Electronic Design

Digital Revolution Delivers Enhanced Audio Playback Systems

A digital revolution is under way in audio. It began with digital mastering, analog-input Class-D amplifiers, and the CD, but the momentum is increasing. Today, partially digital systems are common, although fully digital audio playback systems, with digital data streams that extend from source to speaker, have just recently become available. Now, interest in such systems is growing due to the prevalence of digital content and interest in multichannel systems.

TacT Audio set the standard for digital amplifiers with its Millennium product in 1998. The 1999 Guinness Book of World Records called this the world's first fully digital amplifier. It proved that a digital amplifier could achieve high sound quality.

Digital amplifiers enable form-factor miniaturization with their power efficiency, which allows the use of power supplies of about one-third the size and power, and heatsinks of about one-tenth the size of those used in equivalently rated analog amplifiers. Lightweight products with minimal supplemental cooling requirements result.

Since the release of the Millennium, significant activity has focused on the deployment of digital amplifier technology at different performance levels, for various market needs. High-performing systems have been measured to have flat total harmonic distortion plus noise (THD+N) curves versus frequency, at around 0.015%, and actually dropping to about 0.002% above 10 kHz. These amplifiers maintain this performance nearly up to their peak output power. The noise floor has been measured in the ­140- to ­150-dB range.

For the consumer market, cost-competitive chips implemented with consumer-grade components still show excellent performance. For example, some systems have been measured to have the same flat THD+N curves, with values right at 0.08% for 30-W, 0.02% for 10-W, and 0.035% for 1-W operation. Here, a noise floor in the ­125- to ­130-dB range is more common. Consumer products using these new chips are already on the market.

While the audio systems with the highest sound quality are still analog, digital systems generally offer many advantages over analog systems: simpler design and verification; greater flexibility, reliability, and testability; reduced part count, time-to-market, and cost per feature; elimination of costly, low-tolerance parts; simplified production; rapid prototyping; performance stability with temperature fluctuation and other environmental effects; the ability to use the same hardware in multiple systems; and the ability to reconfigure systems according to application, even in the field and on-the-fly.

Digital systems also present challenges not encountered in analog audio systems. However, modern digital signal processing techniques have solved the problems formerly associated with digital audio processing: limit cycles, audible quantization noise, pops, clicks, and zipper noise found in the digital volume controls and tone controls.

As with any rapidly expanding industry, the audio sector has seen a wide variety of products offered to deal with its processing needs. Fully programmable DSPs have been mainstays, and this will continue due to their absolute flexibility and their ability to meet the changing needs of audio standards.

Besides DSPs, second-generation digital audio processors are now available. Although less flexible than fully programmable DSPs, they are attractively priced and specifically designed to excel at audio processing tasks. Typical audio functions, such as bass and treble controls, volume control, dynamic range compression, and configurable equalization filters, are built in. Others can be hosted by downloading the appropriate coefficients. High-precision digital audio processors can even handle low-frequency, high-Q filtering at sample rates up to 192 kHz.

Examples of implementable functions include mixing/multiplexing, a wide variety of virtual 3D algorithms, parametric equalization, volume-tracking loudness compensation, center or subwoofer channel synthesis, delay, dither, spectrum analyzer, and sound effects. Because these functions are configurable, the system designer has complete control over the features and the flexibility to voice systems as desired. Moreover, these devices require no user-developed software. Configuration is achieved through coefficients that are simple to generate and download via manufacturer-provided software tools. This allows rapid prototyping, use of the same hardware in multiple products, and reduced product development time.

The highly efficient, fully digital amplifier with advanced digital audio processing provides high-fidelity amplifier sound with the advantages of high-performance digital crossovers, speaker driver equalization, and volume controls. By implementing this technology, the audio market can provide very efficient, modestly priced, feature-rich, high-performance audio systems that are easily tailored to single-user or multiple-user preferences. This flexible technology lets systems designers make choices that will lead to innovative new end products.

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