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# isallpass() - Signal Processing

```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

System Object StateCoefficient Data TypeRule
Unlocked'Same as input'The function assumes that the coefficient data type is signed,
16 bit, and autoscaled. The function performs fixed-point analysis
based on this assumption.
Unlocked'Custom'The function performs fixed-point analysis based on the setting
of the CustomCoefficientsDataType property.
Locked'Same as input'When the input data type is 'double' or 'fixed',
the function assumes that the coefficient data type is signed, 16-bit,
and autoscaled. The function performs fixed-point analysis based on
this assumption.
Locked'Custom'The function performs fixed-point analysis based on the setting
of the CustomCoefficientsDataType property.

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.flag = isallpass(hd) returns true if
the filter object hd is an allpass filter.```

### Syntax

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

### Example

```Allpass FiltersOpen This Example
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)
fvtool(b,a)

flag =

1

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)
fvtool(b,a)

flag_isallpass =

1```