Feb 192022

So, I’ve been wanting to do this for the better part of a decade… but lately, the cost of more capable embedded devices has come right down to make this actually feasible.

It’s taken a number of incarnations, the earliest being the idea of DIYing it myself with a UHF-band analogue transceiver. Then the thought was to pair a I²S audio CODEC with a ESP8266 or ESP32.

I don’t want to rely on technology that might disappear from the market should relations with China suddenly get narky, and of course, time marches on… I learn about protocols like ROC. Bluetooth also isn’t what it was back when I first started down this path — back then A2DP was one-way and sounded terrible, HSP was limited to 8kHz mono audio.

Today, Bluetooth headsets are actually pretty good. I’ve been quite happy with the Logitech Zone Wireless for the most part — the first one I bought had a microphone that failed, but Logitech themselves were good about replacing it under warranty. It does have a limitation though: it will talk to no more than two Bluetooth devices. The USB dongle it’s supplied with, whilst a USB Audio class device, also occupies one of those two slots.

The other day I spent up on a DAB+ radio and a shortwave radio — it’d be nice to listen to these via the same Bluetooth headset I use for calls and the tablet. There are Bluetooth audio devices that I could plug into either of these, then pair with my headset, but I’d have to disconnect either the phone or the tablet to use it.

So, bugger it… the wireless headset interface will get an upgrade. The plan is a small pocket audio swiss-army-knife that can connect to…

  • an analogue device such as a wired headset or radio receiver/transceiver
  • my phone via Bluetooth
  • my tablet via Bluetooth
  • the aforementioned Bluetooth headset
  • a desktop PC or laptop over WiFi

…and route audio between them as needs require.

The device will have a small LCD display for control with a directional joystick button for control, and will be able to connect to a USB host for management.

Proposed parts list

The chip crisis is actually a big limitation, some of the bits aren’t as easily available as I’d like. But, I’ve managed to pull together the following:

The only bit that’s old stock is the LCD, it’s been sitting on my shelf gathering dust for over a decade. Somewhere in one of my junk boxes I’ve got some joystick buttons also bought many years ago.

Proposed software

For the sake of others looking to duplicate my efforts, I’ll stick with Raspberry Pi OS. As my device is an ARMv6 device, I’ll have to stick with the 32-bit release. Not that big a deal, and long-term I’ll probably look at using OpenEmbedded or Gentoo Embedded long-term to make a minimalist image that just does what I need it to do.

The starter kit came with a SD card loaded with NOOBS… I ignored this and just flashed the SD card with a bare minimum Debian Bullseye image. The plan is I’ll get PipeWire up and running on this for its Bluetooth audio interface. Then we’ll try and get the hardware bits going.

Right now, I have the zero booting up, connecting to my local WiFi network, and making itself available via SSH. A good start.

Data sheet for the LCD

The LCD will possibly be one of the more challenging bits. This is from a phone that was new last century! As it happens though, Bergthaller Iulian-Alexandru was kind enough to publish some details on a number of LCD screens. Someone’s since bought and squatted the domain, but The Wayback Machine has an archive of the site.

I’ve mirrored his notes on various Ericsson LCDs here:

The diagrams on that page appear to show the connections as viewed from the front of the LCD panel. I guess if I let magic smoke out, too bad! The alternative is I do have two Nokia 3310s floating around, so harvest the LCDs out of them — in short, I have a fallback plan!

PipeWire on the Pi Zero

This will be the interesting bit. Not sure how well it’ll work, but we’ll give it a shot. The trickiest bit is getting binaries for the device, no one builds for armhf yet. There are these binaries for Ubuntu AMD64, and luckily there are source packages available.

I guess worst case scenario is I put the Pi Zero W aside and get a Pi Zero 2 W instead. Key will be to test PipeWire first before I warm up the soldering iron, let’s at least prove the software side of things, maybe using USB audio devices in place of the AudioInjector board.

I’m going through and building the .debs for armhf myself now, taking notes as I go. I’ll post these when I’m done.