Nikolai Golovchenko wrote:
>
> Wow! I missed this feature at first. Just tweak the output when
> you have time, and who cares about PWM or saw-tooth waves! :)
>
>  Nikolai


Yes it appealed to me also. The ability to return to
the pwm routine at any time and refresh it and still get
a fair pwm analogue output.

I suggested a sawtooth wave for a number of reasons. The
problems with true random numbers is they are totally
unpredictable. When I worked as a games programmer
*everything* uses random numbers, chance of player being
injured, chance of getting the goodies, etc.

No games programmers use real randomness. You *absolutely
will* get long strings of good or bad numbers based only
on luck. Generating good psuedo-random numbers is a an
art, but the basis of that art is ensuring for an EQUAL
biasing of all samples. In other words, if you take 100
samples, about 10 should be under 10%. About 10 should
be from 10% to 20%, etc etc. This is vitally important
in the real world.

With a pwm dependant on the random number, it is quite
possible that in 1000 samples (1 second?) that all the
random samples will be low. This gives a ridiculously long
RC time constant to try and filter it, and chance of
scary errors when a lot of "bad" samples happen. Which
is a matter of pure luck! There will be times when 1000
sequential samples happen with NONE of them over 25%
value. This would be a pwm disaster. And it WILL happen.

Games programmers use a more "sawtooth" random approach,
where EVERY possible sample must occur in a short time
period, so in one second EVERY possible value occurs a
lot of times. Taking a random (psuedo random) sample
somewhere in there will provide a much more usable random
value than a pure random number.

This occured to me straight away when the random pwm
idea was mentioned. You really don't want proper randomness
or you introduce problems, hence the discussions here
over the last couple of days. Using a high frequency
sawtooth style wave sampled at irregular frequency or the
appropriate aliasing frequency will give a more predictable
and much more reliable result than a true random waveform
which sould be high or low for an unlimited number of cycles
based upon luck.

If the sawtooth frequency is significantly higher than
the sampling frequency, the result will be very random
with excellent weighting. If the sawtooth is generated in
hardware even better.

For a timing-insensitive pwm I really think this would be
a better method. Yes, you have to deal with aliasing, but
is that a bigger problem than the alternatives?? I don't
think so... :o)
-Roman

PS. The preferred way to generate good psuedo random
numbers is to generate numbers between 0 and 255 (for
eaxample) and place in a 256 unit lookup table where
there is only ONE of each number, but the postions
are random. Then you can read 256 random numbers
in sequence and have perfect weighting. As a games
programmer I coded many psuedo-random generators and
testers, and this is a  good place to start. Maybe a
search of games programming sites would reveal some
useful info. People think they are dopey kids but
they are just as good at what they do as any aerospace
programmer.

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