WM8728
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DEVICE DESCRIPTION
INTRODUCTION
The WM8728 is a high performance DAC designed for digital consumer audio applications. Its
range of features makes it ideally suited for use in DVD players, AV receivers and other high-end
consumer audio equipment.
WM8728 is a complete 2-channel stereo audio digital-to-analogue converter, including digital
interpolation filter, multi-bit sigma delta with dither, switched capacitor multi-bit stereo DAC and
output smoothing filters. The WM8728 includes an on-chip digital volume control, configurable
digital audio interface and a 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.
Control of internal functionality of the device is by either hardware control (pin programmed) or
software control (2 or 3-wire serial control interface). The MODE pin selects between hardware
and software control. The software control interface may be asynchronous to the audio data
interface. In which case control data will be re-synchronised to the audio processing internally.
Operation using a master clock of 256fs, 384fs, 512fs or 768fs is provided, selection between
clock rates being automatically controlled in hardware mode, or serial controlled when in software
mode. Sample rates (fs) from less than 8ks/s to 96ks/s are allowed, provided the appropriate
master clock is input. Support is also provided for up to 192ks/s using a master clock of 128fs or
192fs.
The audio data interface supports right justified, left justified and I2S (Philips left justified, one bit
delayed) interface formats along with a highly flexible DSP serial port interface. When in hardware
mode, the three serial interface pins become control pins to allow selection of, input data format
type (I2S or right justified), input word length (20 or 24 bit) and de-emphasis functions.
In DSD mode, a separate bitstream data input pin is required for each of the channels, plus a 64fs
dataclock MCLK. These signals are applied via pins DIN and LRCIN and the signals routed
internally into the DAC circuits, under control of the SCKDSD mode select pin (19) See Figure 3.
Additionally a Phase Modulation scheme is supported, whereby the audio data is transmitted as a
Manchester encoded bitstream. This has the advantage of removing the significant spectral audio
energy from the datastream, minimising digital signal corruption of the analogue outputs. In order
to simplify decoding of this Phase modulated data, a 2x speed clock (BCKIN) is used to sample
the incoming data. See Figure 4.
The device is packaged in a small 20-pin SSOP.
CLOCKING SCHEMES
In a typical digital audio system there is only one central clock source producing a reference clock
to which all audio data processing is synchronised. This clock is often referred to as the audio
system’s Master Clock. The external master system clock can be applied directly through the
MCLK input pin with no software configuration necessary for sample rate selection.
Note that on the WM8728, MCLK is used to derive clocks for the DAC path. The DAC path
consists of DAC sampling clock, DAC digital filter clock and DAC digital audio interface timing. In
a system where there are a number of possible sources for the reference clock it is recommended
that the clock source with the lowest jitter be used to optimise the performance of the DAC.
WM8728 always acts as a slave and requires clocks to be inputs.
WOLFSON MICROELECTRONICS LTD
PP Rev 1.2 April 2001
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