Re: digital differentiator using remez

[Paul Kienzle]

On Feb 8, 2007, at 2:17 AM, Ron Crummett wrote:

> Hi -
>
> I am trying to design a 4th order wideband differentiator using the
> remez function.

See below for a patch.

>   I type in
> > > b = remez(4, [0 0.75], [0 0.75*pi], [], 'differentiator');
>
> And I get this response:
>     error: invalid conversion from matrix to real vector
>     error: remez: need one weight for each band [=length(band)/2]
>     error: evaluating assignment expression near line 22, column 3

I think this error is because 'differentiator' returns true
for is_matrix_value(), but doesn't want to be converted to
a vector for some reason.  I get around this in the new code
by checking first if it is a string.

> My only guess is that I need something in the weighting entry, so I
> change it to
> > > b = remez(4, [0 0.75], [0 0.75*pi], 1, 'differentiator');
>
> Which gives me the message:
>     error: remez: griddensity is too low; must be greater than 16
>     error: evaluating assignment expression near line 22, column 3

I was trying to be clever: scalar => grid density,
vector => weights; unfortunately this breaks when
you only have one band and the vector is of length 1.

I couldn't stomach the firpm solution to this problem:
put grid density in a cell array, and instead force
you to type ...,'bandpass',griddensity) if you want
to set the grid density.

> A third attempt:
> > > b = remez(4, [0 0.75], [0 0.75*pi], 1, 'differentiator', 64);
>     error: remez: incorrect argument list
>     error: evaluating assignment expression near line 22, column 3

Same problem as above.

> At this point I am out of ideas of what I need to change.  Has anyone
> had any success with this?  I am running 2.9.6 on Kubuntu breezy.  
> Thanks.


I'm posting the following patch to octave-forge cvs:

Index: remez.cc
===================================================================
RCS file: /cvsroot/octave/octave-forge/main/signal/src/remez.cc,v
retrieving revision 1.1
diff -c -p -r1.1 remez.cc
*** remez.cc    20 Aug 2006 13:58:25 -0000      1.1
--- remez.cc    9 Feb 2007 03:33:35 -0000
*************** Frequency is in the range (0, 1), with 1
*** 817,823 ****
    OCTAVE_LOCAL_BUFFER (double, weight, numbands);
    for (i=0; i < numbands; i++) weight[i] = 1.0;
    if (nargin > 3) {
!     if (args(3).is_real_matrix()) {
        ColumnVector o_weight(args(3).vector_value());
        if (o_weight.length() != numbands) {
        error("remez: need one weight for each band [=length(band)/2]");
--- 817,825 ----
    OCTAVE_LOCAL_BUFFER (double, weight, numbands);
    for (i=0; i < numbands; i++) weight[i] = 1.0;
    if (nargin > 3) {
!     if (args(3).is_string())
!       stype = args(3).string_value();
!     else if (args(3).is_real_matrix() || args(3).is_real_scalar()) {
        ColumnVector o_weight(args(3).vector_value());
        if (o_weight.length() != numbands) {
        error("remez: need one weight for each band [=length(band)/2]");
*************** Frequency is in the range (0, 1), with 1
*** 825,834 ****
        }
        for (i=0; i < numbands; i++) weight[i] = o_weight(i);
      }
-     else if (args(3).is_string())
-       stype = args(3).string_value();
-     else if (args(3).is_real_scalar())
-       density = NINT(args(3).double_value());
      else {
        error("remez: incorrect argument list");
        return retval;
--- 827,832 ----
*************** Frequency is in the range (0, 1), with 1
*** 837,843 ****
    if (nargin > 4) {
      if (args(4).is_string() && !args(3).is_string())
        stype = args(4).string_value();
!     else if (args(4).is_real_scalar() && !args(3).is_real_scalar())
        density = NINT(args(4).double_value());
      else {
        error("remez: incorrect argument list");
--- 835,841 ----
    if (nargin > 4) {
      if (args(4).is_string() && !args(3).is_string())
        stype = args(4).string_value();
!     else if (args(4).is_real_scalar())
        density = NINT(args(4).double_value());
      else {
        error("remez: incorrect argument list");
*************** Frequency is in the range (0, 1), with 1
*** 845,854 ****
      }
    }
    if (nargin > 5) {
!     if (args(5).is_real_scalar()
!       && !args(4).is_real_scalar()
!       && !args(3).is_real_scalar())
!       density = NINT(args(4).double_value());
      else {
        error("remez: incorrect argument list");
        return retval;
--- 843,851 ----
      }
    }
    if (nargin > 5) {
!     if (args(5).is_real_scalar()
!       && !args(4).is_real_scalar())
!       density = NINT(args(5).double_value());
      else {
        error("remez: incorrect argument list");
        return retval;