Hi Ian,
> Tom, I'm not sure I used the correct terminolgy here in this context
> ("ringing"). Ringing, as I understand it, is repetition of the basic
> response of the filter at harmonics of the filter's centre frequency, so
> if one set up a narrow-band filter at say, 300 Hz, an analogue filter
> would show damped responses at intervals of 300 Hz. DSP filters are not
> supposed to do this because these responses can be suppressed in software,
> or are automatically suppressed by the FFT algorithm used.
Ringing occurs at the exciting frequency, or near it. Any high- Q
filter will ring, and one with broad skirts will generally ring the least
for a given advertised bandwidth.
DSP filters are no more immune than analog filters to this problem.
If you hear someone saying they have a 100 Hz filter that does not
ring, you can bet the filter's skirts are gradually sloped.
A sudden change in phase shift with frequency generally indicates
less than ideal filter design or construction.
> Group delay does not exist in DSP filters because the system is not
> operating in realtime, and does not use frequency dependant/propogation
> dependant components such as inductors and capacitors; the whole process
> is completed before anything reaches the output.
>
> Therefore in DSP filters there cannot be ringing or group delay, no
> matter what is processed - unless the dynamic range of the ADC (and DAC
> stage?) is exceeded..
Not so Ian. DSP filters have or can have substantial group delay. It
may be easier to optimize software, but the problems are all still
there.
Exceeding the range of the D-A or A-D conversion does not cause
ringing per se, but it certainly causes distortion products!
The bandwidth of the filter, and slope of the skirts, affects ringing
just as it does an analog filter. I don't write software for digital
filters, but I do evaluate the results in communications systems
and DSP based filters have the same problems as any other filter.
If you don't want it to ring as bandwidth approaches the minimum
required by the signal, it needs to have gently sloped skirts.
The sales pitch for DSP filters reminds me of the sales pitch for
certain antennas, like the "lossless linear loading" that is actually
more lossy than even a reasonably good inductor.
73, Tom W8JI
w8ji@contesting.com
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