def pitch_from_midinum(m):
    """Return pitch of midi note number

    Midi note numbers go from 0-127, middle C is 60.  Given a note number
    this function computes the frequency.  The return value is a floating
    point number.

    """
    if m is None:
        return None
    return 440.0 * 2.0 ** ((m - 69.0) / 12.0)
