Digital Signal Processor

Computerworld - Every modern computer has a microprocessor in it, but not many have a digital signal processor (DSP). Since the CPU is a digital device, it clearly processes digital data, so you might wonder what the difference is between digital data and a digital signal. Basically, signal refers to communications—that is, a continuous stream of digital data that might not be stored (and thus might not be available in the future) and that must be processed in real time.

The digital signals can come from almost anywhere. For example, downloadable MP3 files store digital signals that represent music. Some camcorders digitize the video signals they generate and record them in a digital format. And the more sophisticated cordless and cellular phones typically convert your conversation into a digital signal before broadcasting it.

Variations on a Theme

A DSP differs markedly from the microprocessor that serves as the CPU in a desktop computer. A CPU's job requires it to be a generalist. It has to orchestrate the operation of diverse pieces of computer hardware, such as the hard disk drive, the graphics display and the network interface, so they work together to perform useful tasks.

This agility means that a desktop microprocessor is complex—it must support key features such as memory protection, integer arithmetic, floating-point arithmetic and vector/graphics processing.

As a result, a typical modern CPU has several hundred instructions in its repertoire to support all of these functions. This requires that it have a complex instruction-decode unit to implement the large instruction vocabulary, plus many internal logic modules (termed execution units) that carry out the intent of these instructions. As a result, a typical desktop microprocessor contains tens of millions of transistors.

In contrast, a DSP is built to be a specialist. Its sole purpose is to modify the numbers in a digital signal stream—and do it quickly. A DSP's circuits consist mainly of high-speed arithmetic and bit-manipulation hardware that can rapidly modify large amounts of data.

As a consequence, its instruction set is much smaller than that of a desktop microprocessor—perhaps no more than 80 instructions. This means that the DSP needs only a slimmed-down instruction-decode unit and fewer internal execution units. Moreover, any execution units that are present are geared toward high-performance arithmetic operations. Thus, a typical DSP consists of only several hundred thousand transistors.

As a specialist, a DSP is very good at what it does. Its myopic focus on math means that a DSP can continuously accept and modify a digital signal, such as an MP3 music recording or a cell phone conversation, without stalling or losing data. To help improve throughput, DSPs have extra internal data buses that help shuttle data among the arithmetic units and chip interfaces faster.