[sdiy] Optical Pickup

[Mark Rivera]

Mark Rivera wrote:
> 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?

Eh, I should watch my terminology, I didn't mean variance as in σ^2, 
just +/-.