Radio scan controller?

[Jeremy Brookes]

>>Bear with me folks, this is probably going to sound a little wacked, but
>I'm
>open to honest opinions/debate.  A friend of mine and I were discussing the
>prospect of using MIDI to control radio frequency scanning -- that is, to
>produce the sort of "scratch" sound effect you get by rapidly rotating the
>FM radio dial from 88.1 to 107.9 and back again as if it were a DJ
>turntable.  (If you have to ask why anyone would want to do that, just
>remind yourself that I warned you I was wacked.)  Anyhoo...in a fit of
>brainstorming, I came up with the following thoughts:


>Until recently, the knob on a radio was attached to a multi section, ganged
>variable capacitor, which controls the local oscillator, and the RF
>pre-selector.  This could be replaced by varicap diodes which are voltage
>controlled, although making them track together like the ganged variable
>cap did won't be easy.
>
>Modern, digitally controlled radios use a microprocessor to send commands
>to a PLL to select the frequency.  This type of PLL is usually not designed
>to make fast frequency changes.  Probably be a major hack to get it to do
>what you want.


And quite often the PLL is designed to change in channel steps (normally
10kHz or 12.5kHz) thereby making it difficult to sweep smoothly across
channels. It might be possible though to hack the radio at a lower level.
The output of the PLL will be fed to a VCO (an RF one - not much musical use
here). By breaking this link and injecting your own voltage you might be
able to sweep the radio. You'd most likely lose the ability to accurately
tune to a station. But maybe this won't work as the PLL is part of the
demodulation?