WM8746
Production Data
DEVICE DESCRIPTION
INTRODUCTION
WM8746 is a complete 6-channel stereo audio digital-to-analogue converter, including digital
interpolation filter, multi-bit sigma delta with dither, and switched capacitor multi-bit stereo DAC and
output smoothing filters.
The device is implemented as three separate stereo DACs in a single package and controlled by a
single interface. Each DAC has its own data input DIN0/1/2, and LRCIN, BCKIN and SCKI are
shared between them. An additional LRCIN2 input is provided to allow for the front channels in a
surround system to be run at higher sample rate than the other 4 channels (ie. 192kHz for front
channels and 96kHz). In this mode the same SCKI is used for all channels, the front channels being
run at twice the over-sampling rate of the other channels.
Control of internal functionality of the device is by either hardware control (pin programmed) or
software control (3-wire serial control interface). The MODE pin selects between hardware and
software control. In software control mode, an SPI type interface is used. This interface may be
asynchronous to the audio data interface. Control data will be re-synchronised to the audio
processing internally.
Operation using a system clock of 256fs, 384fs, 512fs or 768fs is provided, selection between clock
rates being automatically detected. Sample rates (fs) from less than 8kHz to 96kHz are allowed,
provided the appropriate system clock is input. Support is also provided for up to 192kHz using a
system clock of 128fs or 192fs.
The audio data interface supports right, left and I2S 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 (16, 20, 24, or 32-bit)
and de-emphasis functions.
AUDIO DATA SAMPLING RATES
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 SCKI input pin
with no software configuration necessary. Note that on the WM8746, SCKI 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.
The system clock for WM8746 supports audio sampling rates from 128fs to 768fs, where fs is the
audio sampling frequency (LRCIN) typically 32kHz, 44.1kHz, 48kHz, 96kHz or 192kHz. The system
clock is used to operate the digital filters and the noise shaping circuits.
The WM8746 has a system clock detection circuit that automatically determines the relationship
between the system clock frequency and the sampling rate (to within +/- 32 system clocks). If greater
than 32 clocks error, the interface defaults to 768fs and maintains the output level at the last sample.
The system clock should be synchronised with LRCIN, although the WM8746 is tolerant of phase
differences or jitter on this clock. Table 4 shows the typical system clock frequency inputs for the
WM8746.
SAMPLING
RATE (FS)
SYSTEM CLOCK FREQUENCY (MHZ)
128fs
192fs
256fs
384fs
512fs
768fs
(LRCIN)
32kHz
4.096
5.6448
6.144
6.144
8.467
8.192
11.2896
12.288
24.576
12.288
16.9340
18.432
36.864
16.384
22.5792
24.576
24.576
33.8688
36.864
44.1kHz
48kHz
9.216
96kHz
12.288
24.576
18.432
36.864
Unavailable Unavailable
192kHz
Unavailable Unavailable Unavailable Unavailable
Table 4 System Clock Frequencies Versus Sampling Rate
March 2006, PD Rev 4.0
10
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
