Scala for Machine Learning - Second Edition

The hidden Markov model (HMM)

A HMM is indeed a Markov process (also known as a Markov chain) for observations with a discrete time. The main difference with the Markov processes is that the states are not observable. A new observation is emitted with a probability known as the emission probability, each time the state of the system or model changes.

There are now two sources of randomness:

• Transition between states
• Emission of an observation when a state is given

Let's reuse the boxes and balls example. If the boxes are hidden states (non-observable), then the user draws the balls whose color is not visible. The emission probability is the probability b_ik =p(o_t = color_k | q_t =Si) to retrieve a ball of the color k from a hidden box I, as described ...