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Wolfson combines analog and digital tricks to extend portable playback times

Wolfson Microelectronics has 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 optimise audio quality and minimise power consumption so that audio playback of up to 40 hours can potentially be achieved on a single charge.

With much additional functionality within portable devices and battery life not getting any longer, Wolfson's cto Peter Frith explains that the playback time for audio devices is no longer than several years ago. Whilst digital techniques have significantly minimised the toll on power consumption attributable to audio – the processor power required to decode a compressed audio file has reduced from 50mW to 12mW – the pressure has been on to reduce the power consumption of the analog portion of the circuitry. Notes Frith: “A few years ago, DAC playback was 26mW. Our WM8956 brought DAC to headphone power consumption down to 10mW, and we have now reduced this to 4.5mW.”

Crucial to this achievement have been some fundamental changes to Wolfson's DAC architecture. Frith refers to the paradox that a higher voltage level, requiring a higher current source, needs to be maintained in order to achieve a small signal to noise ratio. Wolfson's SmartDAC employs capacitor switching to preserve signal to noise and minimise current input. Says Frith: “Our technology is proprietary, but the result is a smaller reference supply but with a bigger output.” The SmartDAC also avoids the requirement for a dedicated audio supply.

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 just 3% being typical. The primary reason for this has been the disparity between average audio playback and peak. Whilst the amplifier of choice must suit the peak, this means that outside peak operation, it is consuming far more power than necessary. According to Frith, if a 3V rail is supplying the amplifier, then as much as 2.9V could be wasted. Wolfson solves the problem by combining an adaptive dual charge pump and DC servo architecture. This capless charge pump is an evolution from class H and G techniques. Wolfson calls it Class W, as it adds a control algorithm to forecast and optimise amplifier efficiency for low power consumption in playback mode, depending on music content signal levels.

Finally, Wolfson 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 Wolfson acquired with Oligon 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.

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