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Signal Modifiers: Standard Filters

filter2, zfilter2

  kr      filter2    ksig, iM, iN, ib0, ib1, ..., ibM, ia1, ia2, ..., iaN
  ar      filter2    asig, iM, iN, ib0, ib1, ..., ibM, ia1, ia2, ..., iaN
  ar      zfilter2   asig, kdamp, kfreq, iM, iN, ib0, ib1, ..., ibM, ia1,ia2, ..., iaN


Description

General purpose custom filter with time-varying pole control. The filter coefficients implement the following difference equation:

  (1)*y(n) = b0*x[n] + b1*x[n-1] +...+ bM*x[n-M] - a1*y[n-1] -...- aN*y[n-N]

the system function for which is represented by:

           B(Z)      b0 + b1*Z-1  + ... + bM*Z-M+1
  H(Z)  =  ----  =  --------------------------
           A(Z)       1 + a1*Z-1  + ... + aN*Z-N

Initialization

At initialization the number of zeros and poles of the filter are specified along with the corresponding zero and pole coefficients. The coefficients must be obtained by an external filter-design application such as Matlab and specified directly or loaded into a table via GEN01. With zfilter2, the roots of the characteristic polynomials are solved at initialization so that the pole-control operations can be implemented efficiently.

Performance

The filter2 opcodes perform filtering using a transposed form-II digital filter lattice with no time-varying control. zfilter2 uses the additional operations of radial pole-shearing and angular pole-warping in the Z plane.

Pole shearing increases the magnitude of poles along radial lines in the Z-plane. This has the affect of altering filter ring times. The k-rate variable kdamp is the damping parameter. Positive values (0.01 to 0.99) increase the ring-time of the filter (hi-Q), negative values (-0.01 to -0.99) decrease the ring-time of the filter, (lo-Q).

Pole warping changes the frequency of poles by moving them along angular paths in the Z plane. This operation leaves the shape of the magnitude response unchanged but alters the frequencies by a constant factor (preserving 0 and p). The k-rate variable kfreq determines the frequency warp factor. Positive values (0.01 to 0.99) increase frequencies toward p and negative values (-0.01 to -0.99) decrease frequencies toward 0.

Since filter2 implements generalized recursive filters, it can be used to specify a large range of general DSP algorithms. For example, a digital waveguide can be implemented for musical instrument modeling using a pair of delayr and delayw opcodes in conjunction with the filter2 opcode.

Examples

A first-order linear-phase lowpass linear-phase FIR filter operating on a k-rate signal:

  k1      filter2    ksig, 2, 0, 0.5, 0.5   ;; k-rate FIR filter

A controllable second-order IIR filter operating on an a-rate signal:

  a1      zfilter2   asig, kdamp, kfreq, 1, 2, 1, ia1, ia2 ; controllable a-rate   ; IIR filter

Deprecated Names

The k-rate version of filter2 was originally called kfilter2. As of Csound version 3.493, this name is deprecated. filter2 should be used instead of kfilter2. The opcode determines its rate from the output argument.

Author

Michael A. Casey
M.I.T.
Cambridge, Mass.
1997


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Signal Modifiers: Standard Filters