Abbreviation for Host TDM (Time Division (or Domain) Multiplexing). HTDM is a special subset of TDM that allows certain host-based plug-ins and software synthesizers to operate within a TDM Pro Tools environment. You can think of it as a sort of cross between regular TDM, RTAS, and Direct Connect. While RTAS is also a host-based environment those plug-ins aren’t integrated as tightly into the Pro Tools software as HTDM plug-ins. For example, HTDM plug-ins can utilize the automation built into Pro Tools, as well as working directly with the Digidesign control surfaces (RTAS or Direct Connect solutions need to use MIDI for automation and control, and Direct Connect requires you to switch to the Direct Connect “client” applications when accessing them). HTDM plug-ins do NOT use the processing power of the TDM DSP Chips. In that sense they work like regular host-based plug-ins with their commensurate drain on CPU resources and latency issues. There is some DSP usage, however, as they require some overhead to be integrated with the system and to provide a place for the audio streams to get in and out.