Wishful Coding

Didn't you ever wish your computer understood you?

The only good open source software is for software developers

The rest is all inferior clones of commercial software.

When I think of really high-quality open source software, 90% of it are compilers, databases and libraries. Tools for software developers by software developers. There are exceptions (Firefox comes to mind), but as they say, the exception proves the rule.

Outside commercial projects that happen to be open source (Android comes to mind), open source software is largely driven by a “scratch your own itch” mentality. However, this poses a problem when software developers don’t have the itch, and people with the itch are not software developers.

I have recently begun to see the world from the perspective of academia and electrical engineering, and it came as a bit of a shock to me how many of the tools that are in common use are bloated commercial Windows GUI software, compared to nimble open source command-line tools I was used to.

Many of them cost hundreds if not thousands of Euros, take up gigabytes of RAM and storage, are a pain to use, and are still the best or only option available. I can only imagine the horrors of working in a non-tech industry.

I don’t think there is an easy solution. If I’m solving a problem for someone else, I probably want to get paid. So it seems the only plausible model is commercial software that happens to be open source.

The other option is either teaching people with an itch to code, or make people who code have the itch. Broaden your interests, y’all!!! </rant>

Pepijn de Vos

LM13700: Voltage Controlled Everything

When making a modular synth, everything has to be voltage controlled. So how do you make a voltage controlled amplifier, a voltage controlled oscillator, or a voltage controlled filter? One way is with an operational transconductance amplifier.

The LM13700 is like a swiss army knife of voltage control. Its datasheet is completely packed with refference circuits for voltage controlled everything. To get familiar with its operation, I built a few of the circuits on breadboard.

Voltage controlled amplifier

Basically an OTA is like an opamp with current output, but it’s frequently used without feedback. To make the differential pair more linear, biasing diodes are used at the input. But the linear range is still limited to a few dozen millivolt. What makes it voltage controlled is that the current gain is controlled by IABC, which is the tail current of the differential pair.

For my test circuit I hooked the current gain up to a button with an RC network connected to it, so it does a nice attack and decay when pressed and released.

State variable filter

Then I fed the output of my VCA into this beautiful state variable filter. What is cool about state variable filters is that they can have low-pass, high-pass and band-pass outputs from the same signal. Each OTA basically forms a Gm-C filter. Put simply, a resistor’s current depends on the voltage you put on it, and so does the current of the OTA depend on its input voltage.

For the above video, I output white noise and a low-frequency sine from the MyDAQ. The white noise goes through the VCA controlled by my RC button envelope, and through the band-pass output of the state variable filter, controlled by the slow sine wave.

Pepijn de Vos

Microrack: A Small Modular Synthesizer

Inspired by the Modulin, I’ve been making my own synthesizer, starting with a Game Boy violin, adding pressure sensitivity, and adding analog delay.

Over the past weeks I’ve been thinking about how I want to connect everything together. I knew I wanted to make it modular, but also that it had to be small enough to become a portable instrument, and hopefully easy to prototype and not too expensive. So I came up with what I call Microrack, a compact mixed-signal bus that is electronically compatible with CV. I typed up a rough description here. In short, it uses a bus with analog multiplexers for audio, and an I2C bus for control signals.

I started by making the power supply and base board. Ideally you’d have something more efficient and powerful, but I started with a simple half-wave rectifier into linear regulators. The I2C lines are exposed to an external Arduino board that will control the user interface and the digital bus. Here is a rough schematic. One thing that is regrettably absent is any sort of current limit or fuse.

power supply schematic

Then I started working on the first module. I decided to start a little drum machine based on a noise source, a filter, and an envelope generator. The drum machine was mostly driven by the idea to make white noise in discrete logic. The heart of this module is a linear feedback shift register, implemented with two 74HC595 shift registers and a 4030 quad XOR gate.

linear feedback shift register

The shift clock of the registers is driven by an atmega328p. The output clock of the last shift register is driven by a NOT-wired XOR gate to close the feedback loop. The output clock of the first shift register is driven by the atmega at a lower rate, to sample the noise. The outputs of the first shift register are fed to a R-R2 resistor ladder.

resistor ladder

So by controlling the shift clock and the output clock, the bandwidth and randomness of the noise can be controlled. The DAC output is then fed into an opamp to translate from [0 5] V to [-5 +5] V, which is then output via the analog multiplexer. I’m pretty happy with the result.


Except then I fried the atmega.

Pepijn de Vos