Motorboating in Space

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Or, how to stop Zaxxon going thump-thump-thump.

‘Motorboating’ has been a problem in electronics almost as long as electronics has existed. It gets its name from a characteristic thumping or buzzing noise, reminiscent of a motor boat’s engine. It’s a problem which usually occurs in audio amplifiers, and it happens either because of a design error or faulty components. Sometimes a change in an amplifier’s operating environment, such as a radio battery running down, can cause it. It’s loud, annoying, and can even damage speakers,

The reason for the noise is feedback. If an amplifier drives a signal into a loudspeaker, the power for that signal has to come from its power supply. Its power supply, especially if it’s a run-down radio battery, isn’t perfect. Drawing power from it makes its output drop in voltage for a moment. Unfortunately, electronic circuits aren’t perfect either. Their behaviour is strongly affected by their power supply. Connect such a circuit to such a power supply and amplifier, and you have a vicious circle: circuit sends a signal to amplifier, amplifier sends it to speaker and draws more power, power supply affects circuit, which makes another signal which gets sent to amplifier, and so on. It’s called feedback because the output signal feeds back into the input, via an unorthodox route. The circle of feedback can lead to the regular buzzing noise – the motorboating.

Recently I have restored a Zaxxon arcade game circuit board, which dates from 1982 (actually, it’s a bootleg, but the circuit is largely the same). I got it working well, but with one big problem: the sound was accompanied by a constant thumping noise which wasn’t supposed to be there. Here’s a short movie of how it sounded. It’s especially noticeable at the start and end of the clip.

Fans of the game will know that Zaxxon has very distinctive sound. Many video games at the time used digital techniques, often using standard chips, to generate their sound, which gives them a characteristic bleepy quality. Zaxxon is different. It uses what amounts to an analogue synthesizer: a magnificent assembly of timers, oscillators, amplifiers and filters. It has a lot in common with the kind of instruments used in pop music at the time. It makes a glorious, raucous noise.

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But this kind of analogue circuitry has a problem, especially when it’s cheaply built using early 1980s technology: it’s very sensitive to its power supply. Any variation in the power supply basically gets straight to the synthesizer’s output. What’s more, Zaxxon’s loudspeaker amplifier runs from the same power supply as the synthesizer. This lot is a recipe for motorboating, and that’s exactly what happened to my game.

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Of course, we have to assume that it all worked properly when it came out of the factory, but then it would have been running from an official Zaxxon power supply. The one I use in my arcade game test rig may not be as good as the original one, but it’s good enough for most things, and I wasn’t going to change it just to fix this problem. So I had to come up with a modification to keep apart the amplifier power and the synthesizer power.

The traditional cheap and cheerful way of keeping power supplies apart, known as decoupling them, is simply to put a resistor and capacitor between them, like this:

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This decoupling means that variations in one power supply have a smaller effect on the other. It works well, and has been used in millions of electronic devices from the earliest days of radio. However, a certain amount of power is always lost in the resistor. Many circuits don’t mind this, or can be designed to handle it. I tried this approach with Zaxxon,and it turned out that the sound synthesizer doesn’t cope well with a reduced supply voltage. Many of the effects, especially explosions, became disappointingly quiet. I had to find another way.

Arcade games typically use two power supplies: 5 volts for their digital circuits, and 12 volts for the sound amplifier. This gave me an idea: how about using the 5 volt supply to run the audio synthesizer, keeping it neatly separate from the amplifier? Clearly the synthesizer wouldn’t just work from 5 volts: I’d already had trouble with it running from about 10 volts in the decoupling experiment. However, there was a solution. It would be possible to boost the 5 volt power supply up to 12 volts using, aptly, a boost converter. Boost converter modules are cheap and readily available thanks to low-cost far eastern manufacturing. The one I chose had a conveniently adjustable output voltage. It didn’t take long to wire it up. I’d already separated the amplifier supply from the synthesizer, and so I just had to take a wire from the existing 5 volt supply to the sound board, check my work and switch on.

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It worked! The sound was now perfect, with no strange thumping effects, and everything seemed to be at the right volume. It remained only to make the modification more solid, and there was even a handy spare hole  to mount the boost converter in. Job done!

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