Scott Soderlund wrote:

Snip

Looking at the method above, I don't understand the meaning of the
'GAIN' variable (and more specifically the number it is set to). Is
this likely the cutoff frequency? Also, in the next to the last line
of the method, where yv[2] = ..., there are two numbers in this line
that I have no idea what they mean. The number "-0.9479259375" and
the number "1.9469976496". My guess is that the number
"-0.9479259375" is the lowest frequency possible (zero Hz) and that
"1.9469976496" is the highest frequency possible. This would make
sense if the GAIN variable was the cutoff frequency. Again, that is
my best guess! Can someone help with my understanding???

No, the filter has some gain in the passband, which has been calculated
during the design, by dividing the input by this the code ensures that
the output level at high frequency is the same as the input.

The other numbers are the coefficents derived from the recurrence relation
which are derived from the Z plane poles & zeros.
The Z plane pole zero diagram is derived by warping the S plane pole & zero
positions to account for the discrete time nature of the filter.

Digital filters are a HUGE sunject and there is lots of literature out
there.

There is an interactive design program for some of the possibilities at:
http://www-users.cs.york.ac.uk/~fisher/mkfilter which is a very easy
way to design 1 off simple filters.

In fact I would not be surprised if this is the original source of
your code.

Regards, Dan.
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