First Order, One Zero filter using the following formula:
y( n ) = A0 * x( n ) + A1 * x( n - 1 )
where y(n) is Output, x(n) is Input and x(n-1) is Input at the prior sample
Setting A1 positive gives a low-pass response; setting A1 negative gives a
high-pass response. The bandwidth of this filter is fairly high, so it often
serves a building block by being cascaded with other filters.
If A0 and A1 are both 0.5, then this filter is a simple averaging lowpass
filter, with a zero at SR/2 = 22050 Hz.
If A0 is 0.5 and A1 is -0.5, then this filter is a high pass filter, with a
zero at 0.0 Hz.
A thorough description of the digital filter theory needed to fully describe
this filter is beyond the scope of this document. Calculating coefficients is
non-intuitive; the interested user is referred to one of the standard texts
on filter theory (e.g., Moore, "Elements of Computer Music", section 2.4).