Tag Archives: oscillator

littleBits Control Signals

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I got the itch to experiment with analog audio processing and finally unpacked the old littleBits synth modules. Folks hack the Korg Monotron series, so why not hack littleBits modules instead? The modules are inexpensive when compared with Monotron and are easily reconfigurable while experimenting.

Since I last wrote about littleBits (circa 2017), Sphero purchased the littleBits company in 2019. Fortunately, they retained the littleBits forum.

Not so good, neither Sphero nor littleBits provide precise documentation about synth module functionality or the input and output signal characteristics. Precise information is needed especially when interfacing modules with the outside world including module synth gear. Timing information, in particular, is needed.

We do know a few things about littleBits, however. littleBits modules normalize input and output signals to a 0 to 5 Volt range. Both digital and analog signals are normalized. Normalization facilitates the plug-and-play module architecture and you can freely interchange analog for digital and vice verse.

Background

Before diving in, here is a little background information about the signal types and terminology commonly used in modular synthesis.

“Control voltage (CV)” is an analog signal which controls continuous functions like oscillator pitch generation, envelope and filter modulation, etc. CV sweeps continuously across an operational range, e.g., 0 to 5 Volts.

“Gate” is a digital signal. It is an ON/OFF signal. A keyboard, for example, asserts gate when a key is pressed and drops gate when the key is released. Gate indicates a condition, e.g., a key is pressed. The leading and trailing edge of the gate indicates a change in the condition.

“Trigger” is a digital signal similar to gate. However, trigger is usually a short digital pulse. Trigger is intended to indicate an event, like a clock tick, not just the presence or absence of a condition. Trigger signals often control synchronization.

Of course, electrons are electrons and one is free to combine CV, gate and tigger in any manner. Not all mixtures are meaningful (useful), however.

Details about CV, gate and trigger vary from manufacturer to manufacturer. Moog, for example, use the linear Volts per octave convention. On the other hand, old Yamaha and Korg synths use the Hertz per Volt convention. Maximum and minimum voltages level may differ by manufacturer and so on.

My goal here is understanding the convention used by littleBits.

littleBits MIDI and oscillator modules

I decided to start from the front of the synthesis signal chain and work back. The first stage in the synthesis chain is the littleBits MIDI module. A close look at the MIDI module signals in action should tell us how littleBits implement basic synthesizer control (CV, gate and trigger).

littleBits MIDI module

The MIDI module has a USB-B device port that presents itself to the USB-A host as a class-compliant MIDI device. The MIDI module supports both USB MIDI IN and USB MIDI OUT. However, the module operates in one mode (IN or OUT) at a time. The mode is selected by its mode switch (duh!).

  • IN mode: Receives MIDI messages from the host.
  • OUT mode: Sends MIDI messages to the host.

This blog post focuses on IN mode.

IN mode converts incoming MIDI note messages to two signals:

  • Gated control voltage (gated CV)
  • Trigger

Although littleBits call the digital output “Trigger,” it really is a gate signal, as we shall see.

littleBits Oscillator module

The littleBits Oscillator module is a pretty simple affair. The sole input is the (gated) control voltage which changes the pitch. The sole output is either square or saw wave as selected by the waveform switch.

The test rig

Here’s my test and measurement approach.

The littleBits signal chain consists of a power module connected to the MIDI module which drives a littleBits oscillator module. I split the gated CV signal sending it to both the oscillator and a proto module. The oscillator output is sent to a speaker module, giving me aural feedback. Hey, is this thing on?

MIDI module/Oscillator test circuit

The GND and gated CV signal are sent from the proto module to a Gabotronics Xminilab oscilloscope. I attached another proto module to the MIDI module “trigger out.” The GND and “trigger out” from that proto module are went to the second channel of the oscilloscope. Thus, I can monitor both the gated CV and “trigger out” and see the timing relationships between the signals.

SONAR/Oscilloscope test rig

The Xminilab front panel user interface is a little fiddly. So, I connected the oscilloscope to a PC running the Gabotronix oscilloscope application. This arrangement makes it sooooo much easier to configure the oscilloscope and to capture screen shots.

