BUG: freqz does not work on multi-dimensional array, despite what documentation claims

[sliedes]

Describe your issue.

freqz documentation seems to imply that it supports multidimensional inputs as long as the first dimension is the filter coeffients:

b: array_like

Numerator of a linear filter. If b has dimension greater than 1, it is assumed that the coefficients are stored in the first dimension, and b.shape[1:], a.shape[1:], and the shape of the frequencies array must be compatible for broadcasting.

However, freqz fails when given a 2-dimensional array.

Reproducing Code Example

import scipy
import numpy as np

scipy.signal.freqz(np.zeros((128, 10)))

Error message

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/sami/.virtualenvs/tmp/lib/python3.10/site-packages/scipy/signal/_filter_design.py", line 474, in freqz
    h = (npp_polyval(zm1, b, tensor=False) /
  File "/home/sami/.virtualenvs/tmp/lib/python3.10/site-packages/numpy/polynomial/polynomial.py", line 754, in polyval
    c0 = c[-1] + x*0
ValueError: operands could not be broadcast together with shapes (10,) (512,)

SciPy/NumPy/Python version information

1.9.3 1.23.4 sys.version_info(major=3, minor=10, micro=6, releaselevel='final', serial=0)

[tupui]

Hi @sliedes, thank you for reporting. The documentation could probably be improved, if you have a look at the broadcasting example at the end of the docstrings, you need to be careful with the shape (provided this makes any sense, the following would work scipy.signal.freqz(np.zeros((128, 10, 1)))). Does that help? https://scipy.github.io/devdocs/reference/generated/scipy.signal.freqz.html

We could maybe improve the error messaging here.

[tinaoberoi]

Hi @tupui ,
Is anybody working on this? If not would love to take this up.

[tupui]

Hi @tinaoberoi, you are more than welcome to work on this. We don't do issue/work assignment, we just make sure there is just 1 PR open for a given task so there is no competing work.