LFO – Low Frequency oscillator
An LFO looks a lot like a VCO, but the difference is that an LFO is slow. You can modulate the VCO’s pitch with an LFO, but if your VCO supports it, the LFO can also cycle through waveshapes, add or remove FM modulation from another VCO, etc. An LFO could also modulate the VCO’s volume, but most VCO’s don’t offer volume control because that’s the job of the Voltage Controlled Amplifier.
The ADSR Envelope can be used to control how the filter changes over time. ADSR stands for Attack Decay Sustain Release. Conventional synths have one or more ADSR sliders. Typically you’d want ADSR on your VCO (to make a sound snappy or slowly rising or falling), but the VCO just does BZZZ. Some VCO’s like the Mutable Instruments Plaits have a LEVEL CV connection so it can directly accept voltage from an ADSR and it saves a lot of patching. But normally, the ADSR is connected to the Amplifier, where the ADSR is going to play with the volume, panning or both. Second, you’d want an ADSR on the filter, so you can shape how its color evolves over time.
Eurorack has many ADSR solutions with full control over the four stages. For some it’s even possible to change it into two AD envelopes, or even four D envelopes. Sequencers often use short and snappy sounds so in most cases a bit of D is enough.
Complex LFO / Looping Envelope
In eurorack you can also use an ADSR envelope and loop it. A normal LFO provides a sine, block, sawtooth and random waveshape. A complex would give you full control of what happens within one loop cycle. This opens up many possibilities like lagging an event until you think it’s fun to fire it. So instead of ADSR you get a WADSR (wait). The Mutable Instruments Stages is a cool example of such functionality.
Make Noise Maths
And then there’s Maths. At first look, you’ll have no idea what it is and what it does. It is completely alien. Basically it’s an analog computer circuit with four lanes of movement. You can input both sound and control voltages. There are many outputs for the mathematical outcomes. It can provide basic LFO stuff, a full ADSR, two AD’s, it can create a sub bass from a sound, it can extract harmonics. Maths provides the slowest LFO on the market with a rise time of 25 minutes. But if you speed it up, it becomes audible sound – a VCO. It’s not your best partner for pitched sounds, but it can double as a double VCO. if you need some extra noise or beef. It can trigger other gear at the end of a stage, the list goes on and on and on. Maths should be in EVERY modular rig. You can save a lot of money by tying a few of those together.