Making Beautiful Music
Researchers combine power efficiency with high-quality sound

By Doug Peterson

Walk into Philip Krein’s laboratory and you can hear beautiful classical music coming from a loudspeaker—near-perfect sound. But hook an oscilloscope to the terminals of the speaker and you see harsh-looking square waves going in.

“It doesn’t make sense,” says CSL professor Krein. “The signal makes it look like the amplifier is blasting the speaker with power.”

But it actually does make sense, thanks to Krein’s work.

Krein and his team, collaborating with researchers at Motorola, have found a way to combine high-efficiency power with high-quality sound in ways that have never been done before. The result will be a new surround-sound audio chipset, based on this technique and soon to be released by Motorola.

A typical audio amplifier is only about 10-percent efficient in power consumption, Krein says. But by using pulse width modulation (PWM), his research team made it possible for an amplifier to reach about 90-percent efficiency in power consumption—and still retain high sound quality.

In the past, he notes, people did not believe that PWM could be used for this type of application without sacrificing quality.

“It was assumed you could never do this at a quality high enough for home theater or other serious audio applications,” he says. “That’s why when PWM is used, it is found in cell phone speakers and telephone systems in which the quality of sound doesn’t have to be as high.”

The key to power efficiency with PWM, Krein explains, is to process all of the power through switches. Switches do not lose energy; they manage energy. CSL and Motorola researchers have found a way to operate those switches so the audio signal reaching the loudspeakers is not degraded.

According to Krein, “The quality level is very close to the theoretical limit of what you can get out of an audio CD, for example.”

Another feat accomplished by the research team in this work is that they have made the process truly digital. Today, commercially available amplifiers often claim to be digital. But, in truth, they are digital up to a certain point before the signal is converted to analog.

With the new system, the signal remains in digital form the entire way through the process as the sound makes its way to the loudspeaker. The only analog part of the system is the loudspeaker itself.

Although several research groups across the country are doing this kind of work, the CSL and Motorola team was the first to reach this high level of quality in a complete digital system.

The next step, which will also be funded by Motorola, is to apply this precise process to a much wider set of applications in power control—high-quality power supplies, motor control, telecommunications systems and any type of portable equipment, such as cell phones.

PWM is a classical communications approach that researchers have used for close to 50 years, Krein says. It is well known and well understood.

But as he goes on to note, “What people didn’t know is just how far you could push it in terms of quality.”