I have a friend called Chris who is a big fan of the band China Drum. Many years ago he challenged me to program their song Last Chance as a custom ringtone on his Nokia phone and, being vaguely musical, I obliged.
Time has moved on since then. With his hitherto rock-star hair cut to a respectable length, he is now the CEO of a company providing disease model human cells. And he owns a Tesla, something he likes to remind me about from time to time. Now it turns out that one of the silly things you can do with a Tesla is program the lights to flash to make a custom light show for a piece of music of your choice. You can probably see where this is heading.
Since a) I was going up to meet him (and his Tesla) in Yorkshire last weekend and b) I wasn’t sure how long the “I’ve been busy” excuse would work in response to the “Where’s my light show?” question, I figured I’d probably better actually try to do the damn thing.
Problem 1 – I don’t have the song
Sadly I didn’t have a copy of China Drum’s Goosefair album, so I didn’t have the audio. But I do have Linux, and a Spotify subscription, and the command-line ncspot client. I reasoned that if I could play the audio, I could surely also record the audio, it just might mean losing the will to live while trying to understand how Pulseaudio works (or maybe it’s PipeWire now, or who knows?)
Cutting a long and tedious story short, by doing some mystical fiddling about, I was able to send the output of
ncspot to Audacity and thereby record myself a WAV of the song. In the interests of remaining on the right side of the law, I made good faith attempts to locate an original copy of the album, but it seems to be out of print. So I now have a copy via eBay.
Problem 2 – I don’t have the application for building the light show
To build a light show, one needs an open-source application called xLights. Since this doesn’t appear to be in the Ubuntu package repository, I had to build it from source. For some reason, I’m a bit averse to installing random libraries and things on my machine, but fortunately there is a “build it in Docker” option which I used and seemed to work successfully, except that I couldn’t figure out how to get at the final built application! It existed as a file in the filesystem of the docker container, but since the executing script had finished, the container wasn’t running, and there seemed to be no obvious way to get at it (it is entirely possible that I was just being stupid, of course). In the end, I reasoned that any file on a container filesystem has to be in my
/var/lib directory somewhere, and with a bit of poking around, I located the
xLights-2023.08-x86_64.AppImage file and copied it somewhere sensible.
Problem 3 – I don’t know where the beats are
I followed various instructions and got myself set up with a working application, a fresh musical sequence project for the file, and the
.wav imported and displaying as a waveform.
The way xLights works is that you start with a bunch of horizontal lines representing available lights, you set up a bunch of timing markers which present as vertical lines thus dividing the work area up into a grid, and you can create light events in various cells of the resulting grid and the start / stop transitions of each light are aligned with the timing markers (though they can subsequently be moved). Thus you need a way of creating these markers. Fortunately, it is possible to download an audio plugin to figure these out for you. After doing this, you end up with a screen looking something like this:
Problem 4 – I can’t copy and paste
So I started filling things in with the idea that if I got something that I was happy with for a couple of bars, I could copy and paste it elsewhere rather than having to enter every note manually. Unfortunately, I ran into an unexpected problem, which is that the timing on the track is very variable. For example, I picked a couple of channels and used one to show where the “1” beats were in each bar and put lights on “2”, “3” and “4” in the other. But if you then try to copy and paste this bar into two bars, then four, then eight and so on, you quickly get out of sync with the beat lines, because the band speed up as the song goes on. Also, I couldn’t see any obvious option in
xLights to quantise a track (i.e. to adjust the starts and ends of notes to match a set of timing marks).
Fortunately, the format that
xLights uses to save these sequences is XML-based. Therefore I was able to write a Scala application to read in the sequence, make a note of all of the timestamps corresponding to the timing marks, and then shuffle all starts and ends of notes to the nearest timestamp. Actually, originally I wrote a thing to try to regularise the timestamp markers (i.e. keep the same number of them but make the spacing uniform) and quickly realised that the resulting markers were woefully out of sync with the music, which is when I realised the tempo of the song was variable.
Problem 5 – I can’t count
I dimly remembered from the Tesla light-show instructions that there was a limit of 3500 lights in a show, so I had to make sure the total number didn’t go above that. I couldn’t see an obvious way to do this in
xLights, but I could see that it was just a question of running a suitable XPath expression against the XML file. So I fired up oXygen and wrote one (the correct XPath – see below – is
And so my development cycle was basically:
- Stare at lots of blobs on the grid, adding and removing them and generally twiddle until it seems satisfactory
- Run the Scala application to quantise it all to the timing marks, thus dealing with any slight mismatches caused by copying-and-pasting
- Check in oXygen how many lights I’ve used (editing the raw XML was also useful to copy between channels e.g. to make the rear left turn indicators do the same as the front left turns)
Problem 6 – I can’t read instructions
Having got something I was reasonably happy with, I double-checked the instructions. Oh no! I am an idiot! It doesn’t say 3500 lights, it says 3500 commands where a command is “turn light on” or “turn light off”. So I now have twice the number of allowed lights, and drastic editing would be needed (naturally, this was the Friday night before I was due to drive up with it, after a number of late nights working on it).
Fortunately, I had also been an idiot (very slightly) with my counting XPath; because of the way the XML format works, each timing mark was being counted as an event. So having tweaked it not to count the timing marks, I had around 2600 lights, which is rather more than the 1750 budget but less bad than the 3400ish I started with.
So I had to scale things back a bit. I dropped the tracks I’d been using for the “1 2 3 4” beat. I removed some of the doubling up between channels. I dropped some of the shorter notes in tracks where I’d been trying to reproduce the rhythm of the vocals. I simplified bits and generally chopped it about until I had something that met the requirements.
Problem 7 – I don’t have a USB stick
Well, I didn’t at the start of the project. I did by the end; the smallest one I could find in Curry’s was 32GB. Which is ridiculous overkill for the size of the files needed (the audio was 27M, the compiled lightshow file was 372K). But, well, whatever…
And that was it. I followed the instructions about what needed to be on the stick (a folder called “LightShow” with that exact capitalisation containing “lightshow.wav” and “lightshow.fseq”) and took it with me to Yorkshire. Chris plugged it into the Tesla. It ran successfully. Hooray!
I’m not sure there’s a great moral to this story (other than maybe “read the instructions carefully”) but it was a fun challenge. Thanks to China Drum for a great song, Tesla for building the light show feature, and the
xLights authors for an open-source application that made building the light show possible.