Different DSPs For Different Jobs

One way to classify DSP devices and applications is by their dynamic range. The dynamic range is the spread of numbers, from small to large, that must be processed in the course of an application. It takes a certain range of values, for instance, to describe the entire waveform of a particular signal, from deepest valley to highest peak. The range may get even wider as calculations are performed, generating larger and smaller numbers through multiplication and division. The DSP device must have the capacity to handle the numbers so generated. If it doesn't, the numbers may "overflow," producing invalid results. The processor's capacity is a function of its data width (i.e. the number of bits it manipulates) and the type of arithmetic it performs (i.e., fixed or floating point).

A 32-bit processor has a wider dynamic range than a 24-bit processor, which has a wider range than 16-bit processor. And floating-point chips have wider ranges than fixed-point devices. Each type of processor is suited for a particular range of applications. 16-bit fixed-point DSPs such as typically used for voice-grade and telecom systems (such as cell-phones), since they work with a relatively narrow range of sound frequencies. On the other hand, hi-fidelity stereo sound has a wider range, calling for a 16-bit ADC or 24-bit ADC, and a 24-bit fixed-point DSP like the Motorola DSP563xx series. In this case, the ADC's 16-bit or 24-bit width is needed to capture the complete high-fidelity signal (i.e. much better than a phone); the DSP thus must be 24 bits to accommodate the larger values resulting when the signal data is manipulated.) Applications requiring still greater dynamic range include image processing, 3-D graphics, and scientific and research simulations; such applications typically a 32-bit floating-point processor.

DSP Evolution

Just a decade and a half ago, digital signal processing was more theory than practice. The only systems capable of doing signal processing were massive mainframes and supercomputers and even then, much of the processing was done not in real time, but off-line in batches. For example, seismic data was collected in the field, stored on magnetic tapes and then taken to a computing center, where a mainframe might take hours or days to digest the information.

The first practical real-time DSP systems emerged in the late 1970s and used bipolar "bit-slice" components. Large quantities of these building-block chips were needed to design a system, at considerable effort and expense. Uses were limited to esoteric high-end technology, such as military and space systems. The economics began to change in the early 80s with the advent of single-chip MOS (Metal-Oxide Semiconductor) DSPs. Cheaper and easier to design-in than building blocks, these "monolithic" processors meant that digital signal processing could be cost-effectively integrated into an array of ordinary products. The early single-chip processors were relatively simple 16-bit devices, which, teamed with 8- or 10-bit ADCs, were suitable for low-speed applications, general-purpose coders such as talking toys, simple controllers, and vocoders; (voice encoding devices used in telecommunications).

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