Intermission Sound Control with Raspberry Pi

A few times a year I volunteer to an arts organization and serve as their “house sound and light guy.” I usually run sound and do the lights for the traveling acts if they don’t have a tech with them. The auditorium is in a rural Minnesota area, but even so, we try to host a professional quality event for our concert goers and guest artists.

Small details such as pre-show and intermission music in the lobby area before the show make a big difference. Often I toss a CD of royalty-free light music into my ‘go bag’ before I leave home – to play through the house and lobby speaker systems. Sometimes I put on my “radio voice” and do the lobby announcements.

Getting the right blend of background music, voice-over, and a pleasing overall level can be tricky. There is an existing “Intermission Signal Bell” built into the lobby amplifier system…. but no ones uses it because it sounds like something out of the stone age.

Intermission Sound Control with Raspberry PiI’d often thought: “how could I automate this task and make it stupid-crazy simple?” Wouldn’t it be nice to make it easy and available so that ANYONE could do it and get consistently good results? That way, any teacher or student at the school auditorium could just press a button and have prelude or intermission music start up in the lobby for their concert or program.

So that was the beginning of the need – what follows is my solution.

The Pi

When I started experimenting with the Raspberry Pi (rPi), I realized that it would be the perfect little device to automate my music and announcement task. Granted, I maybe could have made it work using a less expensive Arduino, but for the cost difference, I’m not sure that the technical “hoops” would have would have been worth it. Some of the inspiration for the device I had in mind came from the Richardson and Wallace article “Simple Soundboard” in issue 33 of Make: magazine. There are some key differences however.

The Raspberry Pi lent itself very well to all of the requirements I had in mind: 1) I could store the playback audio files on the SD Card file system, 2) I could use the GPIO pins for both the trigger switches and for a front panel “ready” light, and 3) the rPi could easily play back audio files through its audio out jack. All I had to do was put the pieces together into a simple and rugged case.

The Pieces

There are five main components that comprise my Pi intermission/announce unit listed on the right of the page. The total investment is probably in the $130 range. Although many of the parts were dug up and found rattling around in my parts/junk bins and scavenged off eBay.

Construction

I wanted the device to mount unassumingly into the rack that all the other auditorium hardware was in, so that meant I needed a rack mount case. I was surprised to find that rack mount project enclosures were really expensive no matter where I looked, even on my favorite bargain shopping hangout: eBay. It was far more economical to just buy some sort of rack mount JUNK that didn’t work, rip the guts out, and then recycle and use the rack case. For this project, the case from on old “Alesis MIDIVerb II” worked great. After removing all of the original buttons and switches – I glued a strip of aluminum sheet metal over the front to cover up the existing holes and mount the new buttons onto.

In the Case

I also didn’t want a “wall wart” or any sort of external power supply box, so I decided to look for power options that would allow me to mount a standard C14 power socket into the back of the rack case and feed the AC mains from that into switching power supply modules which would then feed the devices. The AC power supply modules were purchased on eBay. They are pretty simple: 120V input and 5V and 24V outputs each.

Intermission Sound Control with Raspberry PiI picked up a few RDL Audio Distribution Amp modules on an eBay auction for next to nothing. I see now that they are rather expensive if purchased retail (around $130 each), so all I can suggest is to keep your eyes open for eBay bargains on those! You can still snag an STA-DA3 for around $30 each used on eBay. I used simple Plumber’s Strap to fasten it into the rack case.

I had some trouble with double triggering with the announcer button and so I put a 0.1uF capacitor across the switch terminals. With that and the debounce function of the GPIO event detection routine, the problem went away. Also, the ground of the LED resistor is tied to pin 14 on the Pi header which is generally labeled as “DNC” for “Do Not Connect,” but it appears that many of the DNC pins are just grounded anyhow, so they make a convenient spot to hook into when a ground is needed. I guess they call them DNC because they are planning to use those pins in future revisions of the Pi board and don’t want people getting used to using them. So keep an eye out for that change!

 

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