Can we design Parks-McClellan Lowpass Filter with only sampling frequency of 12KHz and cutoff frequency of 4KHz and order of 30? I went through Matlab help, where we need parameter like passband cutoff frquency, stopband cutoff frequency, passband ripple e.t.c.,
The minimal specification for a FIR lowpass filter designed by the Parks McClellan algorithm is the passband edge, the stopband edge, and the filter order. You don't need to specify passband and stopband ripples; they are the result of the design process. What you can do is specify passband and stopband error weights, which let you specify the relation between passband and stopband approximation errors (a larger weight reduces the error).