Most folks shopping for a home theater receiver or amplifier are bound to have their eyes peeled for a single number: the power rating. Ideally, this spec will tell you how much juice a particular amp can deliver to a given set of speakers under normal conditions. But the problem is, multichannel audio systems used for watching movies and listening to surround sound music are often subjected to conditions that go beyond what generally passes as normal. And then there's the issue of variations among manufacturer power ratings; some companies offer a generous amount of information about their products, while others serve up specifications that seem almost deliberately obscure.
If the power ratings for some receivers and multichannel amps lack detail, the situation gets even worse - much worse, in fact - when you check out home-theater-in-a-box systems, which generate the biggest sales of audio gear in the current bigscreen HDTV era. A typical power rating for an HTiB system might read "Total Power: 200 watts." But how does that power get distributed among the various satellite speakers and the subwoofer? More important, what are the measurement conditions? (For example, over what bandwidth and at what distortion level is the specification arrived at?) Obviously, some level of standardization for power specifications among audio products would be a welcome thing.
Power Ratings: The Current State
Most of the measurement techniques used by surround sound receiver and multichannel amplifier manufacturers are detailed in CEA 490-A, the Consumer Electronics Association's Standard Test Methods of Measurement for Audio Amplifiers (shown at right), which was published in 2002. This standard was itself developed in response to the Federal Trade Commission's revamping of its 1974 rule governing Power Output Claims for Amplifiers Utilized in Home Entertainment Products - an effort that also sought to include the crazy new world of multimedia speaker systems for computers. Although the CEA standard doesn't cover those devices, it does seek to standardize the measurement of "output power of multichannel amplifiers used in home theater applications" and to create a "standard language for primary ratings of amplifiers that allows consumers to make an 'apples-to-apples' comparison between various brands and models of amplifiers." Compared to the relatively threadbare FTC rule, the CEA standard provides a more comprehensive set of recommended measurement practices for rating audio gear. Unfortunately, while the FTC's rule carries some legal weight, the CEA standard is a set of guidelines that depends on voluntary compliance from audio manufacturers.
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