Because Jonnie Common‘s installation is going to involvve eight channels of transmitted FM, it will be an octophonic performance, hence the working title for the transmitter is now officially Octomitter!

So. I have all 8 boards working nicely, built to Guts v0.3.2 spec, which is almost ready for release, pending the instructable.

I spent some time testing them yesterday with some interesting results:

• All have to be on at the same time, else the transmitting frequencies vary.
• The 9v Oomlout transformer interferes with the signal a little, producing a whining noise, presumably because of the PWM used to step down the voltage.
• This whine varies in pitch depending on how many transmitters are in circuit parallel with the power source. Switching 1 to 8 transmitters on in sequence produces an almost major scale. Almost!
• It takes a while to find all the sweet spots for each transmitter’s output frequencies, such that the harmonics don’t sit on top of each other, and they are not competing with national radio stations.
• The best tactic in calibrating is simply Be Methodical, as with most things like this! Switch all on, tune to desired frequencies, then tweak the ones that aren’t playing ball. Note down everything.

Next up: testing in situ with Jonnie’s material, hopefully.