WM8731_06 Datasheet, PDF (20/63 Pages) Wolfson Microelectronics plc – Portable Internet Audio CODEC with Headphone Driver and Programmable Sample Rates

WM8731 / WM8731L
Production Data
DEVICE DESCRIPTION
INTRODUCTION
The WM8731/L is a low power audio CODEC designed specifically for portable audio products. It’s
features, performance and low power consumption make it ideal for portable MP3 players and
portable mini-disc players.
The CODEC includes line and microphone inputs to the on-board ADC, line and headphone outputs
from the on-board DAC, a crystal oscillator, configurable digital audio interface and a choice of 2 or 3
wire MPU control interface. It is fully compatible and an ideal partner for a range of industry standard
microprocessors, controllers and DSPs.
The CODEC includes three low noise inputs - mono microphone and stereo line. Line inputs have
+12dB to -34dB logarithmic volume level adjustments and mute. The Microphone input has -6dB to
34dB volume level adjustment. An electret microphone bias level is also available. All the required
input filtering is contained within the device with no external components required.
The on-board stereo analogue to digital converter (ADC) is of a high quality using a multi-bit high-
order oversampling architecture delivering optimum performance with low power consumption. The
output from the ADC is available on the digital audio interface. The ADC includes an optional digital
high pass filter to remove unwanted dc components from the audio signal.
The on-board digital to analogue converter (DAC) accepts digital audio from the digital audio
interface. Digital filter de-emphasis at 32kHz, 44.1kHz and 48kHz can be applied to the digital data
under software control. The DAC employs a high quality multi-bit high-order oversampling
architecture to again deliver optimum performance with low power consumption.
The DAC outputs, Microphone (SIDETONE) and Line Inputs (BYPASS) are available both at line
level and through a headphone amplifier capable of efficiently driving low impedance headphones.
The headphone output volume is adjustable in the analogue domain over a range of +6dB to –73dB
and can be muted.
The design of the WM8731/L has given much attention to power consumption without compromising
performance. It includes the ability to power off selective parts of the circuitry under software control,
thus conserving power. Nine separate power save modes be configured under software control
including a standby and power off mode.
Special techniques allow the audio to be muted and the device safely placed into standby, sections
of the device powered off and volume levels adjusted without any audible clicks, pops or zipper
noises. Therefore standby and power off modes maybe used dynamically under software control,
whenever recording or playing is not required.
The device caters for a number of different sampling rates including industry standard 8kHz, 32kHz,
44.1kHz, 48kHz, 88.2kHz and 96kHz. Additionally, the device has an ADC and DAC that can operate
at different sample rates.
There are two unique schemes featured within the programmable sample rates of the WM8731/L:
Normal industry standard 256/384fs sampling mode may be used, with the added ability to mix
different sampling rates. Also a special USB mode is included, whereby all audio sampling rates can
be generated from a 12.00MHZ USB clock. Thus, for example, the ADC can record to the DSP at
44.1kHz and be played back from the CODEC at 8kHz with no external digital signal processing
required. The digital filters used at for both record and playback are optimised for each sampling rate
used.
The digitised output is available in a number of audio data formats I²S, DSP Mode (a burst mode in
which frame sync plus 2 data packed words are transmitted), MSB-First, left justified and MSB-First,
right justified. The digital audio interface can operate in both master or slave modes.
The software control uses either 2 or 3-wire MPU interface.
A crystal oscillator is included on board the device. The device can generate the system master clock
or alternatively it can accept an external master clock from the audio system.
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PD Rev 4.3 August 2006
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