Using Node.js on a Raspberry Pi to listen to MIDI messages from an Avid S6L console to trigger HTTP requests or run scripts

Back in the summer, I posted about a project I had recently finished, which involved sending HTTP requests to a server that would then relay a MIDI output message based on the request that was sent.

We’ve been using that software (dubbed midi-relay) since then to be able to control our Chroma-Q Vista lighting desks remotely across vlans by using stream decks running Companion. It works pretty well, especially since the midi-relay software is configured to run directly on the lighting consoles upon startup. We have even set up a few crontab entries to send CURL commands to the light desks to turn them on at certain times when we don’t want to be on-site just to press a button.

In anticipation of completing my most recent project, “LiveCaption“, which takes audio and transcribes it to text in real-time, I started working on midi-relay 2.0: listening to MIDI input and using that to trigger a response or action.

I figured it was time this thing had a logo.

In both auditoriums at my church, we have Avid S6L audio consoles. These consoles can do a lot, and like most consoles, they have GPIO pinouts to allow you to trigger things remotely, whether as an action originating from the sound console, or externally that then triggers something on the console like recalling a snapshot, muting an input, etc.

Screen Shot 2019-11-19 at 4.23.54 PM
Stock photo of the console I found on the Internet.
These are (some of) the I/O pins on the S6L console. It has GPIO and MIDI ports. We use the footswitch input for setting tap tempo.

I started looking at the possibility of using the GPO pins on the console to trigger an external action like sending an HTTP request to Ross Dashboard, Companion, etc. However, there are only 8 GPO pins on this audio board, so I knew that could be a limiting factor down the road in terms of the number of possible triggers I could have.

The S6L also has MIDI In and Out, and through the Events section of the console, it can be used as either a trigger (MIDI In) or an action (MIDI Out) on just about anything.

Photo Nov 19, 1 28 22 PM
The Events page on an Avid S6L console. All kinds of things can be used as triggers and actions here! In this particular event, I’ve created a trigger that when the Snapshot “Band” is loaded, it sends MIDI Out on Channel 1 with Note 22 (A#0) at Velocity 100. MIDI-Relay then listens for that MIDI message and sends an HTTP POST request to the LiveCaption server to stop listening for caption audio.

We already have a snapshot that we load when we go to the sermon/message that mutes things, sets up aux sends, etc. and I wanted to be able to use that snapshot event to automatically start the captioning service via the REST API I had already built into LiveCaption.

In the previous version, midi-relay could only send Note On/Off messages and the custom MSC (MIDI Show Control) message type I had written just for controlling our Vista lighting consoles. With version 2.0, midi-relay can now send MIDI out of all of the channel voice MIDI message types:

  • Note On / Note Off
  • Polyphonic Aftertouch
  • Control Change
  • Program Change
  • Pitch Bend
  • Channel Pressure / Aftertouch

It can also send out:

  • MSC (MIDI Show Control), which is actually a type of SysEx message
  • Raw SysEx messages, formatted in either decimal or hexadecimal

And, midi-relay can now listen for all of those channel voice and SysEx messages and use it to trigger one of the following:

  • HTTP GET/POST (with JSON data if needed)
  • AppleScript (if running midi-relay on MacOS)
  • Shell Script (for all OS’s)

There are a few software and hardware products out there that can do similar things, like the BomeBox, but I wanted to build something less-expensive and something that could run on a Raspberry Pi, which is exactly how we’ve deployed midi-relay in this case.

Photo Nov 19, 1 27 32 PM
Here is the Raspberry Pi running midi-relay, connected to the MIDI ports on the S6L via a USB to MIDI interface. It tucks away nicely at the back of the desk.

Now we can easily and automatically trigger the caption service to start and stop listening just by running the snapshots on the audio console that we were already doing during that transition in the service. This makes it easier for our volunteers and they don’t really have to learn a new thing.

Here’s a video of it in action:



If you’d like to check out version 2.0 of midi-relay, you can download both the source code and binaries from GitHub:

The documentation is pretty thorough if you want to use the API to send relay messages or set up new triggers, but you can also use the new Settings page running on the server to do all that and more.

Screen Shot 2019-11-19 at 4.21.28 PM
From the Settings page, you can view available MIDI ports, add/delete Triggers, view detected midi-relay hosts running on the network, and send Relay messages to other hosts.

And if you’re a Companion user for your stream deck, I updated the module for Companion to support the new channel voice MIDI relay messages as well! You’ll need to download an early alpha release of Companion 2.0 to be able try that out. Search for “Tech Ministry MIDI Relay” in Companion.

Here’s a list of the Raspberry Pi parts I used, off Amazon:

Photo Nov 19, 3 52 56 PM

I hope this is helpful to you and your projects! If you need any help implementing along the way, or have ideas for improvement, don’t hesitate to reach out!


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