[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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