7 shows the input CV value set by the Modulator Wheel below the Lower Threshold. Only one Thor button is lit, because the input signal is below the Upper Threshold, and the gate is closed (switch is off). The left DDL shows zero. The right DDL shows the inverted output (switch on).

So what can this device be used for?

That is actually a really good question!

One idea is to use a CV input value from a Malström LFO. With several Dual Edge CV Gates, you could divide the LFO range into sections, where each section enables a particular instrument. In this example file, the LFO changes through several waveforms, having a distinct effect on the sound. Note that there is some overlap between each section, so that sometimes more than one SubTractor is audible at a time. It's a bit cheesy, but it works.

Example 1 RNS
Example 1 OggVorbis

Another use may be to select different instruments or effects depending on the velocity of a MIDI note, or to select different effects or signal paths in response to the amplitude envelope of another audio source such a drum loop.

Example 2 RNS
Example 2 OggVorbis

This device is really a solution looking for a problem - a building block towards larger, more complex configurations. So if you have any good ideas for creative uses of this device, I'd really love to hear about them.


  1. Great post. I currently have a situation where I want to switch between the greater of two CV streams. Basically, I have two CV streams as input, and the output would be the greater of the two at any given point in time. Could you use the Dual Edge CV Gate to implement this the logic to switch between the two streams?

  2. SnowCrash - good question. There are a few ways you could implement this 'max' function - one way would be to calculate the difference (using Thor to invert one CV and 'add' them together). If you then put this difference through the Thor Shaper in "hard clip" mode, maximum drive (see my Shaper article), then you'll get a high CV value if one CV is greater than the other, and a low CV value if vice-versa. Then you could use this CV Gate (or perhaps just a Thor route) to select one or the other. There's probably an easier way to do it though... :)