Filter Design Steps

The user must do three things:

1. Make up the filter specifications in terms of band edges and desired values.
2. Select the filter order using a MATLAB to predict the order from the specs. For example, ellipord will predict the order of an elliptic filter.
3. Do the actual design. Call a function like ellip which in turn will first call ellipap to generate the analog prototype and then call bilinear to transform the filter from analog to digital. Since functions exist for the design of analog prototypes, MATLAB can also design the classical analog filters.

Give a reference.