#!/usr/bin/env python
# -*- encoding: utf-8 -*-
'''Filter construction and loading.
--------------------------------
`resampy` provides two pre-computed resampling filters which are tuned for either
high-quality or fast calculation:
- `kaiser_best` : 64 zero-crossings, a Kaiser window with beta=14.769656459379492,
and a roll-off frequency of Nyquist * 0.9475937167399596.
- `kaiser_fast` : 16 zero-crossings, a Kaiser window with beta=8.555504641634386,
and a roll-off frequency of Nyquist * 0.85.
These filters can be used by calling `resample` as follows:
>>> resampy.resample(x, sr_orig, sr_new, filter='kaiser_best') # High-quality
>>> resampy.resample(x, sr_orig, sr_new, filter='kaiser_fast') # Fast calculation
It is also possible to construct custom filters as follows:
>>> resampy.resample(x, sr_orig, sr_new, filter='sinc_window',
... **kwargs)
where ``**kwargs`` are additional parameters to `resampy.filters.sinc_window`_.
'''
import scipy.signal
import numpy as np
import os
import pkg_resources
import six
import sys
FILTER_FUNCTIONS = ['sinc_window']
__all__ = ['get_filter'] + FILTER_FUNCTIONS
[docs]def sinc_window(num_zeros=64, precision=9, window=None, rolloff=0.945):
'''Construct a windowed sinc interpolation filter
Parameters
----------
num_zeros : int > 0
The number of zero-crossings to retain in the sinc filter
precision : int > 0
The number of filter coefficients to retain for each zero-crossing
window : callable
The window function. By default, uses Blackman-Harris.
rolloff : float > 0
The roll-off frequency (as a fraction of nyquist)
Returns
-------
interp_window: np.ndarray [shape=(num_zeros * num_table + 1)]
The interpolation window (right-hand side)
num_bits: int
The number of bits of precision to use in the filter table
rolloff : float > 0
The roll-off frequency of the filter, as a fraction of Nyquist
Raises
------
TypeError
if `window` is not callable or `None`
ValueError
if `num_zeros < 1`, `precision < 1`,
or `rolloff` is outside the range `(0, 1]`.
Examples
--------
>>> # A filter with 10 zero-crossings, 32 samples per crossing, and a
... # Hann window for tapering.
>>> halfwin, prec, rolloff = resampy.filters.sinc_window(num_zeros=10, precision=5,
... window=scipy.signal.hann)
>>> halfwin
array([ 9.450e-01, 9.436e-01, ..., -7.455e-07, -0.000e+00])
>>> prec
32
>>> rolloff
0.945
>>> # Or using sinc-window filter construction directly in resample
>>> y = resampy.resample(x, sr_orig, sr_new, filter='sinc_window',
... num_zeros=10, precision=5,
... window=scipy.signal.hann)
'''
if window is None:
window = scipy.signal.blackmanharris
elif not six.callable(window):
raise TypeError('window must be callable, not type(window)={}'.format(type(window)))
if not 0 < rolloff <= 1:
raise ValueError('Invalid roll-off: rolloff={}'.format(rolloff))
if num_zeros < 1:
raise ValueError('Invalid num_zeros: num_zeros={}'.format(num_zeros))
if precision < 0:
raise ValueError('Invalid precision: precision={}'.format(precision))
# Generate the right-wing of the sinc
num_bits = 2**precision
n = num_bits * num_zeros
sinc_win = rolloff * np.sinc(rolloff * np.linspace(0, num_zeros, num=n + 1,
endpoint=True))
# Build the window function and cut off the left half
taper = window(2 * n + 1)[n:]
interp_win = (taper * sinc_win)
return interp_win, num_bits, rolloff
[docs]def get_filter(name_or_function, **kwargs):
'''Retrieve a window given its name or function handle.
Parameters
----------
name_or_function : str or callable
If a function, returns `name_or_function(**kwargs)`.
If a string, and it matches the name of one of the defined
filter functions, the corresponding function is called with `**kwargs`.
If a string, and it matches the name of a pre-computed filter,
the corresponding filter is retrieved, and kwargs is ignored.
Valid pre-computed filter names are:
- 'kaiser_fast'
- 'kaiser_best'
Returns
-------
half_window : np.ndarray
The right wing of the interpolation filter
precision : int > 0
The number of samples between zero-crossings of the filter
rolloff : float > 0
The roll-off frequency of the filter as a fraction of Nyquist
Raises
------
NotImplementedError
If `name_or_function` cannot be found as a filter.
'''
if name_or_function in FILTER_FUNCTIONS:
return getattr(sys.modules[__name__], name_or_function)(**kwargs)
elif six.callable(name_or_function):
return name_or_function(**kwargs)
else:
try:
return load_filter(name_or_function)
except (IOError, ValueError):
raise NotImplementedError('Cannot load filter definition for '
'{}'.format(name_or_function))
def load_filter(filter_name):
'''Retrieve a pre-computed filter.
Parameters
----------
filter_name : str
The key of the filter, e.g., 'kaiser_fast'
Returns
-------
half_window : np.ndarray
The right wing of the interpolation filter
precision : int > 0
The number of samples between zero-crossings of the fitler
rolloff : float > 0
The roll-off frequency of the filter, as a fraction of Nyquist
'''
fname = os.path.join('data',
os.path.extsep.join([filter_name, 'npz']))
data = np.load(pkg_resources.resource_filename(__name__, fname))
return data['half_window'], data['precision'], data['rolloff']