Acoustic Model Types

CMUSphinx supports different types of the acoustic models: continuous, semi-continuous and phonetically tied (PTM).

The difference between PTM, semi-continuous and continuous models is the following. We use mixture of gaussians to compute the score of each frame, the difference is how do we build such mixture. In continuous model every senone has it’s own set of gaussians thus the total number of gaussians in the model is about 150 thousand. That’s too much to compute the mixture efficiently. In semi-continuous model we have just 700 gaussians, way less than in continuous and we only use them with different mixtures to score the frame. Due to the smaller number of gaussians semi-continuous models are fast, but because of more hardcoded structure they are also a bit less accurate.

PTM models is a gold middle here. It uses about 5000 gaussians thus providing better accuracy than semi-continuous, but it is still significantly faster than continuous thus it can be used in mobile applications. Accuracy of PTM model is almost the same as accuracy of continuous model.

In modern packages of pocketsphinx and sphinx4 PTM models are used by default.

To understand the model type you need to look in feat.params file inside the model. PTM models have -model ptm there. Continuous model have -feat 1s_c_d_dd. You can also infer the type from the model name, semi-continuous models usually have “semi” in the name.