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>> which case it shuts itself off until the voltage comes back up again.
>
> I always thought they were just wired up to work in the "on" position but
> not the "start" position.
Before cars turned into computers, the radio system typically was just
connected directly to the battery through a fuse, and has an "ignition"
signal line to indicate whether it should be on or not (this signal is on in
both the "on" and "start" position, obviously).
Car manufacturers give all sorts of detailed specs for radio systems,
detailing exact input voltage patterns and what the system must or must not
do. For older radio systems they only had to work down to 10 V or
something, and when the voltage dropped below that they had to behave
sensibly (ie not blurt out massive clicking noises or static). This was
purely because it is cheap and simple to make a radio work with a supply
between 10-16V compared to 0-16V (yes, newer radios do have to theoretically
work with a 0 volt supply for a fraction of a second).
Recently though, especially with these systems where the engine is switched
off every time you stop, the radio system is required to keep working during
engine start events. There are quite detailed specs about what the radio
must "survive" in terms of supply voltage fluctuation, because the supply
voltage is anything but smooth in a car.
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