[sdiy] Optical Pickup

[Mark Rivera]

Dave Manley wrote:
> 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)

base sine:  (3300+312.5 / 2) = 1806.25 Hz
variance:   +/- 1493.75 Hz
mod sine:   25 Hz

Kind of a funky tone isn't it? Since it pulses, maybe it is intended for 
a electrically driving a ringer bell as mentioned?