Investigations into what a BBD Chorus unit *really* does

Posted by & filed under BBDs+Delays, Chorus, Flanger, Stompbox stuff, Synth DIY.

I’ve been curious about Chorus for a while, since I’ve been working on and off with chorus design myself. There were a few things I didn’t understand, like what the relationshp is between the modulation LFO’s waveshape and the frequency modulation of signals going through the chorus. You’d think that if you use a sinewave to modulate the BBD clock, you’d get a sinewave modulation of frequency, right? Wrong! So what do you get? At this point, I realised that… Read more »

Fourier Analysis for non-mathematicians

Posted by & filed under Synth DIY.

Years ago, I discovered that it was possible to analyze a waveform and discover what harmonic components it was composed of, and that the magical technique to do this was called “Fourier Analysis”. I tried to find out all I could, but all the references I could find were aimed at university level mathematicians, and I was only 12 or 13. As far as I could see, none of the books explained the technique, rather they just splurged unhelpful equations… Read more »

Useful design equations for the PT2399

Posted by & filed under Stompbox stuff.

The PT2399 datasheet gives some data showing the relationship between the resistor on pin 6, the clock frequency, and the delay time. Unfortunately, it doesn’t offer any design equations relating these three variables. So, can we derive some? Yes, we can! They’re all included here. First, a quick summary of what I’ve learned, if you want to cut to the chase: Delay msecs = (11.46 * Resistance KΩ) + 29.70 Delay msecs = (683.21 / Clock Freq MHz) + 0.08… Read more »

Single VCA crossfader

Posted by & filed under Projects, Synth DIY, VCAs + Amplifiers.

There are a number of situations where it is useful to be able to crossfade between two different signals with a single control. Examples might be a wet/dry control on a delay effect, or a waveform knob on an oscillator that goes from ramp to square continuously. The simple and obvious way to do this is with a dual-gang pot. Each gang controls one signal, and it’s easy to wire so that as one increases in level, the other decreases,… Read more »

Voltage Controlled Looping Envelope Generator

Posted by & filed under Envelope Generators, Projects, Synth DIY.

This project is a development of my earlier VCADSR envelope generator. Whilst that chip had both GATE and TRIGGER inputs, this chip replaces the TRIGGER input with a MODE CV that allows selection of normal ADSR, Gated looping, or full LFO-style Looping modes. Standard ADSR mode generates the shape you expect, and allows CV control of envelope depth and overall time. Gated looping mode loops while the Gate is high. When the Gate goes low, the envelope goes to the… Read more »

Voltage-controlled digital oscillator VCDO1

Posted by & filed under Projects, Synth DIY.

A single chip 1V/Oct voltage controlled digital oscillator, with 20 waveforms in a scannable wavetable. The VCDO uses a PIC 16F1847 microprocessor to create a versatile and simple voltage-controlled 8-bit digital wavetable oscillator. All oscillator parameters are controlled by 0-5V control voltages. The chip has a native exponential control response, making interfacing with 1V/Oct control voltages very simple, even without an exponential convertor. Scanning through the wavetable with the Waveform knob. Sub Osc is a simple sine 2 octs down:… Read more »

Tap Tempo LFO (TAPLFO V2D)

Posted by & filed under Projects, Synth DIY.

This simple Tap Tempo LFO is based on the PIC 16F684. You can set the LFO frequency by tapping a tempo on a button connected to the chip, or by changing the Tempo CV. With this chip you can build tap tempo effects units or stompboxes, tap tempo controlled drum machines or tap tempo sequencer clocks. All of these are described in the datasheet below. The LFO can produce 8 waveforms, including a random (sample & hold) wave. All the… Read more »

Moog 911 ADSR Envelope Generator clone

Posted by & filed under Envelope Generators, Projects, Synth DIY.

A clone of a classic using my PIC code by J-P Desrochers Jean-Pierre Desrochers wanted to copy the response of the Moog 911 envelope generator using more modern technology. He asked me if it would be possible to tweak the response of my PIC envelope generator to do this, and I told him that it would, since the control mapping is stored in a table in the code. JP worked out the required table values and a front panel design…. Read more »

White Noise Source

Posted by & filed under Noise, Projects, Synth DIY.

A pure white audio noise generator using an 8 pin PIC 12F675 This pages describes a simple digital noise source that produces pure white noise over the whole audio band – out to more than 40Khz, in fact. With filtering, you can derive pink noise and other colours. The circuit diagram included below was developed from the famous Polyfusion noise generator, and can produce white, pink and infra-red (low frequency) noise. The noise generator 12F675 PIC microprocessor uses a dual… Read more »

Practical LFSR random number generators

Posted by & filed under Noise, Synth DIY.

The linear feedback shift register is one of the most useful techniques for generating psuedo-random numbers. I’ve used this method for creating noise generators and as an element in the random modulation generators I spent a long time developing for my Protowave synth. If you’re not really clear how an LFSR works, have a look at one of the many pages online (links below). This page isn’t here for that. In short, an LFSR takes a series of bits from… Read more »