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Determine whether filter is allpass


flag = isallpass(b,a)
flag = isallpass(sos)
flag = isallpass(d)
flag = isallpass(...,tol)


flag = isallpass(b,a) returns a logical output, flag, equal to true if the filter specified by numerator coefficients, b, and denominator coefficients, a, is an allpass filter. If the filter is not an allpass filter, flag is equal to false.

flag = isallpass(sos) returns true if the filter specified by second order sections matrix, sos, is an allpass filter. sos is a K-by-6 matrix, where the number of sections, K, must be greater than or equal to 2. Each row of sos corresponds to the coefficients of a second order (biquad) filter. The ith row of the sos matrix corresponds to [bi(1) bi(2) bi(3) ai(1) ai(2) ai(3)].

flag = isallpass(d) returns true if the digital filter, d, is an allpass filter. Use designfilt to generate d based on frequency-response specifications.

flag = isallpass(...,tol) uses the tolerance, tol, to determine when two numbers are close enough to be considered equal. If not specified, tol, defaults to eps^(2/3). Specifying a tolerance may be most helpful in fixed-point allpass filters.


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Create an allpass filter and verify that the frequency response is allpass.

b = [1/3 1/4 1/5 1];
a = fliplr(b); 
flag = isallpass(b,a)
flag = logical


Create a lattice allpass filter and verify that the filter is allpass.

k = [1/2 1/3 1/4 1/5];
[b,a] = latc2tf(k,'allpass');
flag_isallpass = isallpass(b,a)
flag_isallpass = logical


Introduced in R2013a