Inside DSP Articles

Inside DSP on Audio: Ear To The Ground

By , 1/5/2004

Over 20 years after the compact disc was introduced, we stand on the threshold of a revolution in consumer digital audio: digital audio is becoming one of the most prevalent, exciting technologies in consumer electronics. Enabling this transformation are advances in electronic components, the introduction of new types of digital audio content, and widespread network connectivity.

Home Audio Gets Surrounded
The major consumer audio applications can be loosely grouped into three categories: home, automotive, and portable audio. The first of these categories, home audio, is the largest and best-established digital audio market. For example, the aforementioned compact disc is now so commonplace that even inexpensive clock radios contain built-in CD players.

Looking forward, the growth in home audio markets will likely fall into two categories, which we call the "quality" and "convenience" segments. In both cases, the coming growth will largely be a response to new audio formats and delivery mechanisms.

The "quality" markets are those that are focused on high-quality, multi-channel sound. The main force behind these markets is the increasing availability of television and movie content with multi-channel audio. The advent of the DVD played a key role in moving 5.1-channel audio systems into the mainstream, and it is likely to continue doing so.

Three other important sources of multi-channel audio content are subscription television, digital broadcast television, and game consoles. Subscription television (cable and satellite) already offers programming with multi-channel audio, allowing viewers to hear the Sopranos argue in glorious surround sound. Multi-channel audio will become more widely available as broadcast television transitions from the current analog standard that offers stereo sound, to digital television, which supports the same 5.1-channel audio found in DVDs. In addition, the latest video game consoles incorporate surround-sound features.

One of the more interesting multi-channel audio products is the "home-theater-in-a-box" system. As the name suggests, these systems combine a DVD player, A/V receiver, and amplifier into a single chassis, and then add a set of speakers to the mix. Advances in digital amplifiers, also known as "Class D" amplifiers, have been critical to the development of these systems. Class D amplifiers are up to two times more power-efficient than analog amplifiers. This means products based on Class D amplifiers can use smaller, less expensive power supplies and heat sinks—savings that can lead to sleek, inexpensive audio systems.

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Class D amplifiers do have drawbacks. First, digital amplifiers are more expensive than their analog counterparts. Consequently, digital amplifiers are cost-competitive only when their higher cost can be offset by lower costs for power supplies and other components. Second, digital amplifiers typically have higher harmonic distortion and noise (THD+N) than competing analog amplifiers. This excludes Class D amplifiers from most high-end audio systems. Finally, the Class D amplifiers use high-frequency switching techniques that can introduce electromagnetic interference (EMI). This EMI can cause the radio tuner to malfunction and can cause other problems.

The Internet Sets New Expectations
While these forces are encouraging the adoption of high-quality, multi-channel home audio system, the widespread availability of music on the Internet is creating a new market for "convenience" audio products. Despite the best efforts of the recording industry, the Internet still offers a vast library of music for free download. In addition, legally sanctioned services like Apple's iTunes are offering a growing selection of music. In parallel with (and partly because of) this growth of online music, subscriptions to broadband services and the adoption of home networking have reached critical mass. The combination of easily accessible content and ubiquitous network connections are prompting a new set of consumer expectations.

Specifically, the "convenience" markets are driven by consumers desire to access their music wherever, whenever, and on whatever device they choose. One early trend resulting from this push is the addition of MP3 support to a broad range of CD and DVD players. While this capability certainly does enable consumers to move their music between devices, it is hardly the most convenient way to do so.

Emerging products address this problem by bringing the network to the audio device. Some of these products continue the PC-centric worldview that currently characterizes convenience audio markets. For example, several vendors now offer DVD players that can access audio content on a PC's hard drive. Forthcoming products may cut the PC out of the loop. For example, the much heralded (if slow to arrive) home "media servers" promise to serve as a central repository and distribution hub for digital content.

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