The filter coefficients, which the polyphase mode of the scaler uses, may be specified at compile time or at run time.
At compile time, you can select the coefficients from a set of Lanczos-windowed sinc functions, or loaded from a comma-separated variable (CSV) file.
At run time, you specify the coefficients by writing to the Avalon-MM slave control port.
When the coefficients are read at run time, they are checked once per frame and double-buffered so that they can be updated as the IP core processes active data without causing corruption.
Figure 71. Lanczos 2 Function at Various PhasesThe figure below shows how a 2-lobe Lanczos-windowed sinc function (usually referred to as Lanczos 2) is sampled for a 4-tap vertical filter.
Note: The two lobes refer to the number of times the function changes direction on each side of the central maxima, including the maxima itself.
The class of Lanczos N functions is defined as: