Particles at the inputs of a star enables it. Stars without any inputs are always enabled. "Statically scheduled" means that the firing order of the stars is determined once, during the start-up phase. The firing order will be periodic. The SDF domain is one of the most mature in Ptolemy, having a large library of stars and demo programs. It is a simulation domain, but the model of computation is the same as that used in most of the code generation domains. A number of different schedulers, including parallel schedulers, have been developed for this model of computation.
go()method of a star is called a firing. Particles are called tokens. In a digital signal processing system, a sequence of tokens might represent a sequence of samples of a speech signal or a sequence of frames in a video sequence.
When an actor fires, it consumes some number of tokens from its input arcs, and produces some number of output tokens. In synchronous dataflow, these numbers remain constant throughout the execution of the system. It is for this reason that this model of computation is suitable for synchronous signal processing systems, but not for asynchronous systems. The fact that the firing pattern is determined statically is both a strength and a weakness of this domain. It means that long runs can be very efficient, a fact that is heavily exploited in the code generation domains. But it also means that data-dependent flow of control is not allowed. This would require dynamically changing firing patterns. The Dynamic Dataflow (DDF) and Boolean Dataflow (BDF) domains were developed to support this, as described in The Almagest.
Consider a simple connection between three stars, as shown in figure
LevDur. This behavior is quite different from the matrix stars, which operate on particles where each individual particle represents a matrix.
FIRCxstars optionally perform such a sampling rate conversion, and with an appropriate choice of filter coefficients, can interpolate between samples. Other stars that perform sample rate conversion include
Commutatoror a single signal can be split into subsignals at a lower sample rate using the
Suppose for example that star B in figure 3-1 is an
FFTCxstar with its parameters set so that it will consume 128 samples and produce 128 samples. Suppose further that star A produces exactly one sample on each output, and star C consumes one sample from each input. In summary,
FFTCx star and 128 firings each of stars A and B.
It is not always possible to solve the balance equations. Suppose that in figure
3-1 we have
Delays are indicated in Pigi by small green diamonds that are placed on an arc. Most of the standard palettes of stars have the delay icon at the upper left. The delay has a single parameter, the number of samples of delay to be introduced. In the SDF domain, a delay with parameter equal to one is simply an initial particle on an arc. This initial particle may enable a star, assuming that the destination star for the delay arc requires one particle in order to fire. To avoid deadlock, all feedback loops much have delays. The SDF scheduler will flag an error if it finds a loop with no delays. For most particle types, the initial value of a delay will be zero. For particles which hold matrices, the initial value is an empty Envelope, which must be checked for by stars which work on matrix inputs. Initializable delays allow the user to give values to the initial particles placed in the buffer. Please refer to
2.12.8 on page 2-57 for details on how to use initializable delays.