roponents of signal processing in the digital realm insist that its representative ones and zeros are immune to noise.

However, the analog signal conditioning and conversion at the front end and the digital-to-analog conversion and amplification at the back end are places where a lot of good DSP work can come apart at the seams. Nowhere is that truer than with digital audio, where highly sensitive analog front ends and often, many pounds of heat-sinking steel and hundreds of watts of power dissipation are required to do justice to the work of DSP filters, equalizers and special decoders.

But consumer electronic system builders, algorithm developers, Internet service providers, Web site portals and semiconductor manufacturers are all joining forces on ways to extend the dimensions of the audio signal-processing chain.

Several forces are driving the digitization of the audio chain, according to Slip Lockhoof, audio marketing manager for Motorola's Semiconductor Products Sector (SPS) in Phoenix. The most obvious trend is the convergence between computers and entertainment-particularly Internet audio and Internet radio-to provide a new range of digital source material.

"Napster the Internet-based music exchange service changed the complexion of Internet audio," confirmed Will Strauss, principal market analyst at Forward Concepts Co. (Tempe, Ariz.).

Because of Napster, an infrastructure for transferring 3-Mbyte and 5-Mbyte audio files was put in place, he said. That includes CD read-write drives as well as flash-memory formats. Flash-card MP3 decoders will soon be incorporated in car audios and home cassette decks. It is only a matter of time before they move into DSP-based cell phones and video camcorders. "Even if Napster shuts down tomorrow, the bell has already rung," Strauss said.

Another driver is the proliferation of sophisticated audio-processing algorithms. Those include algorithms that promote ever higher fidelity and more realistic sound experience with multiple channels. Promoters of the ultra-realist 10.2-channel audio sound stage, such as TMH Corp.'s Tomlinson Holman, believe they are on a seven- to 10- year proselytization mission. But DSP-based enhancements to existing multichannel decoders won't take 10 years to catch on, said Lockhoof, who thinks this more limited version will only take a year or two.

Decoding algorithms, such as Dolby Prologic-II (an improved five-channel matrix decoder for stereo source material) and DTS-ES (a matrix decoder that adds a further surround-sound channel to a 5.1-channel source) are appearing on high-end receivers.

"Never too late to think about new algorithms and standards," said Lockhoof.

The proliferation of new decoding algorithms-6.1- and 7.1-channel extensions to the original, 5.1-channel Dolby Digital and DTS encoding-is helping promote 32-bit Sharc DSP business, said Ken Nevard, product manager for digital audio at Analog Devices Inc. (Norwood, Mass.). The parts that will be sold into integrated home theater boxes are extensions of parts that are being sold into high-end systems now, he said.

But does that mean every home must succumb to a proliferation of gadgetry? "Most people don't like 10 boxes stacked on their TVs," Lockhoof pointed out. "It's something called 'the wife acceptance factor.' " Thus, AV (audio-video) mini-systems-some promoted as "home theater in a box" -are increasingly popular with consumers, said Lockhoof. The market growth is not in the $1,000 box, said Lockhoof; it's in consumer units that cost $500 or less.

Increasingly however, as more than one technologist has suggested, the key audio spec is no longer frequency response, or wattage, but Mips (millions of instructions per second).

"The more Mips, the more features," confirmed Lew Pacely, marketing vice president for consumer electronics at Cirrus Logic Inc. (Austin, Texas). Cirrus is Motorola's arch competitor in the Japanese audio-video receiver (AVR) market. DSP-based, all-digital Japanese AVRs are just 18 months away, he said. Many of them will integrate the DVD player as well as Dolby Digital and DTS decoders.

Another semiconductor technology, digital amplifiers, makes it possible to integrate decoders, receivers and speaker drivers in the same $500 box, said Lockhoof. His employer, Motorola, recently entered the digital amplifier business through a partnership with B&K Components Ltd. (Buffalo, N.Y.). The Class D amplifiers dramatically increase the efficiency of audio speaker drivers, thus reducing the size and space (and fan requirements) for transformers and heat sinks (albeit with a higher requirement for capacitor filters).

