No power waste in the all new Digital Signal Processors!
ACCORDING to a report by Samuel K Moore, news editor of the IEEE Spectrum Magazine, hearing aids, power converters, medical implants, and telecommunications could benefit from continuous-time digital signal processing.
Digital signal processorsthose practically ubiquitous (available everywhere) circuits that make cellphone conversations understandable and MP3 players possiblecome in a great many varieties, but until recently, there was one variety no one had even thought to make! Called continuous-time digital signal processing (CT DSP), it has the ability, unique among such circuits, to consume dynamic power in proportion to the intensity of the signal it processes.
When there's no signal, such as during the silent spots of a cellphone conversation, the processor is practically inactive. But when the signal appears, it kicks into gear. Its inventor, Yannis Tsividis, Professor of Electrical Engineering at Columbia University, says the device's miserly management of power could make it attractive for small systemssuch as biomedical implants and remote sensorsthat deal with "bursty" signals in need of real-time processing. Industrial firms are also interested in using the technology for telecommunications and power conversion.
"It's a different way of looking at a problem that people have looked at for a long time," says Rajit Manohar, an expert on clockless circuits and professor of electrical engineering at Cornell University. "There are a lot of benefits to the approach."
A signal can be either analog or digital, and the system that processes it can do so either continuously or in discrete time. But digital signal processing has always been practically synonymous with discrete time. "This has been bothering me for many years," says Tsividis.
In conventional discrete-time DSPs, the clock signal that triggers the sampling must have a frequency of at least twice the highest frequency of interest in a signal. But the clock has to go on nonstop at that frequency whether or not there's a signal and whether or not that signal has any high-frequency component to it at a given time, which is a waste of power.
The new CT DSP, by contrast, has no clock. Instead, when the signal changes by a set amount, the CT DSP produces a digital pulse. If it changes by that amount again, a second pulse occurs, and so on. A separate digital signal marks whether the input is increasing or decreasing. Those digital pulses are put through a programmable circuit that digitally filters the signal in continuous time and then converts it back to analog.
Aside from the potential power advantage of a CT DSP, it just might make things sound better. Conventional DSPs suffer from two types of spurious signals: one, called aliasing, happens because the input signal mixes with the clock frequency. With no clock, CT DSPs are free of aliasing. The other, quantization error, is produced by the inexactness of turning the analog input into a digitized signal. In a conventional DSP, quantization error is spread over all frequencies, but for a CT DSP, it occurs only at multiples of the frequencies in the signal, so it's easier to filter out and there's less of it to begin with. Also, such "harmonic error" is less objectionable to the ear!
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