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Codec stretches audio playback times, reduces power use

Posted: 12 Jun 2008 ?? ?Print Version ?Bookmark and Share

Keywords:codec? MEMS? power? DAC?

Wolfson Microelectronics recently launched the first in the company's next generation of low-power audio devices, an audio codec designed to extend playback time. Targeted at portable devices, the WM8903 combines several new technological fixes to optimize audio quality and reduce power consumption so that audio playback of up to 40h can potentially be achieved on a single charge.

With additional functionality within portable devices and battery life not getting any longer, Peter Frith, chief technical officer, Wolfson, said the playback time for audio devices is no longer comparable to several years ago. While digital techniques have significantly reduced the toll on power consumption attributable to audiothe processor power required to decode a compressed audio file has reduced from 50mW to 12mWthe pressure has been on to minimize the power consumption of the analog portion of the circuitry. He added that DAC playback was 26mW. "Our WM8956 brought DAC to headphone power consumption down to 10mW, and we have now reduced this to 4.5mW," he noted._

Wolfson DAC design
Crucial to this achievement have been some fundamental changes to Wolfson's DAC architecture. Frith was referring to the paradox that a higher voltage level, requiring a higher current source, needs to be maintained to achieve a small signal to noise ratio. Wolfson's SmartDAC employs capacitor switching to preserve signal to noise and minimize current input. "Our technology is proprietary, but the result is a smaller reference supply but with a bigger output," Frith said. "The SmartDAC also avoids the requirement for a dedicated audio supply," he added.

A second major design change for this codec addresses the headphone load. According to Frith, there has been considerable inefficiency in the headphone drivers, with 3 percent being typical. The main reason for this was the disparity between average audio playback and peak. While the amplifier of choice must suit the peak, the outside peak operation is consuming far more power than necessary. Frith noted that, "If a 3V rail is supplying the amplifier, then as much as 2.9V could be wasted."

Solving the problem
Wolfson solved the problem by combining an adaptive dual charge pump and DC servo architecture. The capless charge pump is an evolution from class H and G techniques. The company called it Class W, as it added a control algorithm to forecast and optimize amplifier efficiency for low-power consumption in playback mode, depending on music content signal levels.

Finally, it has reduced unwanted noise in the headphone amplifier output during power-up and power-down by employing clamping circuitry that holds the headphone output to ground during power up.

With the use of digital microphone technology accelerating, such as the MEMS microphones that Wolfson acquired with Oligon Ltd in 2007, the WM8903 integrates a digital microphone interface as well as six configurable analog inputs for stereo microphone recording and connection to various analog-audio sources.

- Vanessa Knivett
Analog Designline Europe

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