WM9704M

Production Data

WOLFSON MICROELECTRONICS LTD

PD Rev 3.2 January 2001

14

VARIABLE SAMPLE RATE SUPPORT

The DACs and ADCs on this device support all the recommended sample rates specified in the Intel

Rev 2.1 specification for both audio and modem rates. Default rates are 48ks/s. If alternative rates

are selected, the AC’97 interface continues to run at 48k words per second, but data is transferred

across the link in bursts such that the net sample rate selected is achieved. It is up to the AC’97

Rev 2.1 compliant controller to ensure that data is supplied to the AC link, and received from the AC

link, at the appropriate rate.

The device supports ‘on demand’ sampling. That is, when the DAC signal processing circuits need

another sample, a sample request is sent to the controller which must respond with a data sample in

the next frame it sends. For example, if a rate of 24ks/s is selected, on average the device will

request a sample from the controller every other frame, for each of the stereo DACs. Note that if an

unsupported rate is written to one of the rate registers, the rate will default to the nearest rate

supported. The register will then respond when interrogated with the supported rate the device has

defaulted to.

ADCs are controlled similarly but with one difference: Normally the left and right channel ADCs

sample at the same rate.

AUDIO

SAMPLE RATE

CONTROL VALUE

D15 TO D0

MODEM

SAMPLE RATE

CONTROL VALUE

D15 TO D0

7200

1C20

8228.57 (57600/7)

2024

8000

1F40

8400

20D0

11025

2B11

9000

2328

16000

3E80

9600

2580

22050

5622

10285.71 (72000/7)

282D

44100

AC44

12000

2EE0

48000

BB80

13714.28 (96000/7)

3592

19200

4B00

24000

5DC0

Table 2 Variable Sample Rates Supported

The following table shows which registers control which DAC rates, versus Mode and ID selected

MODE

ID

FRONT DAC RATE REGISTER

REAR DAC RATE REGISTER

ADC RATE

REG

00 and 01

2Ch

10

2Eh

Rev 2.1 mode (00)

11

2Ch (centre) and 30h (LFE)

32h

00 and 01

2Ch

10

2Eh

Modem mode (11)

11

2Ch (centre) and 30h (LFE)

40h

32h (40h if

PHONE

IP selected)

Table 3 Variable Rate Register Location Versus Mode and ID

GAIN CONTROL REGISTER LOCATION VERSUS MODE AND ID

Depending on what mode and ID the device is operated in, the various gain control registers have

locations in the register map that may change.

In Modem mode, the rear volume is mapped always to location 46h, and the rear mixer is fixed

muted so analogue sources cannot be summed into the Tx modem signal.