Nyquist (Nth) digital filters
This Mathworks documentation gives a good overview about the different parameters: http://uk.mathworks.com/help/dsp/examples/fir-nyquist-l-th-band-filter-design.html?requestedDomain=uk.mathworks.com

This tutorial goes into more detail: http://www.analog.com/media/en/training-seminars/tutorials/MT-002.pdf

And this one may be useful to clarify any doubt in the concepts presented before: http://www.lumerink.com/courses/ECE697A/docs/Matlab%20Filter%20Design%20and%20Implementation.pdf

hope this helps