Tom Rauch wrote:
>> One of the great advantages of DSP is the lack of ringing in narrow
>> bandwidths because of near zero group delay, and reduced operator
>> fatique resulting from the noise reduction.
>>
>> Ian ZS6BTE
>
>
> Hi Ian,
>
> I don't understand how group delay is directly related to ringing. I
> see no connection, unless group delay varies wildly with frequency.
> A changing group delay in the passband may indicate the filter has
> internal problems, either in design or construction.
>
> Other than poorly designed filters, I see very little difference in
> "ringing" between crystal and DSP filters.
>
> People have the misplaced notion that CW will not "ring" in a DSP
> filter. Rise and fall times are limited by filter bandwidth, and that is
> what usually makes a filter "ring" on a smooth signal. A very
> narrow filter, no matter what design, is going to "soften" rise and
> fall times and make a signal "ring".
>
>
> 73, Tom W8JI
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.
All analogue RF filters exhibit group delay, and it has nothing to do
with signal amplitude (or waveform rise/fall time, unless the rise/fall
time is fast enough to have a high frequency component substantially
exceeding the bandwith of the filter). Group delay is a non-linear
relationship between time and individual frequencies in a band of
frequencies passing through the filter, typically the higher frequencies
suffer more delay, I think. The audio manifestation of this is similar
to ringing; in the video domain it's easily described as an echo
-"ghosting" - ringing.
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..
Help me, someone.
Ian ZS6BTE
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