Sunday, April 24, 2011

Review: Oakleysound EFG envelope follower

I recently purchased an Oakleysound EFG, pre-assembled from Krisp1 in the UK. (Oakley itself does not sell assembled modules.) This is a combination preamp, envelope follower, and gate generator. The module was on sale for (if I recall correctly) £110, which after exchange to American currency and shipping, worked out to a total cost of $195 US. Shipping time was about a week, and the module arrived well packed and protected.

This particular assembly is based on the issue (version) 4 board from Oakley. The one I bought is formatted as a 1U width MOTM-format module:

Oakleysound EFG, as assembled by Krisp1. Note that this panel layout is different from the 1U panel shown on the Oakley Web site.

What is an Envelope Follower?

So what is an envelope follower? Well, you know what an envelope generator is. It creates a signal whose (primary) purpose is to represent the volume of a sound over time; the signal is intended to be used as the control input to a VCA. An envelope follower, rather than creating an envelope from scratch, accepts an audio input and then outputs a control voltage that is proportional to the volume of the input audio at any given moment. The louder the volume of the input signal, the higher the control voltage output.

The Oakley EFG also contains a gate generator, that turns on when the input signal rises above a set level, and turns off when it drops below that level. For instance, as an alternative to having a VCA track the input level of your source, you could use an envelope generator connected to the VCA to re-shape the input signal's envelope. For this, you connect the EFG's gate generator output to the gate input of your envelope generator. The EFG's gate serves as an alternative to the normal gate signal that you would get from a keyboard connected to the synth.

EFG Features and Controls

An obvious application of an envelope follower is to make a VCA follow the envelope of some sound produced externally from the synth, such as the sound of a conventional instrument picked up from a microphone or electromagnetic pickup (e.g., electric guitar). These instruments and devices produce signal levels that are far lower than the signal levels used in a modular synth. Towards that end, the EFG incorporates a high-gain, high-impedance preamp, into which you can plug in a dynamic microphone, a self-powered condenser microphone, or an electric guitar/bass directly. The preamp has a very wide gain range, so that it can also accept line-level signals. It provides its output directly to the envelope follower's input; the preamp output is also available directly at a panel jack. A red "PEAK" LED on the panel indicates the onset of clipping in the preamp.

The envelope follower itself follows whatever signal is input to the preamp. A green "FOLLOW" LED on the panel lights up to indicate the control voltage being output; brighter indicates a higher voltage. This version of the EFG provides only one control for the envelope follower itself: a panel switch that selects slow or fast tracking. (Apparently the board can accommodate a pot that allows the envelope generator tracking speed to be varied.)

The gate generator outputs a gate signal that indicates the presence of a signal at the audio input. The gate is at 5V when a signal is present, and at ground when the signal is absent; this is pretty much the standard for modular gear. (It might or might not work with other synths; for instance, it won't work with Moog or Yamaha gear without conversion circuits.) The yellow "GATE" LED lights when the gate is active. The primary use of the gate is to control an envelope generator, in the same way that a keyboard gate does. You could then use the envelope generator to re-shape the audio signal. For instance, by setting your envelope to a fast attack, slow decay, and zero sustain level, you could make an instrument that sustains indefinitely (say, a saxophone or a violin) fade out like a piano.

The THRESHOLD control on the panel determines the level that the input signal has to reach in order to be considered "active". The RESPONSE control adds hysteresis to the threshold level; as you turn the control to the right, the turn-on level goes higher while the turn-off level goes lower. You can use this sort of like a noise gate. If your input signal is a source that fades out gradually, like a guitar or piano, you can set the control to a slow setting so that it the gate remains active for a time after the signal fades below the turn-on threshold. It helps prevent the gate from thrashing back and forth when the signal level is near the threshold. I found that when using an electric bass as the source, I had to set this to a high value to prevent the gate from chasing the waveform; in other words, turning on and off with each cycle of the waveform.

Using the EFG

I tried several things with the EFG. My first patch was to plug an electric bass into the EFG's audio input. I routed its audio output to an MOTM-190 VCA configured as a ring modulator, and fed the output of a VCO to the 190's other input. The output of the 190 went to a Dotcom Q109 VCA which served to actually control the level of the output signal.

