Simple Drum Machine Interface for Serge and Eurorack

This video focuses on a little interface I have made that goes between an algorithmic drum machine made in Max and analog modular drum modules that use momentary trigger control signals of approximately 5 volts to trigger the analog sounds. I will show the contruction of the interface, the connections to the drum modules, and relevant parts of the Max patch.

The interface is a separate box, containing an Arduino Uno microcontroller, with the digital output pins connected to 3.5 mm jack sockets and banana sockets in parallel, so that I can use it for Eurorack and Serge modular simultaneously, if I want. On the Serge side, the earth also needs to be connected from the interface to the Serge with a banana cord.

Across each digital output I have put a 1K resistor, in series with a led to earth, to show the beats as they appear at the interface. There is NO other hardware in the box, and all the soldering of the led and resistor components are done straight to the back of the sockets.

Here I have the interface connected to two Erica Synths drum modules, each making two sounds, and a clap like sound made on the Serge by enveloping an oscillator. The oscillator is frequency modulated simulataneously by two types of noise, pink and Sample and Hold source. So five sounds in all, although the interface can trigger up to 8 sounds.

The interface receives the drum beats from the computer via USB using standard serial protocol inside Max. The drum beats are transferred all at once to the interface as a single number of type unsigned char, i.e. one byte, that has a range from 0 to 255. When one of the beats appears or disappeared, the number is simply updated to reflect the current situation.

The unsigned byte is constructed out of bits, and each bit represents one instrument of the rhythm track, for instance bit0 can be kick drum, bit1 snare, bit2 closed hihat, bit3 open hihat, and so on. Although it is completely free and up to me how I use them, since they are all functionally the same.

Let’s look inside the function that combines the bits. First, when the bit arrives from the algorithmic drum machine, it is turned into a one-shot that turns on the value 1 for a fixed duration, that is half the beat length in beats per minute. Then, all the 1’s and zero’s are combined in a pak (PAK), into a list of 8 digits. Then the final value, from zero to 255 is created in an regular expression using expr, where each bit is multiplied by a power of two according to its rank in the unsigned char. Finally, a change object makes sure that repeat values are not wastefully sent to the interface.

The single char is sent to the interface using serial protocol, at a baud rate about twice the normal for MIDI.