[sdiy] Optical Pickup

[Mark Rivera]

>> I have this thing:
>> <http://www.flickr.com/photos/msmiffy/sets/72157609086659400/>

> It looks like the disk has a low to high frequency modulation (the 
> change in the ripple length).  The text on the disk: "312.5 - 3300 c/s 
> at 1500 RPM" implies the output will be modulated from 312.5Hz to 3300Hz 
> and back with a modulation frequency of 25Hz (1500 rev/min * 1min/60sec 
> = 25 rev/sec).  I think this is some sort of 'alert' tone generator - 
> perhaps when the phone has been off-hook for too long.  The min/max 
> freqs match the band pass of a typical phone system.


Spoiler alert ;)

http://lumin.us/msr/forums/phone_thing.mp3