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Digital audio dominates multimedia trends

Posted: 10 Nov 1999 ?? ?Print Version ?Bookmark and Share

Keywords:mp3? mixed signals? dsp? crystal? cirrus?

Stephan Ohr reviews some of the multimedia chips that you will need to build board-level systems.

The recent Analog and Mixed-Signal Conference in California keyed on the three major applications of mixed-signal technology: Communications, Multimedia, and Power Management. To be very honest, the Multimedia track was rather lean. Digital audio and MP3 dominate the multimedia world almost entirely. There is a trend toward higher fidelity, as evidenced by the higher resolution data converters applied to consumer electronics, and the push toward digital audio disks with higher sample rates. The move toward digital amplifiers is certainly worth mentioning, as well as the adoption of serial buses like IEEE 1394 and USB.

All I can do here is review what I know, in hopes of underlining some of the market opportunities available to Asian engineers who'll use multimedia chips to build board-level systems. In digital audio, Crystal Semiconductor (the Crystal Products Division of Cirrus Logic) has its name on nearly everything. Analog Devices is active in digital audio, but seems to be taking its cues from Intel's Platform Architecture Labs. National Semiconductor was one of the original manufacturers?along with Crystal, Analog Devices and Yamaha Semiconductor?to endorse Intel's AC-97 codec specification. Codecs from ESS Technology initially appeared in a majority of Asian-made audio insert cards. However, their primary advantage was low cost, and the market then shifted toward higher quality. Thus, Crystal Semiconductor now dominates the PC-based digital audio market, according to the latest numbers compiled by Mercury Research.

Singapore-based TriTech Microelectronics demonstrated some interesting audio DACs (parts with the ability to decode multi-channel audio, for example) at the Consumer Electronics Show (CES) in January 1999, but subsequently ran into Crystal's legal machinery and was stalled in the USA. It turns out, Crystal owns some key patents on sigma-delta (actually, delta-sigma) conversion techniques, and has successfully sued its competitors (some big ones like Analog Devices) for patent infringement. AKM Semiconductor, the American arm of Japan's Asahi, is one of the few known licensees for Crystal's conversion techniques. You don't put a sigma-delta audio converter on the market without thinking about what Crystal's going to do.

Dr. Steven Harris, Crystal's director of Advanced Product Technology was understandably relaxed when he mapped out the technical differences between the DVD Audio disk proposed by the DVD forum and the Super Audio Compact Disk advocated by Sony and Philips. Both proposals promise to use optical storage techniques to increase the frequency range (up to 96kHz), sample rate (up to 2.88MHz) and dynamic range (120dB?144dB) available for audiophile recording and playback. The DVD format would use the multilayer storage capability available with DVD optical disks to provide six channels of 24bit audio. The problem is that playback would require specialized equipment that would be very expensive for a couple of generations.

The Super Audio Compact Disk (SACD), in contrast, would make modest demands of the optical storage capacity, and use a lossless compression algorithm (called "Direct Stream Transfer") to increase the capacity and bandwidth of the media. Audiophiles might resent the compression for fear that it may distort the original recorded material, but, in Sony-Philips' conception, SACD disks will be playable on conventional audio CD players (and that will likely make it a winner in the consumer market). The wider frequency response and dynamic range, encoded onto the SACD disk, would be accessible to audiophiles with specialized Direct Stream Digital (DSD) decoders and amplifiers.

The reason Steven Harris is so sanguine about this impending standards battle is that his company makes the DACs that will support either standard. The 24bit CS4396 and CS4397 audio DACs support 192kHz sample rates. So also does Analog Devices' AD1853 audio DAC. These parts are indicative of a trend toward bringing professional audio quality to home audio systems. When you think about it, the 120dB dynamic range of these parts does represent amazingly high levels of audio quality.

Ever-new generations

In September, Cirrus Logic (Crystal's parent) announced a technology agreement with Microsoft Corp. regarding the Windows Media Audio format. The promise of Windows Media Audio is better security protection for copyrighted material sold over the Internet, and better compression for CD-quality audio. Material compressed with Windows Media Audio would take half the storage space as the same material compressed with MP3.

The company's "Maverick" processor (the EP7212) represents a single-chip implementation of a Windows Media Audio player. It integrates an ARM core with Crystal's audio DACs, and will likely go into a variety of handheld consumer products, such as pocket organizers, electronic books, and game platforms. The Maverick will also support MP3.

DSP plays a major role in advancing the trend toward audiophile realism both in music playback and home theater systems. The sigma-delta DAC is itself a dedicated-function DSP, which applies "noise shaping" to the digital audio bit stream coming off the CD. It effectively reshapes the audio bit stream to resemble one that's been oversampled 256 times, at (say) 2.5MHz rather than 44.1kHz. Reconstituted this way, the predictable quantization noise appears well outside the audio frequency range (where it is relatively easy to filter). An additional advantage is that the increased precision in time?the expanded time resolution?tends to flatten differences in amplitude. Thus, the differences between one sample slice and the next ceases to be a matter of magnitude and becomes one of direction (up or down), which can be converted to something resembling an analog voltage with a switched capacitor charge pump. Philips calls this a "1-bit" converter, and believes the technique could be applied to decoding SACD.

But DSP in my opinion will appear more prominently in the Dolby AC-3 ("Dolby Digital") decoders, which play back 5.1 channels (left front, right front, left rear, right rear, center front and a directionless subwoofer) in home theater systems. Dolby Digital relies on "acoustic masking" effects (essentially assuming that there is a certain amount of audio information you will just never hear) to superimpose six channels of information on a 192Kbps data stream.

Crystal's CS4923 single-chip AC-3 decoders are beginning to represent a threat to Motorola's DSP56300 general-purpose DSP, which had been widely used in high-end Japanese-made AV receivers, according to Keith Essency, vice president and general manager of Crystal's audio division. Coupled with the 108dB CS4226 six-channel codec, the CS4923 would provide a complete six-channel solution.

But six-channel audio playback will have increased utility beyond home theater. It will help upgrade two-channel stereo in home and even automotive audio playback. Not to understate the role his company plays, Steven Harris believes that multiple playback channels, and the placement of speakers, will have more of an impact on the listener's perceptions of sound quality than any advances in multimedia electronics.

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