[sdiy] square to sine

[Neil Johnson]

Hi,

cheater cheater wrote:
> Dave, I'm going out on a limb here, but this reminds me of a technique
> normally used for asynchronous sample rate conversion, which is
> "flywheel sync".

"flywheel synchronisation" was an early term for the then-new
technique of PLL as used in British TV sets:

http://en.wikipedia.org/wiki/Broadcast_television_systems#Image_polarity

and later used for the 4.something MHz colour burst.

A blast from the past: BBC R&D report from 1969 talks about flywheel
synchronisation:

http://downloads.bbc.co.uk/rd/pubs/reports/1969-44.pdf

The diagram on page 2 of the report is a PLL.

> Basically, you have a model of a flywheel - with
> torque and friction - and each rotation of it is one output wave
> cycle. As clocks come in, you give the flywheel a "push" or "jolt". If
> the jolts stop coming in, the flywheel stops eventually. If they come
> in at a lower frequency, the flywheel spins slower. If you rapidly
> lower the frequency, you can add some friction (for example, friction
> gets -df/dt added to it). It is normally used to prevent jitter, but
> can also be used for syncing two different waveforms together.

You're talking about the characteristics of the various components of
the PLL (vco, phase detector, loop filter, etc).

Cheers,
Neil
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