I started by connecting the EFG's control voltage output to the Q109's control input. With this configuration, I could play the bass and have its output ring modulated against the VCO. The output from the Q109 tracked the envelope of the bass, so that notes played on the bass attacked and decayed the same as if I were playing directly through an amp. That worked well with the envelope follower tracking switch on SLOW; when I put it on FAST, the envelope generator chased the waveform somewhat. That was actually kind of interesting, but not what I was wanting at the moment.

Next, I plugged in an electric guitar. With the MOTM-190 still configured as a ring modulator, I unplugged the control voltage output from the Q109 VCA. I then plugged the EFG's gate output into a Q108 envelope generator, and plugged its output into the Q109. I was able to obtain steel-drum-like sounds by setting the envelope for minimum attack, short decay, and zero sustain. With a longer attack and high sustain level I could get things that sounded like radio interference, or detuned violins; it helps if you turn the RESPONSE control up so that the gate remains active long enough to reach the sustain phase. One thing that doesn't work is to set up for a long release and then mute notes on the guitar: no matter what you set the envelope generator release to, the VCA can't amplify a signal that isn't there! That's one thing you have to get used to when you use an instrument with an envelope follower.

I did find that it was rather difficult to set the THRESHOLD control to get the gate to behave exactly the way I wanted. With a moderate setting of the RESPONSE control, the threshold range between the point where the gate would go off too soon, and the point where it would stay on indefinitely due to noise picked up by the guitar, was pretty small -- between the 2 and 3 positions. Turning the gain up helps some, but then you get into clipping in the preamp section. If I were seriously going to use the guitar as an input to the EFG in a patch, I'd probably put a compressor in line ahead of the EFG.

Technical Details

The EFG circuitry consists of a single board, mounted at a right angle to the panel and secured to it via the three pots, in the style of most MOTM 1U width modules. The build quality is excellent; the soldering is very clean and the leads to the panel-mount components are nicely dressed. The board came equipped with the MOTM 4-pin power connector; there is also provision on the board to install a Dotcom power connector. A MOTM-style power cable, with latching connectors, was shipped with the module.

All ICs are socketed, for easy replacement if needed. The board has ferrite beads to clean up in the incoming power, and diodes to protect against the power cable being plugged in backwards. I couldn't figure out what brand the pots are, but the feel is smooth and I didn't notice any glitches when turning the knobs. The jacks are Switchcraft enclosed jacks, which are top-quality parts.

I did some tests to look at what kinds of voltages were coming out. For the envelope control voltage, with no input, the output is at zero volts, as you might suspect. I set up an input signal and varied the gain so that it was just below the point where the PEAK light comes on; at this level, the envelope follower outputs 6 volts. It will go higher, past 10V, if you let the input stage be overdriven, but of course at this level the preamp's audio out won't be very useable.

The gate outputs 7V when active, and I verified that it will gate an Encore Electronics universal event generator (a module that is known to be picky about its gates). With the gate THRSHOLD and RESPONSE controls set full counterclockwise, the gate threshold occurs at the point where the envelope follower is outputting about 1 volt.

The board has two unused features which could be exploited. There is a provision to add a lag control to the envelope follow control voltage. Also, by cutting a jumper, the preamp section can be separated from the input to the envelope follower and gate generator circuits, which would allow one to use the preamp for an instrument and then put its output through additional processing before inputting it to the follower.


To be honest, I had not previously considered buying any of the Oakley/Krisp1 modules solely because of the sucky (for Americans) dollar/pound exchange rate, which meant that a similar module from a North American source would nearly always cost significantly less. But when Krisp1 had their close-out sale on this module, I couldn't resist. I'm glad I made the purchase. This is going to be a very useful module, both for running conventional instruments through the modular, and for interfacing the modular to other synths. For instance, I could run the output of a polysynth into the EFG and use the control voltage output to control the cutoff of a VCF, adding an envelope-filter capability to the polysynth. Or, using the EFG in combination with a waveshaping circuit, I could make an approximation of a guitar synthesizer. My only comment is the small useable range of the gate THRESHOLD control. Other than that, it's a great module.

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