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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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