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Xenomai-enabled BeagleBone audio cape offers 1ms latency

Mar 15, 2016 — by Eric Brown 2,613 views

A Kickstarter funded “Bela” cape for the BeagleBone Black uses Xenomai Linux real-time extensions to provide analog and digital audio I/O with 1ms latency.

There are a variety of audio capes for the audio-deprived BeagleBone Black, such as Element14’s $56 BeagleBone Audio Cape. Yet, the $76 Bela is unique. Developed by the Augmented Instruments Lab of the Centre for Digital Music at Queen Mary University of London, the cape adds ultra low latency analog and digital audio, as well as digital I/O for sensors. The Bela system is designed primarily for digital musical instruments and interactive audio systems, including battery-powered and gesture-controlled devices.

Bela cape on BeagleBone Black
(click images to enlarge)

The Bela quickly moved past its modest, $6,937 Kickstarter goal cape to surpass $54,000, and the campaign is open until April 1. The Bela cape features 16-bit stereo audio I/O at 44.1kHz, as well as eight analog inputs and eight analog outputs, all at 22.05kHz. (The analog I/O is software configurable to provide 4x channels at 44.1kHz or 2x channels at 88.1kHz.) The cape also supplies two 1W 8-ohm speaker amplifiers and 16x digital I/O pins.

Bela Kickstarter options
(click image to enlarge)

The Bela starts at an early bird price of 55 Pounds ($76). The Bela Starter Kit, which includes a BeagleBone Black SBC, two audio adapter cables, and an 8GB microSD card with the Bela firmware, starts at 99 Pounds ($137). A 135 Pound ($187) Experimenter Kit adds breadboards, sensors, LEDs, and more. It ships in July instead of June for the other packages.

Two Bela detail views
(click images to enlarge)

By tapping the power of the programmable real-time unit (PRU) unit on the TI Sitara AM3358 SoC of the BeagleBone Black, and combining it with an optimized version of the Xenomai real-time framework for Linux, the Bela runs audio code in hard real-time, bypassing Linux to interact directly with the hardware. The Sitara’s PRU comprises dual 32-bit RISC microcontrollers that enable I/O customization. The PRU and Xenomai foundation enables 1-millisecond action-to-sound latency audio and sensor processing, claims the Augmented Instruments Lab.

The Bela’s buffer sizes can be as small as 2x samples, producing latency as low as 1-millisecond from audio in to audio out, or 100-microseconds from analog in to analog out, claims the Augmented Instruments Lab. Every analog and digital pin said to be is automatically sampled at audio rate, “providing precise, jitter-free alignment between audio and sensors.” The result is more “responsive and expressive” audio and more “natural, intuitive and immediate gestural control,” says the lab.

Bela conceptual diagram (left) and chart showing claimed latency vs. other platforms
(click images to enlarge)

A browser-based IDE with a “lightweight and simple Arduino-like API” and an integrated oscilloscope is available for fast startup. Developers can program the Bela with C++ or run PD patches compiled via Enzien Audio’s Heavy Audio Tools. The open source project has already posted software and hardware designs.


Further information

The Bela cape is available starting at 55 Pounds ($76) through Apr. 1, with shipments due in June. More information may be found at the Bela Kickstarter page as well as the Bela website.

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