[time-nuts] ANFSCD - Synchronizing time in home video recorders
Hal Murray
hmurray at megapathdsl.net
Thu Feb 2 19:13:30 UTC 2012
tvb at LeapSecond.com said:
> I'm not sure how well a multi-level leap year algorithm relates Breseham's
> algorithm. I tracked down his 1965 plotter article. There might be common
> ground there.
It's the same math as a DDS.
If Breseham would land exactly on a grid point after N steps, a DDS will
have no long term drift. That means the slope of the Breseham line is N/M
where both N and M are integers. For the intermediate steps, both Breseham
and DDS come as close as possible: 1/2 grid spacing vs 1/2 clock period.
We usually think of DDS as requiring M to be a power of 2 but you don't have
to do it that way. One obvious example is to make M a power of 10 by doing
decimal adds rather than binary. That should work well with a FPGA but I
haven't done it yet. If you start with 10 MHz, that will give you perfect
hits on integer audio frequencies.
--
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