Determine whether filter is allpass


flag = isallpass(b,a)
flag = isallpass(sos)
flag = isallpass(d)
flag = isallpass(...,tol)
flag = isallpass(hs,...)
flag = isallpass(hs,'Arithmetic',arithtype)


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.

flag = isallpass(hs,...) returns true if the filter System object™, hs, is an allpass filter. You must have the DSP System Toolbox™ software to use this syntax.

flag = isallpass(hs,'Arithmetic',arithtype) analyzes the filter System object hs based on the specified arithtype. arithtype can be 'double', 'single', or 'fixed'. When you specify 'double' or 'single', the function performs double- or single-precision analysis. When you specify 'fixed' , the arithmetic changes depending on the setting of the CoefficientDataType property and whether the System object is locked or unlocked. You must have the DSP System Toolbox software to use this syntax.

 Details for Fixed-Point Arithmetic

When you do not specify the arithmetic for non-CIC structures, the function uses double-precision arithmetic if the filter System object is in an unlocked state. If the System object is locked, the function performs analysis based on the locked input data type. CIC structures only support fixed-point arithmetic.


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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