WM8955L

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Product Preview Rev 0.4 May2003

18

DIGITAL TO ANALOGUE CONVERTER (DAC)

Treble and linear bass enhancement mayproduce signals that exceed full-scale. In order to avoid

limiting under these conditions, it is recommended to set the DAT bit to attenuate the digital input

signal by6dB. The gain at the outputs should be increased by6dB to compensate for the

attenuation. Cut-onlytone adjustment and adaptive bass boost cannot produce signals above full-

scale and therefore do not require the DAT bit to be set.

After passing through the tone control filters, digital ‘de-emphasis’ can be applied to the audio data if

necessary(e.g. when the data comes from a CD with pre-emphasis used in the recording). De-

emphasis filtering is available for sample rates of 48kHz, 44.1kHz and 32kHz.

The WM8955L also has a Soft Mute function, which graduallyattenuates the volume of the digital

signal to zero. This function is enabled bydefault. To playback an audio signal, the WM8955L must

first be unmuted bysetting the DACMU bit to zero.

REGISTER

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

7

DAT

0

DAC 6dB attenuate enable

0 = disabled (0dB)

1 = -6dB enabled

3

DACMU

1

Digital Soft Mute

1= mute

0 = no mute (signal active)

R5 (05h)

DAC Control

2:1

DEEMPH

00

De-emphasis Control

11 = 48kHz sample rate

10 = 44.1kHz sample rate

01 = 32kHz sample rate

00 = No De-emphasis

Table 4 DAC Control

The digital audio data is converted to oversampled bit streams in the on-chip, true 24-bit digital

interpolation filters. The bitstream data enters two multi-bit, sigma-delta DACs, which convert them to

high qualityanalogue audio signals. The multi-bit DAC architecture reduces high frequencynoise and

sensitivityto clock jitter. It also uses a Dynamic Element Matching technique for high linearityand

low distortion.

In normal operation, the left and right channel digital audio data are converted to analogue in two

separate DACs. However, it is also possible to disable one channel, so that the same signal (left or

right) appears on both analogue output channels. Additionally, there is a mono-mix mode where the

two audio channels are mixed together digitallyand then converted to analogue using onlyone DAC,

while the other DAC is switched off. The mono-mix signal can be selected to appear on both

analogue output channels (see Analogue Outputs).

REGISTER

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R23 (17h)

Additional (1)

5:4

DMONOMIX[1:0]

00

DAC mono mix

00: stereo

01: mono ((L+R)/2) into DACL, ‘0’ into

DACR

10: mono ((L+R)/2) into DACR, ‘0’ into

DACL

11: mono ((L+R)/2) into DACL & DACR

Table 5 DAC Mono Mix Select