How does an LFO modulate audio?

[Transistor-Transistor]

So I'm still pretty new to audio electronics and I'm still learning about what affects what. The only pedals I've really built are dirt pedals and I want to learn how to build modulation. I was wondering if any of you could explain the circuitry behind an LFO and how it can affect pitch and/or volume in a circuit. Thanks in advance!

[Matthew Sanford]

Through resistance modulation generally, I believe.

Take Tom's filter, for instance. On the audio scheme you can see the LDRs (1/2 vactrol) in the filters, on the LFO scheme the LEDs (other 1/2 vactrol) that modulate them.

[PRR]

Through a "modulator". There are several kinds.

[ElectricDruid]

Besides "changing resistance" which Matthew mentioned and which covers things based on vactrols (LED/LDR) and things based on FETs, there's also "changing gain".
You can control gain using OTAs or VCAs, and you can use tha variable gain to control other parameters in a circuit. So you can build OTA-based phaser stages, or VCA-based filter stages, for example. Of course, the simplest example with those is to use them for direct volume control, in which case if you attach an LFO, you've got tremolo.

Ultimately what the "modulation" is is a particular parameter in the circuit that's being altered automatically. I think of it as adding a control pot, and having that control tweaked for you (by the LFO-elf?!). You *could* even just go as far as sticking a pot into a circuit, and then sticking an arduino-controlled servo on the pot to tweak it for you. It's expensive, bulky, and over-complicated, but it would work and it makes the concept very clear.

HTH

[GGBB]

explain the circuitry behind an LFO

Uncle Doug has a great explanation about how a classic tube LFO works: https://youtu.be/1kV2nz4-fXw?si=GhDB3f7F5hjjzk06&t=158. The design can be used with transistors as well. This is not the only way to build an LFO but the end result is the same - a low frequency AC voltage signal.

how it can affect pitch and/or volume in a circuit

The LFO signal then can be detected by other circuitry to vary some other property like resistance. For example, the voltage can be fed into an LED which causes it to pulse so that it can be detected by an LDR thereby modulating resistance. Or it can be fed into the gate of a FET to vary it's resistance. Resistance modulation is found in many tremolo and phaser circuits. Analog chorus circuits use the LFO signal to manipulate the timer chip that controls the BBD device which modulates the delay time producing a doppler effect variation of pitch.

[Mark Hammer]

All "modulation" is simply changing something about the sound in real time.  LFOs are certainly one way to cause change, but there are others as well.

An LFO provides what is described as "periodic" change.  It affects some other part or parts of the larger circuit in a predictable way, that requires a designated "period" of time.  Normally this is repeated on a continuous basis.

What is referred to as "aperiodic" change can come in different ways.  For instance, there can be a random source of modulation  A good example are "sample & hold" units.  These do not rely on an LFO directly, but rather take a snapshot of a continuously changing noise source, and apply whatever that random source dictates at that moment.  Another approach to aperiodic modulation is use of 2 or more independent unsynchronized LFOs.  Each individual LFO may potentially provide periodic predictable modulation, but when several of them are running at once, and not all at the same speed, the peaks and valleys can occur in unpredictable ways.  Maybe not truly random, but far from predictable and regularly repeated.

Whether periodic or aperiodic, the shape of the modulation can change the feel of the audible result.  Some pedals (e.g., the EHX Pulsar tremolo) provide for a control that can adjust the period from one which continuously descends from some peak, through to something that rises and falls, to something that ascends from some minimum to a peak and then drops.  Many players are fond of the Uni-Vibe.  One of the quirks of the Uni-Vibe is that its use of an incandescent lightbulb, shining on photocells, results in a particular modulation shape that gives a certain very distinctive feel.  One of the quirks of the EHX Small Stone phaser is that the shape of its LFO modulation is different (and perceived as optimal) for the faster and slower speed ranges.

There is nothing that precludes a source of modulation - whatever that might be - from affecting more than one parameter or characteristic of the sound.  Many pedals that have input jacks for expression pedals will allow one to assign whatever modulation source is fed in to whatever parameter or number of parameters one desires.  Traditional wah-wah pedals are essential modulated filters with the foot treadle hard-wired to provide "expression control" of the bandpass filter in the pedal.

Some modulation sources come in different forms.  I have a Source Audio Hot Hand 3 controller.  This uses the same gyroscopic detection that rotates the image on your phone to sense the movement of the sensor, and sits in a little "ring" you wear on you picking hand.  It can be set to use the two movement dimensions of your choosing to control something else that has an expression pedal jack.  When I learned that the expression pedal standard for Line 6 devices was simply a variable 0-10k resistance to ground, I realized I could do that with a photocell, and stuck a photocell on my guitar with double-sided tape in a location that I could cover and uncover with the pinky finger of my picking hand allowing me to "wiggle the pitch" of what I was playing, using my M5 pedal.

So, modulation is an almost infinite thing that can take whatever form one can dream up, that suits the feel and mood of what you're playing, and can make use of the equipment you have.