Wednesday, 15 August 2007

The Singing PC

I'm part way through attempting to knock together an experimental setup I saw many moons ago on a visit to the University Of Hull. The idea is pretty simple: convert sound to light, shine the light onto a detector, then finally convert the light back into sound again. This one is quite interesting from the - possibly boring, sorry :'( - physics point of view, because we are having to deal with different types of energy...

  • Sound
    Sound is a pressure wave - a longitudinal wave - of moving air molecules. I have often read in text books and on the Net that sound is energy but that isn't right. Because sound is air moving about it is technically kinetic energy.
  • Electricity
    This is a complicated one. The concept is quite simple:- all atoms contain electrons and in electrical conductors the electrons are free to move from one atom to another (in electrical insulators the electrons can't move so they don't conduct electricity). Actually how it works is a bit more complicated (involving magnetic fields and things). So although electricity is moving electrons this isn't simply kinetic energy - there is more to it than that.
  • Light
    This is electromagnetic energy - a mixture of electrical and magnetic energy.

We are going to transform energy into these three different types of energy...


Initially we will put together the light-to-sound converter. Here is the parts list (click on the part to jump to the relevant Maplin page)...

  1. Photodiode SFH2030
  2. Mono preamplifier
  3. Norwich 3.5W audio amplifier
  4. 49mm Mylar speaker

...and then there'll be wire, batteries, and all that, to power the thing. Note that the amp and preamp draw about 500mA (according to their spec.) so I powered this with 6xAA batteries (rechargeable, of course!).

At this stage I should admit that I was going to build my own preamp and amp but when I was wondering around Maplin looking for parts and found these I was overcome with a bad bout of idleness on my part. That said, I know that you can make a photo detector out of a piece of heated copper and a drop of brine - an idea for a later project there, I think.

Here is a schematic (click on the image to see an enlargement)...

And here is a picture of the prototype (on my particularly messy desk)...

Your eyes don't see continuous images - in other words what you actually see flickers, but you don't notice it (what you see is also, bizarrely, upside down. Your brain flips the image on the back of your eye the right way up). Your ears have much better resolution, is the term. So when I switched on the light over my desk it buzzed. That's because mains electricity is supplied at 50Hz (50 cycles per second). This means the bulb will be flickering 100 times a second - I can't see that but I can hear it with my new new detector.

I was having a play with the monitor refresh rate on the PC when my eldest lad came in and suggested that we have a look at a Linux project called Tempest For Eliza. This program uses your monitor to broadcast music on an AM radio signal. It does this by flickering the screen, causing your monitor to radiate radio waves that you can pick up on a radio (here is a YouTube video of this). Our idea was that our light detector should be able to pick up that flicker and turn it, in theory, into music.

Here are the results...

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