Vic K2VCO wrote:
It's not in the manual. They are two different types of DSP filters. I
think FIR has slightly less ringing, but IIR has...something else.
Anyway, some people may prefer the sound of one to the other.
FIR means finite impulse response and IIR means infinite impulse
response. Impulse response is how the output responds to a sudden
change in input. Ringing is an impulse response. FIR filters have an
output that only depends on the last n samples, so ringing cannot last
any longer than that. IIR filters have feedback and, give or take
rounding errors and getting lost in the noise, the effect of an input
change lasts for ever, although it will normally get vanishingly small.
If IIR filters ring, they will ring for ever, like analogue filters.
FIR filters can also have a frequency independent group delay, which is
good for digital modes.
The disadvantage of FIR filters is that you need much more computing
power to create an equally sharp filter, as you have to add
contributions from the many more previous sample times.
A simple IIR filter (although one that doesn't ring) would be an
exponential average, where you combine a fixed proportion of the
previous output with (1 - that proportion) times the current input.
A simple FIR filter would be a rolling average, where you average the
last n samples. Note that this requires n calculations, whereas the
exponential average only requires a couple.
--
David Woolley
"The Elecraft list is a forum for the discussion of topics related to
Elecraft products and more general topics related ham radio"
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