Portable media players, like the Apple iPod and its competitors, have been around for many years, so you might think no further improvements are possible. But this doesn’t seem to be the case. Somehow, the companies that develop audio chips for these devices continue to come up with innovative ways to improve their offerings. The benefit to consumers is a more enjoyable listening experience for longer periods of time.
A slew of new audio chips promising high-quality audio and very low power consumption has surfaced since the beginning of the year, destined for a new generation of PMPs and cell phones. We highlight a selection of these chips here.
NEW CODEC ICS
Introduced in February, the WM8900 from Wolfson Microelectronics (www.wolfsonmicro.com) is a high-performance, ultra-low-power audio codec that employs the company’s AudioPlus Smart Power technology (Fig. 1). The chip uses a class G, ground-referenced headphone driver.
The WM8900’s quiescent headphone playback consumes less than 6 mW in voice mode and less than 11 mW in hi-fi mode, extending battery life in portable audio applications. Using a typical 300-mAh battery, the chip can push battery life out to 11 hours during headphone music playback at a typical 2-mW/ch listening level.
Wolfson also designed the WM8900 to address cost and board space through the aforementioned ground-referenced headphone outputs, which remove the need for bulky dc blocking capacitors. The device comes in a low-profile, 0.55-mm high, 40-pin quad flat no-lead (QFN) package—a good choice for slim, portable electronic applications.
The WM8900 class G amplifier architecture is implemented by powering the headphone amplifier with a dual-input, levelshifting, intelligent charge pump. This pump also generates both the positive and negative power-supply rails, ground-referencing the headphone outputs.
Automatic control of the charge pump maintains the most power-efficient operating state during headphone playback with no intervention required from the operator or host software. Also, the ground-referenced class G headphone amplifier eliminates many sources of pops and clicks during power-up, power-down, mute, and unmute to deliver high-quality audio performance and improved bass response.
“The WM8900 introduces class G amplifier technology as a means to deliver ultra-low-power, ground-referenced headphone drive on a hi-fi audio codec. The device helps designers of portable media players and multimedia handsets to meet the challenge of delivering longer battery life with reduced system cost,” said Nat Edington, vice president of marketing at Wolfson.
The codec uses stereo 24-bit, 64x oversampled sigma-delta analog- to-digital converters (ADCs). The multibit feedback and high oversampling rates reduce the effects of jitter and high-frequency noise. The ADC includes digital gain control, digital filtering, and a programmable digital high-pass filter. The ADC full-scale input level is proportional to the analog-supply voltage, AVDD. With a 3.3-V supply voltage, the full-scale level is 1.0 V rms.
Digital audio data is converted to oversampled bit streams in on-chip, true 24-bit digital interpolation filters. The bit-stream data enters two multibit, sigma-delta digital-to-analog converters (DACs) that convert the data to high-quality analog audio signals.
The multibit DAC architecture reduces high-frequency noise and sensitivity to clock jitter. It also uses Wolfson’s Dynamic Element Matching technique for high linearity and low distortion. Analog outputs from the DACs can then be mixed with other analog inputs using the WM8900’s output mixers. This mix is fed to the output drivers for headphone or line outputs.
The master clock can be input directly or generated internally by an integrated low-power, frequency-locked loop (FLL). The WM8900 operates at analog supply voltages down to 2.4 V. In addition, the digital core can operate at voltages down to 1.8 V to save power. Different sections of the chip can also be powered down under software control.
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