The HMM-based Speech Synthesis System (HTS) has been developed by the HTS working group and others (see Who we are and Acknowledgments). The training part of HTS has been implemented as a modified version of HTK and released as a form of patch code to HTK. The patch code is released under a free software license. However, it should be noted that once you apply the patch to HTK, you must obey the license of HTK. Related publications about the techniques and algorithms used in HTS can be found here.