quadratic or cubic interpolator


    Dec 13 2006 | 4:52 am
    Hi, I'm implementing a continuously variable delay line. Unfortunately I am currently using linear interpolation between samples, and it's prety awful. Does anybody know of a good object for quadratic or cubic interpolation of signals?
    Thanks

    • Dec 13 2006 | 12:51 pm
      On Dec 12, 2006, at 11:52 PM, Tim wrote:
      > Hi, I'm implementing a continuously variable delay line.
      > Unfortunately I am currently using linear interpolation between
      > samples, and it's prety awful. Does anybody know of a good object
      > for quadratic or cubic interpolation of signals?
      I don't have an object for this, but I have some C code for an
      allpass interpolation. Would that help? Or do you know how to code
      externals?
      -----
      Nathan Wolek
      nw@nathanwolek.com
    • Dec 13 2006 | 6:16 pm
      I don't know if I'll have time to make an external, but the code may be helpful. How can I get it from you?
      thx
      tim.m
    • Dec 13 2006 | 6:38 pm
      On Dec 13, 2006, at 1:16 PM, Tim wrote:
      > I don't know if I'll have time to make an external, but the code
      > may be helpful. How can I get it from you?
      Here you go. Complete with reference on where I found the technique.
      /
      ************************************************************************
      ********
      float rbb_allpassInterp(float *in_array, float index, long bufferLength,
      float last_out, float *out_ptr)
      inputs: in_array -- name of array of input values
      index -- floating point index value to interpolate
      bufferLength -- length of in_array
      last_out -- value of last output from buffer
      out_ptr -- pointer to output location
      description: performs allpass interpolation on an input array and
      returns the
      results to a location specified by a pointer; implements filter as
      specified
      in Dattorro 2: J. Audio Eng. Soc., Vol 45, No 10, 1997 October
      returns: interpolated output
      ************************************************************************
      ********/
      void rbb_allpassInterp(float *in_array, float index, long bufferLength,
      float last_out, float *out_ptr)
      {
      // index = i.frac
      long index_i = (long)index; // i
      long index_iP1 = index_i + 1; // i + 1
      float index_frac = index - (float)index_i; // frac
      // make sure that index_iP1 is not out of range
      while (index_iP1 >= bufferLength) index_iP1 -= bufferLength;
      // formula as on bottom of page 765 of above Dattorro article
      *out_ptr = in_array[index_i] + index_frac * (in_array[index_iP1] -
      last_out);
      }
      -----
      Nathan Wolek
      nw@nathanwolek.com
    • Dec 14 2006 | 1:49 pm
      thanks for the code. It looks quite simple. Obviously a piece of cake in C (once you know) but the feedback aspect of the algorithm would make it pretty much impossible in MAX/MSP directly, I believe. Is that correct? From other posts I have seen it seems that a minimum feedback delay of 1 signal vector is required?
      Thanks.
    • Dec 14 2006 | 3:00 pm
      On Dec 14, 2006, at 8:49 AM, Tim wrote:
      > Obviously a piece of cake in C (once you know) but the feedback
      > aspect of the algorithm would make it pretty much impossible in MAX/
      > MSP directly, I believe. Is that correct? From other posts I have
      > seen it seems that a minimum feedback delay of 1 signal vector is
      > required?
      Yes. On both counts.
      -------------------
      Nathan Wolek, PhD --- nwolek@stetson.edu
      Assistant Professor of Music Technology
      Stetson University - DeLand, FL