Wednesday, March 02, 2005

Editor's Notes—Volume 38, Number 3, 2004

It’s interesting to observe that the three feature articles in this issue are highly representative of the currents that are basic to our businesses. Each illustrates a facet of what real-world signal processing (and Analog Devices) is all about—digital, analog, and the interface. They exemplify our long-held conviction that, although we live in an analog physical world, analog and digital must cooperate to solve each other’s problems.

Take, for example, JPEG 2000 Image Compression (page 3). The JPEG 2000 standard defines a new image-coding scheme that uses state-of-the-art compression techniques based on wavelet technology. Its architecture is useful for many applications, including Internet image distribution, security systems, digital photography, and medical imaging. The article highlights some of its benefits.

The technology involves applying the highly sophisticated wavelet transformation in a purely digital interpretation scheme for encoding, transmitting, receiving, storing, and selectively using pictorial material, Yet it starts with electrical samples that depend on light intensity (an analog quantity), and is intended for ultimate display in some form by modifications of light from a source to produce pixels of light whose intensity (an analog quantity) correspond more-or-less faithfully to an intended relationship to the original.

Consider now, if you will, Adjustable Cable Equalizer Combines Wideband Differential Receiver with Analog Switches (page 13). Category-5 unshielded twisted-pair cable, like any transmission medium, suffers from dispersion and high-frequency signal loss. This article presents an equalizer design that compensates Cat-5 cable at frequencies to 100 MHz and lengths to 1000 feet, making it suitable for KVM networking and high-resolution video transmission.

The subject of this article is, quite evidently, preserving the integrity of analog signals. But what do the initials, KVM, stand for? Keyboard, video, mouse! What could be more digital? Again, we have a scheme for preserving information, but this time the purpose of the design is to preserve digital information—subjected to the tender mercies of the analog world in the cable. Since KVM suggests computer, the source and destination of the information could both be totally digital in nature—starting with symbols and ending with symbols.

Finally, we have: All About Direct Digital Synthesis—Ask the Applications Engineer–33 (page 8). Direct digital synthesis (DDS) is a method of producing an analog waveform—usually a sine wave—by generating a time-varying signal in digital form and then performing a digital-to-analog conversion. Because operations within DDS devices are primarily digital, they can offer fast switching between output frequencies, fine frequency resolution, and operation over a broad spectrum of frequencies.

The operation speaks for itself! The properties of the waveform to be generated enter in purely symbolic form as numerical information (btw, the French word for “digital” is numérique). And lo! the device’s DAC—in cahoots with the clock that (along with the power supply) is essential to the device’s operation,—emits an analog waveform of the appropriate frequency and phase.

Dan Sheingold []

Analog gets a bad rap, even in popular culture. In movies, TV shows, and magazines, people are told that analog is dirty and old fashioned, and that digital is clean and modern. In an ad from one of the electronics superstores, for example, a guy gets dumped for being “too analog”. In The Teeth of the Tiger by Tom Clancy[i], readers are told that

The world was not digital, after all—it was an analog reality, always untidy, always with loose ends that could never be tied up neatly like shoelaces, and so it was possible to trip and fall with every incautious step. (page 172);
The world, one had to remember, was analog, not digital, in the way it operated. And analog actually meant sloppy. (page 286);
“I think that we can depend on that.” “Yeah, unless he got an unexpected phone call, or he saw something in the morning paper that caught his interest, or his favorite shirt wasn’t properly pressed. Reality is analog, Sam, not digital, remember?” (page 316).

Clancy is right—the world is analog. But that doesn’t make it dirty, unpredictable, or imprecise. While digital signals are limited by finite resolution, analog signals can have infinite resolution, limited only by noise or quantum effects. Analog signal processing can respond nearly instantaneously, without the computational delays inherent to digital signal processing. Analog circuitry can often operate at far lower power levels than digital circuitry providing the same function. Yet it is difficult to maintain the speedy, pristine nature of an analog signal through further signal processing for communications or storage, especially over long distances, in hostile environments, or over extended periods of time. Thus, the world needs precision data converters, high-speed operational amplifiers, power management components—and the expertise to use them.

The images in a digital camera are stored as 1s and 0s, but they are acquired by a CCD analog imager. Processing the CCD signal requires analog functions such as sampling, variable-gain amplification, and A/D conversion. Displaying the image on the liquid-crystal display requires analog functions, such as D/A conversion, filtering, and gamma correction. Many digital cameras include audio functions, and therefore require audio codecs and amplifiers to drive the speakers and microphones.

Wireless communication is also made possible by analog technology. Cellular carriers brag about their all-digital networks, but humans don’t speak in 1s and 0s, and don’t hear that way either. Voices must be digitized by A/D converters and reconstructed by D/A converters. And, while the data being transmitted is digital, the transmission medium is analog. 1s and 0s can’t be transmitted as-is—they must first be modulated onto high-frequency carriers. On the receive side, weak signals must be captured by low-noise amplifiers—and demodulated. Power for all of these functions must be supplied by a small battery that lasts for weeks between charges, can be recharged quickly, and can be used while it is being charged. This requires complex analog power-management techniques.

Analog Devices, with its technologies, products, application notes, data sheets, application seminars, design tools, web site, field application engineers—and publications such as Analog Dialogue—seeks to help foster both the technology and expertise that designers—trained in either analog or digital—can use to cope with the realities of a mixed-signal world.

Scott Wayne []

[i] Tom Clancy, The Teeth of the Tiger. New York: G. P. Putnam’s Sons, hardcover (2003).


Post a Comment

<< Home