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# -*- coding: utf-8 -*-
"""
================
Viterbi decoding
================
This notebook demonstrates how to use Viterbi decoding to impose temporal
smoothing on frame-wise state predictions.
Our working example will be the problem of silence/non-silence detection.
"""
# Code source: Brian McFee
# License: ISC
##################
# Standard imports
import numpy as np
import matplotlib.pyplot as plt
import librosa
#############################################
# Load an example signal
y, sr = librosa.load(librosa.ex('trumpet'))
# And compute the spectrogram magnitude and phase
S_full, phase = librosa.magphase(librosa.stft(y))
###################
# Plot the spectrum
fig, ax = plt.subplots()
img = librosa.display.specshow(librosa.amplitude_to_db(S_full, ref=np.max),
y_axis='log', x_axis='time', sr=sr, ax=ax)
fig.colorbar(img, ax=ax);
###########################################################
# As you can see, there are periods of silence and
# non-silence throughout this recording.
#
# As a first step, we can plot the root-mean-square (RMS) curve
rms = librosa.feature.rms(y=y)[0]
times = librosa.frames_to_time(np.arange(len(rms)))
fig, ax = plt.subplots()
ax.plot(times, rms)
ax.axhline(0.02, color='r', alpha=0.5)
ax.set(xlabel='Time', ylabel='RMS');
##############################################################################
# The red line at 0.02 indicates a reasonable threshold for silence detection.
# However, the RMS curve occasionally dips below the threshold momentarily,
# and we would prefer the detector to not count these brief dips as silence.
# This is where the Viterbi algorithm comes in handy!
#####################################################
# As a first step, we will convert the raw RMS score
# into a likelihood (probability) by logistic mapping
#
# :math:`P[V=1 | x] = \frac{\exp(x - \tau)}{1 + \exp(x - \tau)}`
#
# where :math:`x` denotes the RMS value and :math:`\tau=0.02` is our threshold.
# The variable :math:`V` indicates whether the signal is non-silent (1) or silent (0).
#
# We'll normalize the RMS by its standard deviation to expand the
# range of the probability vector
r_normalized = (rms - 0.02) / np.std(rms)
p = np.exp(r_normalized) / (1 + np.exp(r_normalized))
##############################################
# We can plot the probability curve over time:
fig, ax = plt.subplots()
ax.plot(times, p, label='P[V=1|x]')
ax.axhline(0.5, color='r', alpha=0.5, label='Descision threshold')
ax.set(xlabel='Time')
ax.legend();
#######################################################################
# which looks much like the first plot, but with the decision threshold
# shifted to 0.5. A simple silence detector would classify each frame
# independently of its neighbors, which would result in the following plot:
plt.figure(figsize=(12, 6))
fig, ax = plt.subplots(nrows=2, sharex=True)
librosa.display.specshow(librosa.amplitude_to_db(S_full, ref=np.max),
y_axis='log', x_axis='time', sr=sr, ax=ax[0])
ax[0].label_outer()
ax[1].step(times, p>=0.5, label='Non-silent')
ax[1].set(ylim=[0, 1.05])
ax[1].legend()
###############################################
# We can do better using the Viterbi algorithm.
# We'll use state 0 to indicate silent, and 1 to indicate non-silent.
# We'll assume that a silent frame is equally likely to be followed
# by silence or non-silence, but that non-silence is slightly
# more likely to be followed by non-silence.
# This is accomplished by building a self-loop transition matrix,
# where `transition[i, j]` is the probability of moving from state
# `i` to state `j` in the next frame.
transition = librosa.sequence.transition_loop(2, [0.5, 0.6])
print(transition)
#####################################################################
# Our `p` variable only indicates the probability of non-silence,
# so we need to also compute the probability of silence as its complement.
full_p = np.vstack([1 - p, p])
print(full_p)
####################################
# Now, we're ready to decode!
# We'll use `viterbi_discriminative` here, since the inputs are
# state likelihoods conditional on data (in our case, data is rms).
states = librosa.sequence.viterbi_discriminative(full_p, transition)
# sphinx_gallery_thumbnail_number = 5
fig, ax = plt.subplots(nrows=2, sharex=True)
librosa.display.specshow(librosa.amplitude_to_db(S_full, ref=np.max),
y_axis='log', x_axis='time', sr=sr, ax=ax[0])
ax[0].label_outer()
ax[1].step(times, p>=0.5, label='Frame-wise')
ax[1].step(times, states, linestyle='--', color='orange', label='Viterbi')
ax[1].set(ylim=[0, 1.05])
ax[1].legend()
#########################################################################
# Note how the Viterbi output has fewer state changes than the frame-wise
# predictor, and it is less sensitive to momentary dips in energy.
# This is controlled directly by the transition matrix.
# A higher self-transition probability means that the decoder is less
# likely to change states.
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