The USB MIDI comes from the PC, too. SONAR generates MIDI messages and sends them to the littleBits MIDI module. Test messages are produced from a repeating one measure loop (80 BPM or so). The repeating loop gives me good repeatability.

The signals under test

As noted by other experimenters, littleBits combine gate with CV functionality. When the MIDI module receives a note ON message, it:

  • Asserts the trigger signal, and
  • Drives the gated CV output with a positive voltage proportional to the MIDI note number.

The MIDI note range is C2 to C6 (4 octaves). MIDI note C2 generates a gate CV voltage of 0.2 Volts. From there, the output voltage increases by 1 Volt per octave (1V/oct). Each semi-tone step increases the voltage by 1/12 Volts. The module asserts trigger by raising its output voltage to 5V.

When the MIDI module receives the corresponding note OFF message, it:

  • Drops the trigger signal to 0V, and
  • Drops the gated CV output to 0V.

Notice that the gated CV output is asserted and dropped in parallel with the trigger output. Trigger is always driven to 5V while the gated CV voltage is positive and is proportional to the MIDI note number.

The screenshot below illustrates the operation of these two signals. The top trace (green) is the gate CV voltage. The bottom trace (red) is the trigger voltage. [Click images to enlarge.]

Gated CV (green/top trace) and “trigger” (red/bottom trace)

The MIDI test loop plays C2, C3, C4 and C5 in succession and repeats. The stair steps in the top trace show the effect of each successive note in the loop. The vertical display scale is 2.56V per grid division. You can see that each successive step is 1 Volt higher starting with C2 at 0.2V.

The trigger and gated CV traces are in temporal lock-stop, i.e., they rise and fall together. The width of the trigger signal is always the same width as the gated CV signal. Please recall that “trigger” in synth-speak is normally a fixed-width narrow pulse. That’s why I think the littleBits “trigger” signal is really a gate signal.

So, why do littleBits use a gated CV? Short answer: In conventional use cases, both gate and CV can be sent through a single wire (connection). The synthesist doesn’t have to route two separate wires (connections). The simplified wiring makes life easier for novice users (kids). The synthesist merely lines up a keyboard (sequencer, MIDI module) with an oscillator module and “it just works.” We will see other instances of the “It just works” philosophy in the envelope module, too.

C2, C3, etc. are MIDI note numbers and nice names. However, you’ll need to tune the Oscillator module to obtain the correct musical pitch.

Copyright © 2020 Paul J. Drongowski

Tuning up a littleBits oscillator

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I’m starting to experiment with the littleBits oscillator and synth bits. I’d like to control the oscillators using an Arturia Keystep either through MIDI or through the Keystep’s control voltage (CV) and gate outputs.

Perusing the littleBits forum, I noticed several complaints about how difficult the oscillators are to tune. Setting the basic pitch is not a problem — just turn the PITCH knob. The issue is really intonation. Guitarists are very familiar with intonation, that is, being in tune along the fretboard. The keyboard equivalent is being in tune across the entire keyboard or a given range of keys.

For my initial testing, I assembled a simple littleBits circuit:

Power -> MIDI module -> Oscillator -> Dimmer -> Synth Speaker

The littleBits MIDI module was connected to an Arturia Keystep and for comparison’s sake, a Korg MicroKORG. The MIDI module translates incoming MIDI note on and note off messages into littleBits-compatible CV. The Dimmer is the volume control. I find it much easier to set volume levels, etc. with a full-size pot instead of a trimmer. Also, I strongly recommend putting knobs on the oscillator PITCH pot. It is much easier to set the oscillator pitch accurately when a knob is installed. (Funny how the little things make a big difference.)

My first concern was the actual control voltage being sent to the oscillator. I pulled out my trusty multimeter and measured the Keystep’s pitch (CV) over a wide range of keys (notes). I also measured the CV generated by the littleBits w5 MIDI module. The voltages all look reasonable for 1 volt per octave CV, modulo the limited 3 digit precision of the multimeter. Two notes separated by an octave produced a one Volt difference as expected.

