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Sound reproduction has been one of the last bastions of analog design as an art form. But, for us old "analog hands," the barbarians are at the gates. Digital signal processing is moving in on audio on several fronts. Engineers designing high-end audiophile gear are becoming very clever about using DSP to develop signals that drive tiny speakers with high precision, resulting in astounding sound quality from very small enclosures. Sound-modifying devices—so-called stomp boxes—for electric guitarists that relied on overdriven preamps, analog delay circuits, and voltage controlled filters and amplifiers are being imitated by all-in-one DSP-based devices. Do you want your amplifier to sound like a 1970s vintage wall of Marshalls? Just push a button.

One of the most interesting developments along this line is the emergence of the Class-D audio amplifier IC. First fielded by Linfinity and then Texas Instruments, National Semiconductor is now offering Class-D audio amplifier ICs along with its traditional Class-AB (in other words, "push-pull") devices.

Class-D audio amplifiers bear the same relation to switching regulators that linear audio amplifiers bear to linear regulators. In fact, Class-D audio amplifiers have so much in common with switching regulators, that Micro Linear has an applications note on using a switching regulator as a Class-D amplifier (Application Brief 7, "Using the ML6552 as a Class-D Audio Amplifier").

The Class-D amplifier encodes the audio information in a pulse-width-modulated signal. So a Class-D amplifier requires a simple low-pass filter on its output to block the high-frequency switching noise and smooth the audio output. And there lies the rub. The filter means that the Class-D amplifier needs more components and PC-board real estate than a comparable Class-AB amplifier requires. It must be said in their favor, however, that inductor and capacitor manufacturers have been beavering away at making their components smaller, less expensive, amenable to pick and place machinery, and reflow solderable. But, in price sensitive consumer applications, solutions requiring extra components are tough to justify.

The lure of the high-volume notebook PC market was motive behind bringing out the Class-D IC amplifiers. The pressure to cram higher and higher performance, yet provide longer and longer battery lifetimes has spawned some remarkable developments in power management. It was thought that the higher efficiencies of Class-D amplifiers compared to Class-AB—up to 80% or more compared to 50% or less—would make them a shoe-in for laptop PCs. Another possible high-volume application cited was Boom Boxes.

When these proposals were first advanced I wondered just how compelling they were for these two target markets. How many laptop PC users were playing games or running other applications that made good use of high-quality, loud, 3D sound? My impression is that people use laptops mostly for business applications in which audio is not a factor. And I also wondered how many Boom Box users were worrying about the cost of batteries.

Only time will tell if the Class-D IC amplifier makes significant inroads into the territory of Class-AB amplifiers. My guess is that increasingly more audio will come in the form of highly compressed, multichannel sound. This form of audio will require sophisticated DSP to uncompress the sound, extract the multiple channels as well as perform the volume-control, tone-control, and equalization functions. As a part of the overall job, the DSP processor could generate PWM outputs for each channel. With all the audio processing vacuumed up into the DSP processor, all that would remain in the analog domain would be a simple power stage to drive the loudspeakers. As I said, the barbarians are at the gates.

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The Barbarians are at the Gates

3/18/2000 05:00 AM EST
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