04-rms-tracing.py - Auto-wah effect.
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The auto-wah effect (also know as "envelope following filter") is like a
wah-wah effect, but instead of being controlled by a pedal, it is the RMS
amplitude of the input sound which control it. The envelope follower (RMS)
is rescaled and used to change the frequency of a bandpass filter applied
to the source.

.. code-block:: python

    from pyo import *
    
    s = Server().boot()
    
    MINFREQ = 250
    MAXFREQ = 5000
    
    # Play the drum lopp.
    sf = SfPlayer("../snds/drumloop.wav", loop=True)
    
    # Follow the amplitude envelope of the input sound.
    follow = Follower(sf)
    
    # Scale the amplitude envelope (0 -> 1) to the desired frequency
    # range (MINFREQ -> MAXFREQ).
    freq = Scale(follow, outmin=MINFREQ, outmax=MAXFREQ)
    
    # Filter the signal with a band pass. Play with the Q to make the
    # effect more or less present.
    filter = ButBP(sf.mix(2), freq=freq, q=2).out()
    
    s.gui(locals())