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sos

Convert quantized filter to second–order sections (SOS) form

Syntax

Hq2 = sos(Hq)
Hq2 = sos(Hq, order)
Hq2 = sos(Hq, order, scale)

Description

Hq2 = sos(Hq) returns a quantized filter Hq2 that has second-order sections and the dft2 structure. Use the same optional arguments used in tf2sos.

Hq2 = sos(Hq, order) specifies the order of the sections in Hq2, where order is one of the following options:

  • 'down' — to order the sections so the first section of Hq2 contains the poles closest to the unit circle (L norm scaling)

  • 'up' — to order the sections so the first section of Hq2 contains the poles farthest from the unit circle (L2 norm scaling and the default)

Hq2 = sos(Hq, order, scale) also specifies the desired scaling of the gain and numerator coefficients of all second-order sections, where scale is one of the following options:

  • 'none' — to apply no scaling (default)

  • 'inf' — to apply infinity-norm scaling

  • 'two' — to apply 2-norm scaling

Hq2 is a dsp.BiquadFilter filter System object.

Use infinity-norm scaling in conjunction with up-ordering to minimize the probability of overflow in the filter realization. Consider using 2-norm scaling in conjunction with down-ordering to minimize the peak round-off noise.

When Hq is a fixed-point filter, the filter coefficients are normalized so that the magnitude of the maximum coefficient in each section is 1. The gain of the filter is applied to the first scale value of Hq2.

sos uses the direct form II transposed (dft2) structure to implement second- order section filters.

Examples

[b,a]=butter(8,.5);
Hq = dfilt.df2t(b,a);
Hq.arithmetic = 'fixed';
Hq1 = sos(Hq)

Hq1 =
 
         FilterStructure: 'Direct-Form II Transposed, Second-Order Sections'
              Arithmetic: 'double'                                          
               sosMatrix: [4x6 double]                                      
             ScaleValues: [0.00927734375;1;1;1;1]                           
     OptimizeScaleValues: true                                              
        PersistentMemory: false

See Also

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Introduced in R2011a