Power this: testing audio Ics: SoCs for set-top boxes, television monitors, disk players, and mobile media players have or soon will have HD capability. But another aspect of this evolution--one that could prove even more challenging to SoC designers and test engineers--is that, along with HD video comes a significant increase in the quality of the accompanying audio.(Cover story)

It is a well-publicized trend that HDTV (high-definition TV) is sweeping through the next generation of SoC (system-on-chip) platforms. SoCs for set-top boxes, television monitors, disk players, and--in the near future-mobile media players will have HD capability. But another aspect of this evolution, audio quality, is slipping beneath the radar.

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The problem with high-resolution audio isn't just in the circuit design. In fact, analog-IC designers are producing DACs and amplifiers that are arguably better than anything available during the golden age of discrete-component gear. The problem is in characterization and testing. The quality of high-end audio, as many analog designers will tell you and as audiophiles may insist to the point of religious discourse, is excruciatingly hard to quantify even on the characterization bench and almost impossible to verify in a manufacturing-test environment. SOC designers are now sizing up this new challenge, joining experienced audio-IC designers who have lived with it.

SO WHAT'S THE PROBLEM?

The new test-and-measurement challenge is coming from the convergence of two forces. One, as previously mentioned, is the increasing capacity for digital bit streams to provide quality audio. To be more precise, the increasing quality of the source material is not the problem. One can always convert a good bit stream into a mediocre audio signal. The problem is the growing expectations of consumers. When digital audio means audio from an MP3 bit stream or a similarly lossy source, the underlying issues in the codec usually make it unnecessary to take care with the analog circuitry. Users judged MP3 players against portable cassette players and CD players, which for the most part sounded worse in comparison.

"The quality of outputs for MP3 players has actually been driven by the emergence of high-performance headphones rather than by the source," says Gary Adrig, director of marketing for audio products at National Semiconductor. "As headphones improved, we saw some customers in what had been an undemanding application start to ask for 100dB SNRs [signal-to-noise ratios] and 0.05% THD [total harmonic distortion]."

As content vendors began to move to lower compression ratios--hence, higher bit rates--chip designers had to move to wider datapaths and better DACs to keep the noise floor of the hardware below the inherent noise level of the decoded source. Despite the limitations of MP3, market competition proved that consumers are quite discriminating about sound quality.

With the new HD media, the pace has picked up as audio tracks have jumped right past CD quality of 16 bits at 44.1k samples/ sec to as much as 24-bit, 192k-samples/sec data for DVD Audio. This performance will certainly lead buyers of high-end gear to question the quality of the sound they are hearing. "We are already there in the home-theater market," says Texas Instruments Marketing Manager Kevin Belnap. "Once you reach a minimum level of noise and harmonic distortion, a lot of listener-preference issues, like sound stage and the whole 'vacuum-tube-sound' thing, come into play."

But the critical ear won't stop there. Users of set-top and converter boxes, HD-ready television sets, and even mobile devices are likely to also demand much better audio than they have been hearing. The course of this evolution might be more apparent from one professional-audio developer's experience. …

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