Sample Rate Reducers & Unusual Ring Mods

***Originally these designs were based on the CD4016BE, but they're hard to come by and the Ebay ones are not totally reliable. Therefore I'd recommend swapping in the CD4066, which is basically the same - pinout and everything - only you can pick them up from most component outlets.***

Ever wondered what it would sound like to slice rythmic chunks out of your guitar signal at high speed? Turns out it makes a sample rate reducer, one of my favourite effects.

I knocked one up a while ago for an experiment using an unusual IC for pedal circuitry: the CD4016 voltage controlled switch. Upon further interrogation it transpired I'd basically recreated the Analog Bitcrusher by Colin Raffel, using a 4016 instead of the elusive J-FET.

(Digital downsampling is a different technique and won't be discussed here.)

Here's the schematic for the basic design, which developed into the Penny Arcade.

A basic square wave oscillator made with a 40106 chip feeds the 'enable' pin (13) of the 4016, turning on/off the connection between the guitar input and output (1 & 2). You could increase the pot value for more range, but it gets pretty murky down there and eventually just becomes a tremelo.

This is essentially how a lot of DIY sample rate reducer/aliaser pedals work (sometimes mislabelled "bitcrushers") - by cutting holes out of your signal at audible frequencies. Think of them as really fast tremolos or on/off gates, where the rate is so fast it becomes a note itself.

This is a screenshot from Audacity, playing around with the Penny Arcade. Note the regular "cuts" taken from the waveform as the oscillator opens and closes the 4016 switch.

The effect is similar to a ring mod - as the cuts introduce new notes into the signal - only less pronounced. These notes are the "sum" and "difference" of the note you play and the oscillator frequency. While these new notes make straightforward sense mathematically, they are musically very dissonant.

The alternative (more classic) sample rate reducer method uses a sample-and-hold chip such as the LF398. This is an Audacity screenshot using that chip, with a 40106 oscillator feeding the Logic Input and a guitar signal being modulated.

The oscillator frequency determines the rate at which the chip "samples and holds" the voltage of the guitar signal - keeping the voltage at a constant until the next duty cycle of the oscillator triggers a new sample. The two screenshots look fairly different, but the sound is very similar. The main sonic diiference being that the 4016 method allows some clean signal to come through, whereas the LF398 sounds very effected. The LF398 is also a fair bit more expensive.

Squeeeeel!!!

The major drawback with the above schematic is the carrier signal bleedthrough (the ever-present faint whining of the oscillator). Here's how to cut it out:

This is a simple half-wave rectifier - a kind of envelope follower that tracks the positive edge of the waveform. The capacitor stores current from your guitar signal, returning it to the signal path to smooth over the top edge of your otherwise jaggardy waveform. The resistor allows some of that extra current to flow to ground after the guitar signal stops.

The values of the cap and resistor need fine tuning as they work in tandem to produce the envelope. Increasing one value may mean you have to lower the other to find the sweet spot. The voltage/volume of your original signal also comes into play here.

Once we have a suitable value combo, we can send the half-wave rectifier output to the enable pin of a spare 4016 gate, so that the switch controls the oscillator. Now the carrier signal will only turn on when you play! This is a stripboard layout for the Simple Rate with the envelope follower included (the 1n5817, 1uf & 100k).

Now, if we swap around the oscillator and guitar input on the Simple Rate, so the frequency of your guitar takes cuts out of the oscillator at rapid speeds, we get a much more pronounced synthy ring mod effect, while keeping the fragmented fairydust timbre of a sample rate reducer. Here's how that might look:

The "trippy switch" takes out the capacitor in the half wave rectifier. In this set up the oscillator is turned on/off by severing it's feedback loop, which controls it's pitch. This creates the crazy glissando effects.

It's possible to have the Rong Mod and Simple Rate in one box as shown below, with a DPDT to switch between the two, but as with all designs featuring oscillators for control voltages, the limitations of stripboard mean we might still suffer some bleedthrough.

The Rong Mod uses the same method for the ring mod sound as the Ring Mod Radio, where the radio stands in for the oscillator. You could try it with absolutely any audio signal in place of the oscillator or guitar signal and see what comes out... Or have the guitar signal slice itself up for a smashed glass fuzz sound... Or even use the modulated output signal from the Rong Mod to modulate your original guitar signal. Things start to get a bit blurry at this stage.

Of course, you can build a more traditional sounding ring modulator with a single X-OR gate (there are four in a CD4070) and an oscillator (or any other eternal signal) but the oscillator will be constantly loud as hell without an envelope follower. An AND gate such as the 4081 also has interesting effects.

For clarity, all of these ring mod designs are technically "pseudo-ring modulators" as they do not contain a ring of diodes like the original ring mods of the sixties. However, many modern ring mods are built without the diode ring and I doubt you can hear the difference.

There's a whole bunch of ways to make a ring mod. I even weedled one out of an LM324 op amp, although it leaves a lot to be desired. I often come across them by accident. Happy hunting!