Re: [LAU] [Slightly OT] 312.5Hz

On Sat, Nov 15, 2008 at 11:51:35AM +1030, Matthew Smith wrote:
> Slightly off-topic, but does anyone here know any audio significance of
> 312.5Hz?

No. But the frequency range you described covers audio telephony.

> I have here a module from a piece of equipment dated back to 1968. It
> consists of a 4-pole, 50Hz motor, a printed disc, a lamp and a
> photocell. The printing on the disc (rotor) is a wiggly line of
> changing frequency which - according to the disc - represents a signal
> between 312.5Hz and 3kHz when spinning at 1500rpm. You light the lamp,
> spin the motor, then amplify the signal from the photocell.

I'm an amateur radio operator, license VK2LQZ.

What you describe reminds me of a very low frequency RF oscillator with
an audio tone added in. I'd like to see a photograph of the wiggly
line, as I suspect it is not a simple sine wave, but rather the addition
or multiplication of two sine waves at different amplitudes. The
photograph would need to show the angle covered.

A 50Hz motor means a country where 50Hz is common, such as UK,
Australia, and so forth. A 50Hz synchronous motor can indeed do
1500rpm, with the speed controlled by the grid. The voltage would only
control the starting speed, unless there was deliberate air resistance.

But maybe not RF. I've done some calculations. I cannot get 312.5Hz
out of 1500rpm, there's a remainder ... 1500 rev/min is 25 rev/sec, and
312.5 divided by 25 is 12.5, so the audio frequency is not a multiple of
the rotational frequency.

Alternatively, it is an evacuation siren audio oscillator, with a motor
wired to start slowly.

Does it start slowly?

-- 
James Cameron    mailto:quozl@email-addr-hidden     http://quozl.netrek.org/
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