HTK (Hidden Markov Model Toolkit) is a software toolkit used for building and working with Hidden Markov Model (HMM)-based speech recognition systems. The HTK toolkit also includes tools for manipulating audio data, and it uses its own audio file format for storing audio data called the HTK audio format. The HTK audio format is a binary format used for storing audio data in little-endian byte order. The format includes a header that specifies the number of samples, the sample rate, and the number of channels. The audio data is then stored as a series of 16-bit signed integers, with one integer per sample. While the HTK audio format is primarily used within the context of speech recognition systems, it can be converted to other more widely supported audio formats, such as WAV or FLAC, using conversion tools. It's important to note that the HTK audio format is not typically used for general audio storage or playback and is only intended for use within the HTK toolkit.