A crazy idea

As I cycled to work this morning, I had a crazy idea. It’s just daydreaming and will probably never happen, but it’s fun to think about. I’ve already posted this idea to the linux-audio-user mailing list in case anyone there has any insights on synthesising/sampling engine sounds.

I want to measure how fast I’m turning the pedals on my bike (the cadence) and synthesize/sample the sound of an internal combustion engine.

As far as I can work out, there are three major parts to this.

1. A sensor that can measure my cadence. A simple magnet switch that triggers once a revolution won’t be enough to measure the cadence with sufficient resolution, since my cadence is usually between 50 and 80 rpm, and I will need to sample more than once a second. I would probably need to mount multiple magnets spaced equally around the chainwheel and have a single sensor on the frame. Then I have to get it to supply this information to my control program.

2. I need a control program that can read in the input from my cadence sensor and convert a cadence reading of “66 rpm” into a frequency that should be sampled/synthesised, e.g. “500 Hz” (I’m making these numbers up). It will also need to be able to somehow smooth out the readings, perhaps by interpolation, so when I accelerate, the sound of the revs climbing doesn’t increase in obvious steps. It could also have other logic, e.g. when my cadence is 0 rpm, the sound of the engine is idling rather than off.

3. I need a synthesiser or sampler that can take an input from my control program and make the sound of an engine (or more likely, a sine wave to start with). I’ve never sampled or synthesised on a computer before but this engine-specific sampling technology already exists in video games, such as torcs.

I have absolutely no idea why I would want such a device – just for the fun of building it, I guess. I would like it to work in realtime (rather than later generating the soundtrack from recorded cadence data). The thought of sitting at the traffic lights with my earphones in and then hearing the mighty roar of a V8 as I pull away would be really satisfying…

Any thoughts – useful, interesting, humorous, or otherwise – are welcome!

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