A periodic signal analyser, just collect data in a specific sample rate,
and create a string of bits, it is then saved in a file or process it in
real time comparison.

The comparison is made based on bits on and off, against the pattern, or
"master". What it does is just account how many bits on and off match
from the sample signal against the stored "master" signal.  In a system
like that, there is a minimum quantity of bits to match, and that would
be your "minimum similarity threshold".  More matching bits means the
"similarity" is better, but probably you will never get a 95% matching
noisy signal.

Suppose your sample contains 150 bits, and your "master" contains only
50 bits.  The sample should contain at least 2 or 3 times more bits than
the "master" to make sure the "master" similarity *will be* inside the
sample string. Suppose also that you want 80% similarity, so it means at
least 40 matching bits.

The master (50 bits) should be compared bit by bit, left aligned to the
sample, until you got more than 10 mismatch bits (higher than 10% of
acceptable error), when it happens, you should restart the comparison
sequence with the master aligned to the sample's left bit#2 and so on,
until the last comparison would be done with master aligned to the
sample's left bit #110.  Starting from sample's #111 bit it is
impossible to have a 80% match, since you would have only 39 bits to
compare and 40 is the requirement (80%).

As you can see, a periodic analyser requires a speedy processor to be
able to do (n^2)*2 bit comparisons per sample, where "n" is the length
of the "master" bit string.

--------------------------------------------------------
Wagner Lipnharski - UST Research Inc. - Orlando, Florida
Forum and microcontroller web site:  http://www.ustr.net
Microcontrollers Survey:  http://www.ustr.net/tellme.htm


Dave Johnson wrote:
>
> I want to look for periodic noise in a stream of data (testing a data
> acquisition board for cleanliness), but admit I'm slightly clueless how
> to go about it.
>
> Does anyone know of a decent tutorial on this kind of thing?
>
> Dave Johnson