Production Data

WM9704M

WOLFSON MICROELECTRONICS LTD

PD Rev 3.2 January 2001

13

REV 2.1 COMPLIANT 2-CHANNEL MODE

Features in this mode are as follows:

•

Vendor ID reads back as WML3

•

2 channels of ADC and DAC conversion provided, with all recommended Audio and Modem

sample rates supported via the audio sample rate registers 2Ch and 32h

•

Master/slave ID0/1 are supported

•

Headphone/line level outputs duplicating the main outputs, are supported, with gain control

from register 04h

•

Wolfson 3D stereo enhanced sound supported

•

Master volume control register maps to the location dependant on selected ID: (ID 00 or 01

uses master volume at register 02h, ID 10 uses 38h (surround volume) and ID11 uses 36h

(LFE, Centre volume)

GPIO PINS

The AC’97 Rev 2.1 specification allows for provision of programmable IO pins, up to 16 in number.

Within the 48-pin TQFP package used, provision has been made for 3 pins to be used as GPIO pins.

These pins (numbers 43, 44, 48) are also used as I

When used as GPIO pins, the pins 43, 44, 48 are mapped onto the bits 11,12,13 in the AC-link slot

12. These optional locations may be configured in any way: as inputs or outputs, supporting interrupt

operation etc, offering maximum flexibility to the user. The appropriate GPIO control registers are

supported to control these pins.

Configuration of these pins as GPIO is described in the Control Interface description.

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.

There is no provision for pseudo-stereo effects. Mono signals will have no enhancement applied

(if the signals are in phase and of the same amplitude).

Signals from the PCM DAC channels do not have stereo enhancement applied. It is assumed that

these signals will already have been processed digitally with any required 3D-enhancement effect.

Applying the analogue 3D enhancement will corrupt this digital effect. This is equivalent to setting the

POP bit in register 20h. As a result, the readback value of this bit is fixed as '1', and attempts to

change it will be ignored. POP bit is set to one and cannot be re-set.