Filter Design Toolbox™

convert - Convert filter structure of discrete-time or multirate filter

Syntax

hq = convert(hq,newstruct) hm = convert(hm,newstruct)

Description

Discrete-Time Filters

hq = convert(hq,newstruct) returns a quantized filter whose structure has been transformed to the filter structure specified by string newstruct. You can enter any one of the following quantized filter structures:

'antisymmetricfir': Antisymmetric finite impulse response (FIR).
'df1': Direct form I.
'df1t': Direct form I transposed.
'df2': Direct form II.
'df2t': Direct form II transposed. Default filter structure.
'dffir': FIR.
'dffirt': Direct form FIR transposed.
'latcallpass': Lattice allpass.
'latticeca': Lattice coupled-allpass.
'latticecapc': Lattice coupled-allpass power-complementary.
'latticear': Lattice autoregressive (AR).
'latticema': Lattice moving average (MA) minimum phase.
'latcmax': Lattice moving average (MA) maximum phase.
'latticearma': Lattice ARMA.
'statespace': Single-input/single-output state-space.
'symmetricfir': Symmetric FIR. Even and odd forms.

All filters can be converted to the following structures:

df1
df1t
df2
df2t
statespace
latticearma

For the following filter classes, you can specify other conversions as well:

Minimum phase FIR filters can be converted to latticema
Maximum phase FIR filters can be converted to latcmax
Allpass filters can be converted to latcallpass

convert generates an error when you specify a conversion that is not possible.

Multirate Filters

hm = convert(hm,newstruct) returns a multirate filter whose structure has been transformed to the filter structure specified by string newstruct. You can enter any one of the following multirate filter structures, defined by the strings shown, for newstruct:

Cascaded Integrator-Comb Structure

cicdecim — CIC-based decimator
cicinterp — CIC-based interpolator

FIR Structures

firdecim — FIR decimator
firtdecim — transposed FIR decimator
firfracdecim — FIR fractional decimator
firinterp — FIR interpolator
firfracinterp — FIR fractional interpolator
firsrc — FIR sample rate change filter
firholdinterp — FIR interpolator that uses hold interpolation between input samples
firlinearinterp — FIR interpolator that uses linear interpolation between input samples
fftfirinterp — FFT-based FIR interpolator

You cannot convert between the FIR and CIC structures.

Examples

[b,a]=ellip(5,3,40,.7);
hq = dfilt.df2t(b,a);
hq2 = convert(hq,'df1')
hq2 =
 
         FilterStructure: 'Direct-Form I'
              Arithmetic: 'double'
               Numerator: [0.1980 0.7886 1.4236 1.4236 0.7886 0.1980]
             Denominator: [1 1.4339 1.8021 0.6139 0.2047 -0.2342]
        PersistentMemory: false
                  States: Numerator:  [5x1 double]
                          Denominator:[5x1 double]

For an example of changing the structure of a multirate filter, try the following conversion from a CIC interpolator to a CIC interpolator with zero latency.