This chapter discusses the various ways of controlling the operation of HTK tools along with related aspects of file system organisation, error reporting and memory management. All of the operating system and user interface functions are provided by the HTK module HSHELL. Memory management is a low level function which is largely invisible to the user, but it is useful to have a basic understanding of it in order to appreciate memory requirements and interpret diagnostic output from tools. Low level memory management in HTK is provided by HMEM and the management of higher level structures such as vectors and matrices is provided by HMATH.
The behaviour of a HTK tool depends on three sources of information. Firstly, all HTK tools are executed by issuing commands to the operating system shell. Each command typically contains the names of the various files that the tool needs to function and a number of optional arguments which control the detailed behaviour of the tool. Secondly, as noted in chapter 2 and shown in the adjacent figure, every HTK tool uses a set of standard library modules to interface to the various file types and to connect with the outside world. Many of these modules can be customised by setting parameters in a configuration file. Thirdly, a small number of parameters are specified using environment variables.
Terminal output mostly depends on the specific tool being used, however, there are some generic output functions which are provided by the library modules and which are therefore common across tools. These include version reporting, memory usage and error reporting.
Finally, HTK can read and write most data sources through pipes as an alternative to direct input and output from data files. This allows filters to be used, and in particular, it allows many of the external files used by HTK to be stored directly in compressed form and then decompressed on-the-fly when the data is read back in.
All of the above is discussed in more detail in the following sections.
