Bitbuf

The bitbuf is a portable 8-bit minimalistic live looping machine. Designed as a tool for chip music improvisation and live performances, it aims to provide a set of simple yet versatile features. It's a work in progress, but I've made a prototype which is already quite functional and enjoyable to work with.

The word bit in this context refers to a small fragment (a loop) of a piece of 8-bit music. Buf is computer programming jargon for a piece of memory where you store something for later retrieval. The basic job of this machine is to memorise a loop, and then recall it over and over again.

Presentation video

• lft_bitbuf_presentation (Video, 646.0 MB)

Related tools

Other portable minimalistic loop-based sequencers exist, such as LSDj for the gameboy, Korg DS-10 for the Nintendo DS, nanoloop and recently the DCM8. They have certainly been sources of inspiration. However, I find them to be designed in a way which, to me, is fundamentally unsatisfactory: First, neither of them appear to be useful for live looping, where loops are recorded as part of the performance. Second, they lack support for a number of techniques that are prevalent in chip music, such as the ability to change the current chord in a simple way.

Drawing on my own experiences of 8-bit music, I developed a new kind of sequencer interface to address the specific needs of chiptune improvisation (which I had to figure out as I went along). An overview of the interface is given in the presentation video above. My design goals were to create an interface which would be useful, logical and minimalistic, but not necessarily intuitive to a new user.

Technical details

The bitbuf is based on an 8-bit microcontroller: An ATmega88 with 1 kB of RAM and 8.5 kB of ROM, running at 20 MHz, programmed in C and assembly language. The tight hardware specifications aid in keeping the design minimalistic.

The sound output is provided as a 44.1 kHz mono line out signal, via an external DAC chip (MCP4921). The input is omni-mode MIDI, with support for a very limited set of continuous controllers.

The software architecture is pretty straight-forward: A 44.1 kHz timer triggers an interrupt handler which generates sound from eight oscillators and a current sample level. The worst case execution time of this handler is currently 355 cycles, which at 44.1 kHz corresponds to about 80% of the computational power of the ATmega88. Another timer, at 5 kHz, updates the sample level from a DPCM table (if a drum sample is active), scans the switch and LED matrix, and sets a flag when the playroutine is due. A UART interrupt writes received MIDI bytes into a circular buffer. Meanwhile, in the main context, the MIDI buffer and playroutine flag are polled, and acted upon. These actions include reading and modifying the loop data structure, allocating oscillators to the tracks and live notes, and updating the oscillator parameters.

Future

This is a prototype, and I'm eager to get some feedback. I don't have any long-term plans for the bitbuf yet, but please let me know what you think so far; general improvement suggestions, requests for specific features, opinions.