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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