What texts are you using?
DaveD
KC0WJN
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All spelling mistakes are the responsibilty of the reader (Rick Renz, STK, ca. 1994)
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On Jul 6, 2024, at 09:54, Reginald Beardsley via groups.io <pulaskite@...> wrote:
?I am working on numerical modeling of CCW and am at the point that I need to write the code to implement twin pass band tuning. As this is NOT seismic processing I pulled a small stack of my EE DSP classics and started reading them.
I have always found the descriptions of digital filter design in the EE literature baffling complex. As seismic is done in recorded time it has a great deal of latitude in implementation that a real time DSP implementation lacks. So I presumed that the reason lay in the real time constraint.
Yesterday I realized that the design of a static filter is completely trivial. Specify the frequency domain characteristics and back transform to the time domain. The impulse response is the taps for any arbitrary filter. I also noticed that the filters in the examples were zero phase filters.
In seismic we always use zero phase (aka "linear phase" in EE jargon) filters as interpreting a symmetric waveform is much easier.
The first step in seismic processing is called "designature". This consists of using a recording of the system impulse response to create an all pass filter which removes the phase imposed by causality. Typically the recording system impulse response is measured and then the phase of the source is recorded for marine work using arrays of air guns.
Analog filters are minimum phase as are all physical processes. But the digital design examples I saw were zero phase with a symmetric impulse response. This leads to a non-physical result. The filter output begins *before* the input arrives. I find that rather hard to justify in a communication system. A quick check of the indices revealed that except for a very brief mention of minimum phase and causality the issue is completely ignored. Oppenheim and Schafer devote a few page and Rabiner and Gold a paragraph before dismissing the matter. Another text made no mention of it at all.
Would a DSO which responded to an impulse showing the trace varying *before* the peak of the impulse be acceptable? I can't imagine why anyone would accept that.
So why should it be acceptable in a communication system? Can anyone elucidate the matter?
Have Fun!
Reg
