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USB MIDI interface for the NeXTCube – ISPW board

USB MIDI interface for the NeXTCube – ISPW board

2023-05-16 18:02:14

I’ve just lately restored a NeXTCube with an ISPW Ariel soundcard with the intention to place it within the palms of artists and researchers within the context of a living electronic music instrument heritage project. To make the dice discuss to keyboards, synths or different audio workstations I’ve constructed a MIDI interface for the NeXTcube.

Fig: the NeXTCube with the Ariel ProPort and MIDI enter/output interface.

Not too long ago, I used to be in a position to restore a NeXTCube and set up an early model of MAX – a graphical music programming atmosphere. Nonetheless, an important a part of the system was lacking: there was no option to do MIDI input/output. MIDI is used to attach controllers, keyboards, synthesizers or different musical devices to the audio workstation. The NeXTCube itself has a serial port which allows users to connect MIDI devices. Subsequent to the serial port on the mainboard, the NeXTCube I’m working with additionally has a RS-422 serial port on the ISPW ‘soundcard’. The serial port makes use of RS-422 and mini DIN 8 connectors which give MIDI enter and output. Whereas the MIDI information bytes are transmitted based on spec, the connector and {the electrical} alerts will not be suitable with commonplace MIDI.

Fig: the IRCAM/Ariel ISPW soundcard with mini DIN-8 RS-433 serial port on the best.

For MIDI I/O we’d like a tool which permits to attach the RS-422 MIDI to each legacy MIDI gadgets and to computer systems by way of USB MIDI. If a MIDI occasion arrives from the NeXTCube’s RS-422 it must be handed via to the USB and legacy MIDI ports and the opposite method round. The Teensy platform is good: it helps {hardware} serial and USB MIDI. On this retro-computing mission, it appears wasteful to make use of the 600MHz Teensy 4.0 just for message passing: the Teensy has far more computing energy than NeXTcube however it’s low cost, straightforward to program, obtainable and sensible.

The RS-422 serial port makes use of -6V to 6V logic which must be remodeled to the 0V to three.3V logic for the Teensy microcontroller. A PCB offers this functionality and is related to a {hardware} serial port of the Teensy. The pinout of the RS-422 port was measured by way of a scope and matched the documentation. The Teensy has an usbMIDI mode and may current itself as a typical MIDI machine to a PC. Two opto-isolated legacy MIDI DIN-5 ports have been related to a different {hardware} serial port. The software program on the Teensy conducts the three-way MIDI message passing.

Vid: Max/FTS FM synth reacting to USB MIDI enter.

The electronics have been mounted right into a reused metallic enclosure. The entrance panel of the enclosure was changed by a customized 3D printed panel. The entrance accommodates the RS-422 port, two MIDI DIN 5 ports and a micro usb port both for energy alone or MIDI messages and energy. Be happy to take a look at the OpenSCAD design with a level MINI DIN8 hole.

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With a working MIDI interface for the NeXTcube permits interfacing with MIDI keyboards and controllers. It can be used to measure roundtrip latency. MIDI to sound latency determines how lengthy it takes between urgent a MIDI key and listening to sound. MIDI to MIDI roundtrip latency determines how lengthy it takes to course of, parse and return a MIDI message. For a responsive, dependable system each forms of latencies must be fixed and ideally within the vary of 10ms or under.

Fig: Measured MIDI roundtrip latency on the ISPW board for the NeXTCube.

Measuring the MIDI roundtrip latency exhibits that the system is ready to reply in 3.6+-0.4 ms (N=300). A mix of a MAX patch and Teensy firmware was used to measure this routinely. The MIDI-to-audio latency was measured a couple of instances manually and at all times was round 13ms. These figures present that the system is good for low-latency real-time music making in its default configuration. In MAX the audio buffer sizes could possibly be decreased to attain a fair decrease latency however with the danger of working into buffer underruns and audio glitches.

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