The Keystep generates CV from 0 Volts to 10 Volts. littleBits signals are limited to the range from 0 Volts to 5 Volts. Rather than tempt fate and drive the littleBits oscillator from the Keystep CV output, I decided to put a littleBits CV interface module on order. The CV interface scales CV to the littleBits range — whatever that means. Stay tuned.

The voltage for each note as produced by each CV source (Keystep CV vs. MIDI module) is not the same. For example, the Keystep generates 4.03 Volts for MIDI note C2 while the MIDI module generates 1.20 Volts. Relax. This isn’t a big deal as the basic pitch is easily set by the oscillator’s PITCH control. It’s all relative, man.

I used a Snark guitar tuner to set the pitch and to test intonation. The Snark is an inexpensive small tuner with a microphone input. I like the Snark because it shows the detected note (C, C#, D, etc.) and whether the note is flat or sharp. I don’t like the Snark because the clip-on thingy breaks off almost immediately! The clip-on thingy isn’t necessary for desktop testing, however.

I first tried setting the pitch and intonation by ear. This is where the MicroKORG was really handy. I sent the MicroKORG’s MIDI out to the MIDI-to-CV module and routed the MicroKORG’s audio output to the mixer feeding the studio monitors. I also patched the littleBits audio into the mixer, so I could easily dial up the MicroKorg audio as a pitch reference. Hit a single key on the MicroKORG and both the oscillator and MicroKORG attempt to play the same note. Adjust the PITCH and TUNE knobs appropriately.

The Snark tuner method is much better. The Snark’s display shows when things are sharp or flat. I recommend using a tuner like the Snark instead of matching pitch by ear.

At first I couldn’t get satisfactory intonation beyond a one octave range. This is disappointing and appears to be the place where most forum members gave up. I read the Korg littleBits Synth booklet paying particular attention to the tuning procedure on page 21. One of my biggest complaints about the littleBits system is the lack of detailed documentation and the Synth book was kind of sketchy about tuning and intonation. However, a light did turn on in my head.

I decided to turn the TUNE trimmer fully clockwise (thinking “OFF”). Then, I set the base pitch, i.e., the lowest pitch in the desired range. Next, I checked the highest note in the desired range — about two octaves. Natch, the intonation was bad, so I slowly turned the TUNE trimmer clockwise until the pitch of the highest note was correct. This disturbed the pitch of the lowest note and I had to find a balance between pitchiness at the low and high ends.

The result is fairly playable. I played the chromatic scale from lowest to highest and checked the Snark at each step. The intonation was acceptable.

To make sure this procedure wasn’t a fluke, I tuned the second oscillator using the same method and got the same result. Finally, I wired up the MIDI module driving both oscillators and mixed the audio:

                             |-> Osc ->|
Power -> MIDI -> Split ->|         |-> Mix -> Dimmer -> Speaker
                             |-> Osc ->|

The pitches were danged close and the intonation of the individual oscillators matched quite well. I just needed to adjust the relative tuning to get a good phat sound.

Not bad for “cheap” $16 USD oscillators! It’s rather unfair to blame the oscillator design at this price. Plus, you did say that you want analog synthesis, right? We’ve gotten spoiled by decades of pitch-perfect digital synths. Making the tuning and intonation right is all part of the analog game.

One final thought. The littleBits CV signal driving the oscillators is somewhat under-documented. Voltage Control Lab have a nice analysis. They explain the littleBits OUT signal from the Korg SQ-1. They say that the CV signal plays two roles as a combined gate and pitch control. When the CV is asserted, i.e., the gate is high, the voltage level sets the pitch.

Here’s a more MIDI-centric interpretation. When a MIDI note is off, the CV signal is 0 Volts. The oscillator is silent at 0 Volts. When a MIDI note is on and held, the CV signal sets the pitch. When the MIDI note goes off, the CV signal returns to 0 Volts and silences the oscillator. This appears to be the behavior of the MIDI module (when it is in MIDI IN mode).