Advanced Information
WM9707
DEVICE DESCRIPTION
INTRODUCTION
The WM9707 is fully compliant with Revision 2.1 of the AC’97 specification.
The WM9707 comprises a stereo 18-bit Codec, (that is, 2 ADCs and 2 DACs) a comprehensive
analogue mixer with 4 sets of stereo inputs, phone, 2 microphone, and PC-beep inputs. Additionally,
on-chip reference generation circuits generate the necessary bias voltages for the device, and a
bidirectional serial interface allows transfer of control data and DAC and ADC words to and from the
AC’97 controller. The WM9707 supports 18-bit resolution within the DAC and ADC functions, but the
AC’97 serial interface specification allows any word length up to 20-bits to be written to, or read from,
the AC’97 Codec. These words are MSB justified, and any LSBs not used will simply default to 0.
Normally it is anticipated that 16-bit words will be used in most PC type systems. Therefore, for the
DAC, 16-bit words will be downloaded into the Codec from the controller, along with padding of 0s to
make the 16-bit word up to 20-bit length. In this case, the WM9707 will process the 16-bit word along
with 0 padding bits in the 2 LSB locations (to make 18-bit). At the ADC output, WM9707 will provide
an 18-bit word, again with 0s in the two LSB locations (20-bit). The AC’97 controller will then ignore
the 4 LSBs of the 20-bit word. When the WM9707 is interrogated at Register 00h, it responds
indicating it is an 18-bit device.
The WM9707 has the ADC and DAC functions implemented using oversampled, or sigma-delta
converters, and uses on-chip digital filters to convert these 1-bit signals to and from the 48ks/s 16/18-
bit PCM words that the AC’97 controller requires. The digital parts of the device are powered
separately from the analogue to optimise performance, and 3.3V digital and 5V analogue supplies
may be used on the same device to further optimise performance. Digital IOs are 5V tolerant when
the analogue supplies are 5V, so the WM9707 may be connected to a controller running on 5V
supplies, but use 3.3V for the digital section of WM9707. WM9707 is also capable of operating with a
3.3V supply only (digital and analogue).
An internally generated midrail reference is provided at pin CAP2 which is used as the chip
reference. This pin should be heavily decoupled. Refer to Figure 17 for more details.
The WM9707 is not limited to PC-only applications. The ability to power down sections of the device
selectively, and the option to choose alternative master clock, and hence sample rates, means that
many alternative applications in areas such as telecomms, may be anticipated.
Additional features added to the Intel AC’97 specification, such as the EAPD (External Amplifier
Powerdown) bit, internal connection of PC-beep to the outputs in the case where the device is reset,
are supported, along with optional features such as variable sample rate support and SPDIF output.
3D STEREO ENHANCEMENT
This device contains a stereo enhancement circuit, designed to optimise the listening experience
when the device is used in a typical PC operating environment. That is, with a pair of speakers
placed either side of the monitor with little spatial separation. This circuit creates a difference signal
by differencing left and right channel playback data, then filters this difference signal using lowpass
and highpass filters whose time constants are set using external capacitors connected to the CX3D
pins 33 and 34. Typically the values of 100nF and 47nF set highpass and lowpass poles at about
100Hz and 1kHz respectively. This frequency band corresponds to the range over which the ear is
most sensitive to directional effects.
The filtered difference signal is gain adjusted by an amount set using the 4-bit value written to
Register 22h bits 3 to 0. Value 0h is disable, value Fh is maximum effect. Typically a value of 8h is
optimum. The user interface would most typically use a slider type of control to allow the user to
adjust the level of enhancement to suit the program material. Bit D13 3D in Register 20h is the
overall 3D enable bit. The capability Register 00h reads back the value 11000 in bits D14 to D10.
This corresponds to decimal 24, which is registered with Intel as Wolfson Stereo Enhancement.
Note that the external capacitors setting the filtering poles applied to the difference signal may be
adjusted in value, or even replaced with a direct connection between the pins. If such adjustments
are made, then the amount of difference signal fed back into the main signal paths may be
significant, and can cause large signals which may limit, distort, or overdrive signal paths or
speakers. Adjust these values with care, to select the preferred acoustic effect.
WOLFSON MICROELECTRONICS LTD
AI Rev 2.2 January 2001
11
WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
