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138 lines
4.8 KiB
Python
138 lines
4.8 KiB
Python
# Author: Eric Larson
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# 2014
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"""Tools for MLS generation"""
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import numpy as np
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from ._max_len_seq_inner import _max_len_seq_inner
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__all__ = ['max_len_seq']
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# These are definitions of linear shift register taps for use in max_len_seq()
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_mls_taps = {2: [1], 3: [2], 4: [3], 5: [3], 6: [5], 7: [6], 8: [7, 6, 1],
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9: [5], 10: [7], 11: [9], 12: [11, 10, 4], 13: [12, 11, 8],
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14: [13, 12, 2], 15: [14], 16: [15, 13, 4], 17: [14],
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18: [11], 19: [18, 17, 14], 20: [17], 21: [19], 22: [21],
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23: [18], 24: [23, 22, 17], 25: [22], 26: [25, 24, 20],
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27: [26, 25, 22], 28: [25], 29: [27], 30: [29, 28, 7],
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31: [28], 32: [31, 30, 10]}
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def max_len_seq(nbits, state=None, length=None, taps=None):
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"""
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Maximum length sequence (MLS) generator.
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Parameters
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----------
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nbits : int
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Number of bits to use. Length of the resulting sequence will
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be ``(2**nbits) - 1``. Note that generating long sequences
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(e.g., greater than ``nbits == 16``) can take a long time.
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state : array_like, optional
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If array, must be of length ``nbits``, and will be cast to binary
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(bool) representation. If None, a seed of ones will be used,
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producing a repeatable representation. If ``state`` is all
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zeros, an error is raised as this is invalid. Default: None.
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length : int, optional
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Number of samples to compute. If None, the entire length
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``(2**nbits) - 1`` is computed.
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taps : array_like, optional
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Polynomial taps to use (e.g., ``[7, 6, 1]`` for an 8-bit sequence).
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If None, taps will be automatically selected (for up to
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``nbits == 32``).
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Returns
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-------
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seq : array
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Resulting MLS sequence of 0's and 1's.
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state : array
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The final state of the shift register.
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Notes
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-----
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The algorithm for MLS generation is generically described in:
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https://en.wikipedia.org/wiki/Maximum_length_sequence
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The default values for taps are specifically taken from the first
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option listed for each value of ``nbits`` in:
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http://www.newwaveinstruments.com/resources/articles/m_sequence_linear_feedback_shift_register_lfsr.htm
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.. versionadded:: 0.15.0
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Examples
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--------
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MLS uses binary convention:
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>>> from scipy.signal import max_len_seq
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>>> max_len_seq(4)[0]
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array([1, 1, 1, 1, 0, 1, 0, 1, 1, 0, 0, 1, 0, 0, 0], dtype=int8)
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MLS has a white spectrum (except for DC):
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>>> import matplotlib.pyplot as plt
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>>> from numpy.fft import fft, ifft, fftshift, fftfreq
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>>> seq = max_len_seq(6)[0]*2-1 # +1 and -1
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>>> spec = fft(seq)
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>>> N = len(seq)
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>>> plt.plot(fftshift(fftfreq(N)), fftshift(np.abs(spec)), '.-')
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>>> plt.margins(0.1, 0.1)
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>>> plt.grid(True)
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>>> plt.show()
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Circular autocorrelation of MLS is an impulse:
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>>> acorrcirc = ifft(spec * np.conj(spec)).real
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>>> plt.figure()
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>>> plt.plot(np.arange(-N/2+1, N/2+1), fftshift(acorrcirc), '.-')
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>>> plt.margins(0.1, 0.1)
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>>> plt.grid(True)
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>>> plt.show()
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Linear autocorrelation of MLS is approximately an impulse:
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>>> acorr = np.correlate(seq, seq, 'full')
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>>> plt.figure()
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>>> plt.plot(np.arange(-N+1, N), acorr, '.-')
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>>> plt.margins(0.1, 0.1)
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>>> plt.grid(True)
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>>> plt.show()
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"""
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if taps is None:
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if nbits not in _mls_taps:
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known_taps = np.array(list(_mls_taps.keys()))
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raise ValueError('nbits must be between %s and %s if taps is None'
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% (known_taps.min(), known_taps.max()))
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taps = np.array(_mls_taps[nbits], np.intp)
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else:
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taps = np.unique(np.array(taps, np.intp))[::-1]
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if np.any(taps < 0) or np.any(taps > nbits) or taps.size < 1:
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raise ValueError('taps must be non-empty with values between '
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'zero and nbits (inclusive)')
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taps = np.ascontiguousarray(taps) # needed for Cython
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n_max = (2**nbits) - 1
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if length is None:
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length = n_max
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else:
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length = int(length)
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if length < 0:
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raise ValueError('length must be greater than or equal to 0')
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# We use int8 instead of bool here because NumPy arrays of bools
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# don't seem to work nicely with Cython
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if state is None:
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state = np.ones(nbits, dtype=np.int8, order='c')
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else:
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# makes a copy if need be, ensuring it's 0's and 1's
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state = np.array(state, dtype=bool, order='c').astype(np.int8)
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if state.ndim != 1 or state.size != nbits:
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raise ValueError('state must be a 1-D array of size nbits')
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if np.all(state == 0):
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raise ValueError('state must not be all zeros')
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seq = np.empty(length, dtype=np.int8, order='c')
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state = _max_len_seq_inner(taps, state, nbits, length, seq)
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return seq, state
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