# Hidden Markov Models

## Hidden Markov Models :

A hidden Markov model (HMM) is a statistical model that allows us to predict the likelihood of certain sequences of observations, given a set of underlying hidden states. This can be useful in many applications, such as speech recognition, financial forecasting, and biological sequence analysis.
To understand how HMMs work, let’s first consider a simple example of a dog that can be in one of two states: barking or not barking. We can’t observe the dog directly, so we can only infer its state based on the noises it makes. If the dog is barking, we will hear barking noises. If the dog is not barking, we will hear silence.
Now, let’s say we want to use an HMM to model this dog’s behavior. We can do this by defining a set of hidden states (barking and not barking) and a set of observations (barking noises and silence). We can then use the HMM to estimate the probabilities of transitioning between these hidden states and the probabilities of emitting the different observations, based on the current hidden state.
For instance, if the dog is barking, it is more likely to continue barking than to stop barking, so we would assign a higher probability to the transition from the barking state to itself than to the transition from the barking state to the not barking state. Similarly, if the dog is barking, it is more likely to emit barking noises than silence, so we would assign a higher probability to the emission of barking noises from the barking state than to the emission of silence from the barking state.
Once we have defined our HMM, we can use it to make predictions about the dog’s behavior. For example, if we hear a barking noise, we can use the HMM to calculate the probability that the dog is currently barking, given the previous observations and hidden states. We can then use this probability to make decisions, such as whether to let the dog inside or not.
Another example of how HMMs can be used is in speech recognition. In this case, the hidden states would represent the different phonemes (the smallest units of sound in a language) and the observations would be the acoustic signals produced by the speaker. For example, if the hidden state is the phoneme “b”, the HMM would assign a higher probability to the observation of the sound “b” being produced, compared to the sound “a” being produced.
Using HMMs in speech recognition allows us to take into account the fact that different sounds can be produced by the same phoneme, depending on the context in which it is spoken. For instance, the sound “b” can be produced differently in the words “bat” and “bit”. The HMM would account for these variations and use them to make more accurate predictions about the phonemes being spoken.
Overall, HMMs are a powerful tool for modeling and predicting sequences of observations, given underlying hidden states. They have been applied in a wide range of fields, from speech recognition to financial forecasting, and have proven to be effective in many cases.