Dreams of a Wireless Intercom System

When we put up a show, we usually have communications line set up between the technician group at the mixing station, the band, backstage, and usually the producer as well. For years this has been done with inexpensive PMR446 handheld radios combined with headsets. (You can't put the speaker on during the show or you'll annoy the audience with beeps and squelch tails.) It is a well known fact by now though that the intelligibility of somebody trying to whisper through these radios in a noisy environment, is slightly less than ideal, and perhaps 50% of the transmissions are variations on the theme "repeat please", "come again", and so on.

I once had the misfortune of being wired into a professional system, and since then I were aware of how good it could be. On the other hand, professional grade systems are for professional grade budget outfits, and that's pretty much the end of the story.


Unless I was bitten. And that's how it starts. It's usually how it starts. At least the order of operations have been somewhat sensible so far.
  1. Look at possible hardware platforms
  2. Develop PoC (Proof of Concept) software
  3. Get hardware platform development kit or similar
  4. Port PoC software to development kit
  5. Design dedicated hardware around chosen hardware platform

Hardware platforms

It was a while ago, but I think the initial idea was to get a fast enough arm processor, and something like a nrf24l01 or other complete radio IC, however, we operate in an old building with thick walls, so range would be a problem, unless I designed a whole repeater system or something like that, which is not done at the drop of a hat. I was also unsure about the throughput I would be able to get, if it would be able to support good quality voice transmissions. Instead I opted to try to leverage the already available wifi infrastructure, and to base my hardware on some existing SBC (Single Board Computer)

A number of SBCs were considered, somewhat unsurprisingly, the Raspberry Pi Zero W was one, another was the C.H.I.P Pro. Other evaluated options were less known fruit themed boards that have popped up in the wake of the success of the Raspberry Pi brand, such as the Orange Pi. After reading reports of unreliability of the wifi subsystem of many of the pi clones I pretty much wrote them off.

In the end the Chip won over the Pi, since it for my particular use case comes preloaded with a lot of things that I would otherwise have to supply hardware for myself, such as audio codec and battery management systems. The Chip comes with flash storage as well, compared to the Pi's need for a µSD-card. Another upside of the Chip is that it has a u.fl.-compatible connector for an external wifi antenna, which is needed since I plan to use a metal enclosure.

Both systems have the downside that it's not exactly easy to just buy five of them, the Pi you can only buy one at a time, and the to buy the Chip, standalone from the development kit, you have to talk to sales. I don't want to talk to sales, they have pretty much sold me on the product with the feature set alone.

A trial placement of the bulkiest components and the enclosure, in order to verify that they would fit inside the box. Since then, the project has been stripped of a few non-essential features, and fit into a slightly smaller enclosure. In both cases, the C.H.I.P Pro would be mounted on the other side of the board, underneath the battery holder.
To be continued...

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