1.4 Organization of the Software

The overall organization of the Ptolemy system is shown in figure 1-1.

A typical use of Ptolemy involves starting two UnixTM processes, as shown in figure 1-1(a), by running pigi (Ptolemy interactive graphical interface). The first process contains the vem user interface and the oct design database [Har86], and the other process contains the Ptolemy kernel.

The pigi program, which is aptly named, contains all Ptolemy domains. As such, it is a rather large program that requires a good bit of memory. For the DSP classes, a much smaller version, called ptiny, is sufficient. This is the version you should use.

In Ptolemy, applications are described as interconnections of blocks, much like the block diagrams you find in a DSP textbook. Blocks can be hierarchical, as shown in figure 1-2.

The lowest level of the hierarchy, as far as Ptolemy is concerned, is derived from a kernel base class called Star. A hierarchical block that consists of multiple stars is a Galaxy, and a top-level system representation is a Universe. Unlike physical galaxies, Ptolemy galaxies can also contain other galaxies.

The domain used in our DSP classes, the synchronous dataflow (SDF) domain [Lee87a,b], is one of the most mature in Ptolemy. It is used for signal processing and communications algorithm development, and has particularly good support for multirate algorithms [Buc91]. A dynamic dataflow (DDF) domain, which is not used in our DSP classes, extends SDF by allowing data-dependent flow of control. Many other Ptolemy domains provide alternative modeling and implementation techniques.

A key idea in the Ptolemy project is to mix modeling techniques, implementation languages, and design styles, rather than trying to develop one, all-encompassing technique. The Ptolemy software is used as a laboratory by the Ptolemy research project for experimenting with such mixtures. Such mixtures, however, are not needed for straightforward modeling of DSP algorithms.