[time-nuts] 50/60 Hz clocks

Chris Albertson albertson.chris at gmail.com
Wed Mar 23 23:20:13 UTC 2011


On Wed, Mar 23, 2011 at 3:18 PM, Mark Sims <holrum at hotmail.com> wrote:
>
>> it doesn't have to be THAT complex. A 500000 entry LUT is however expensive.
> Yeah,  might even cost as much as a whole US dollar (ragged as they are these days).  512 kbyte EPROMs can be had for under $1...  connect the outputs to a resistor ladder (might need an output latch),  filter,  voila rather nice sine wave.

I think if you really needed a 500K entry LUT then each entry would
need to be 3 bytes wide or you loose the point of having a large
table.    So you are up to a 1.5 Mbyte table.  That said a sine wave
can be compressed. First off you only need to store 1/4 of the wave.
Then because the function is monotonic you only need the deltas from
the last sample and we assume the first sample is zero.  The wide
table could easy be compressed to 512K

But even so, one can buy a small micro controller with way more than
1.5MB for only a few dollars.   Or for that matter AD will give you
sample DDS chips for free if you ask.


-- 
=====
Chris Albertson
Redondo Beach, California



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