[sdiy] Optical Pickup

[Dave Manley]

Mark Rivera wrote:
>>> 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
Hi Mark - what was the modulation waveform for this sample?

For a second there I was sure this was some emergency broadcast system 
tone, but that was two sines mixed:  853 and 960 Hz. 
(http://www.nationmaster.com/encyclopedia/Emergency-Broadcast-System)

-Dave