Phase Selectivity in DSP Equals Directivity
What can DSP do with a frequency component based on the phase difference?
The most obvious thing is to pass 100% of those frequency components with a phase difference that corresponds with the desired direction. All other components are not passed, but suppressed 100%. You thus create a very effective directivity. In practice, you must state two boundaries between which the phase difference of the frequency component must be in order to be passed. The width of this phase window depends among other things on the constancy of the direction from which the signal arrives. The smaller this window, the more undesired signals will be suppressed and the stronger the directivity is.
Phase selectivity equals directivity.
If we have one or several interfering sources, we can also opt to suppress associated phase differences. In fact, we can therefore suppress more than one interfering source 100%. For each interfering source, we must select a matching window. The smaller the window, the smaller the chance that desired signals will also be suppressed.
Everything coming outside or within the scope of a window can be suppressed 100%.
We have an infinite front-to-back ratio, which can sometimes be too good. We may not hear the signals at all that we want to or should hear. We can make the level of suppression adjustable for this purpose, depending on what is desired.
We can freely select the form of the window, as shown in Figure 8. It is not necessary to pass or suppress 100% of the frequency components that have a phase which comes within a window. We can make this dependent on the phase difference with respect to the middle of the window. The window does not have to be continuous. We may also place a notch in the window against interference that shows about the same phase difference as the desired signal.
The windows are controlled manually, just like a rotator. The center of the window is set to the corresponding desired direction. Depending on whether the source in that direction is the desired signal or an interfering signal, the signal is either suppressed or passed. The width of the window is adjusted for best readability.
Figure 8 — In the window a) pass 100% b) pass depending on phase difference c)
suppress depending on phase difference d) pass with notch e) pass 100% with 95%
suppression outside the window.
Last update: September 24, 2006