Tripath Technologies Inc. (Santa Clara, Calif.) was one of the first to exploit switching amplifier technology. Its products were first implemented in car stereos, but they're now seeing acceptance in compact, single-box DVD players and home theater systems. The Sony DAV-S300 is a DVD/CD player with six 30-watt amplifiers (for 5.1-channel surround sound) and built-in AM/FM tuner. The Sony Playstation II, essentially a game machine with a built-in DVD disk player, also uses Tripath amplifiers.

The list of manufacturers committed to digital amplifier technology is rapidly growing. It now includes Texas Instruments Inc. (Dallas), Cirrus Logic and, through a licensing agreement with Apogee Technology Inc. (Norwood), STMicroelectronics (Phoenix).

A buzzword among these manufacturers is "all digital." That means the digital amplifier will accept and amplify a digital input -for example, the pulse-code modulated (PCM) signal from a compact disk. Ordinarily, an analog input is used to control the duty cycle of a pulsewidth-modulated (PWM) switcher. (The higher the amplitude, the wider the pulse.) The output of the PWM

drives MOSFET switches and turns them on and off more efficiently than the linearly biased (i.e., always-on) A-B amplifiers.

Capacitor banks are required to filter the digital artifacts from the output pulses, and much of the development work in this area is focused on reducing the filter requirement without decreasing the audio fidelity. Both TI and Apogee, for example, are using a phase-shifted zero-crossing to reduce digital spiking.

Tripath Technology had reduced some filter requirements (thus enabling smaller capacitors) by elevating the PWM switching frequency into the 1-MHz to 2-MHz range, and shifting it around with the amplitude of the input-a spread-spectrum technology the company calls "Class T."

Developers insist the goal of all-digital amplifier development is not just to reduce the size, weight and power dissipation of mobile amplifiers, but also to improve their fidelity with digital compensation techniques. "Indeed, we are looking at an entire transformation of the audio signal processing chain," said Skip Taylor, vice president of marketing for PWM products at Cirrus.

"Initially, the low-end Japanese receivers would be stuffed with PWM controllers," Taylor said. But improvements in PWM technology will put its sound quality on the same footing with high-end audio systems. "Both the sound quality, and the price-performance should be just spectacular," Taylor said. This opens up a very wide market. "From 'milliwatts to killiwatts'-We can drive everything from head phones to home theater subwoofers," he said.

No place but up
The proliferation of PWM devices for audio, as well as Internet audio and home theater will drive a near explosion in the use of digital audio ICs.

According to a just-released Forward Concepts report, total audio-chip revenues reached $1.7 billion in 2000 and, growing at a 24 percent compound annual growth rate, are expected to exceed $4.9 billion by the year 2005.

The report, called "The Convergence of Audio 2001: A Chip Market Analysis," says the consumer audio share of this market-fueled by the digital AVRs, integrated set-top boxes, DVDs, digital speakers, and MP3-type portable players -increased to 52 percent in 2000 from 40 percent a year earlier. It will make up almost 80 percent of the audio chip market by 2005, said Mahy Churylo, senior analyst and principal author of the report.

As the tail of the audio processing chain is turning digital, the previously analog front ends-radio broadcasts themselves-will go digital. Texas Instruments and ST Microelectronics are among the companies attempting to ignite this market. It needs an investment on the infrastructure side, according to John Gardner, TI marketing manager for digital radio products.

TI has promoted its portable low-power DSPs, and partnered with RadioScape on software. Right now, the radios are in the $800 to $2,000 range, but TI is hoping its silicon and software will bring the cost down into the $300 range, Gardner said.

"The original Digital Audio Broadcasting wasn't such a success," acknowledged Ricardo Ferrari, director of ST Microelectronics' audio business unit in Agrate, Italy. The company has invested in WorldSpace satellite radio, which provides 96 digital channels; a direct-to-listener digital satellite radio service; and XM Satellite Radio (Washington), which provides 100 channels of digitized CD-quality music. ST is a manufacturing partner to Delphi and Sony, which helped define the XM chip set.

"The use of two satellites allows you to drive clear across the country without losing your station," Ferrari said.