CoCalc -- aliasing.py

Path: examples/think-dsp/code / aliasing.py
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"""This file contains code used in "Think DSP",
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by Allen B. Downey, available from greenteapress.com
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Copyright 2013 Allen B. Downey
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License: GNU GPLv3 http://www.gnu.org/licenses/gpl.html
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"""
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from __future__ import print_function, division
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import thinkdsp
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import thinkplot
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FORMATS = ['pdf', 'png']
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def triangle_example(freq):
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    """Makes a figure showing a triangle wave.
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    freq: frequency in Hz
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    """
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    framerate = 10000
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    signal = thinkdsp.TriangleSignal(freq)
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    duration = signal.period*3
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    segment = signal.make_wave(duration, framerate=framerate)
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    segment.plot()
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    thinkplot.save(root='triangle-%d-1' % freq,
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                   xlabel='Time (s)',
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                   axis=[0, duration, -1.05, 1.05])
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    wave = signal.make_wave(duration=0.5, framerate=framerate)
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    spectrum = wave.make_spectrum()
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    thinkplot.preplot(cols=2)
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    spectrum.plot()
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    thinkplot.config(xlabel='Frequency (Hz)',
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                     ylabel='Amplitude')
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    thinkplot.subplot(2)
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    spectrum.plot()
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    thinkplot.config(ylim=[0, 500],
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                     xlabel='Frequency (Hz)')
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    thinkplot.save(root='triangle-%d-2' % freq)
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def square_example(freq):
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    """Makes a figure showing a square wave.
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    freq: frequency in Hz
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    """
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    framerate = 10000
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    signal = thinkdsp.SquareSignal(freq)
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    duration = signal.period*3
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    segment = signal.make_wave(duration, framerate=framerate)
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    segment.plot()
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    thinkplot.save(root='square-%d-1' % freq,
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                   xlabel='Time (s)',
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                   axis=[0, duration, -1.05, 1.05])
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    wave = signal.make_wave(duration=0.5, framerate=framerate)
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    spectrum = wave.make_spectrum()
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    spectrum.plot()
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    thinkplot.save(root='square-%d-2' % freq,
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                   xlabel='Frequency (Hz)',
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                   ylabel='Amplitude')
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def aliasing_example(offset=0.000003):
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    """Makes a figure showing the effect of aliasing.
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    """
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    framerate = 10000
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    def plot_segment(freq):
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        signal = thinkdsp.CosSignal(freq)
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        duration = signal.period*4
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        thinkplot.Hlines(0, 0, duration, color='gray')
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        segment = signal.make_wave(duration, framerate=framerate*10)
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        segment.plot(linewidth=0.5, color='gray')
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        segment = signal.make_wave(duration, framerate=framerate)
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        segment.plot_vlines(label=freq, linewidth=4)
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    thinkplot.preplot(rows=2)
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    plot_segment(4500)
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    thinkplot.config(axis=[-0.00002, 0.0007, -1.05, 1.05])
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    thinkplot.subplot(2)
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    plot_segment(5500)
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    thinkplot.config(axis=[-0.00002, 0.0007, -1.05, 1.05])
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    thinkplot.save(root='aliasing1',
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                   xlabel='Time (s)',
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                   formats=FORMATS)
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def main():
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    triangle_example(freq=200)
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    triangle_example(freq=1100)
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    square_example(freq=100)
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    aliasing_example()
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if __name__ == '__main__':
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    main()
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