One can model the phonocardiogram signal as a four state HMM. The first state corresponds to the S1 sound, the second state corresponds to the silence during the systolic period, the third state corresponds to the S2 sound, and the fourth state corresponds to the silence during the diastolic period (see Figure 1). This model ignores the possibility of the S3 and S4 heart sounds, because these heart sounds are not germane to the task of recognizing respiration rates from heart sound data.
Additionally, these sounds are difficult to hear and record; therefore, they are most likely not noticeable in our heart sound data.This four state HMM is useful for modeling the sequence of symbols (or labels) of the phonocardiogram; however, it is too simple to accurately model the transitions between sound and silence. One solution is to embed another HMM inside of each of the heart sound symbol states. The embedded HMM models the signal as it traverses a specific labeled region of the signal. Using this combined approach, we can model both the high level state sequence of our signal (S1-sil-S2-sil) and the continuous transitions of the signal. This type of model is similar to how a speech processing system has a highlevel probabilistic grammar to model the transition of words or phonemes, and an embedded HMM for each phoneme [
One can model the phonocardiogram signal as a four state HMM. The first state corresponds to the S1 sound, the second state corresponds to the silence during the systolic period, the third state corresponds to the S2 sound, and the fourth state corresponds to the silence during the diastolic period (see Figure 1). This model ignores the possibility of the S3 and S4 heart sounds, because these heart sounds are not germane to the task of recognizing respiration rates from heart sound data.Additionally, these sounds are difficult to hear and record; therefore, they are most likely not noticeable in our heart sound data.This four state HMM is useful for modeling the sequence of symbols (or labels) of the phonocardiogram; however, it is too simple to accurately model the transitions between sound and silence. One solution is to embed another HMM inside of each of the heart sound symbol states. The embedded HMM models the signal as it traverses a specific labeled region of the signal. Using this combined approach, we can model both the high level state sequence of our signal (S1-sil-S2-sil) and the continuous transitions of the signal. This type of model is similar to how a speech processing system has a highlevel probabilistic grammar to model the transition of words or phonemes, and an embedded HMM for each phoneme [
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