WM8941 datasheet(66/96 Pages) WOLFSON

66 / 96 page

WM8941

Pre Production

PP, Rev 3.3, December 2007

COMPANDING

The WM8941 supports A-law and

µ-law companding on both transmit (ADC) and receive (DAC)

sides. Companding can be enabled on the DAC or ADC audio interfaces by writing the appropriate

value to the DAC_COMP or ADC_COMP register bits respectively. If packed mode companding is

desired the WL8 register bit is available. It will override the normal audio interface WL bits to give an

8-bit word length. Refer to Table 44 Audio Interface Control for setting the output word length.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

DAC_LOOPBAC

Digital loopback function

0=No DAC loopback

1=Loopback enabled, DAC audio interface

output is fed directly into ADC audio

interface input.

4:3

DAC_COMP

DAC decompanding

00=off

01=reserved

10=µ-law

11=A-law

2:1

ADC_COMP

ADC companding

00=off

01=reserved

10=µ-law

11=A-law

Companding

control

ADC_LOOPBAC

Digital loopback function

0=No ADC loopback

1=Loopback enabled, ADC data output is

fed directly into DAC data input.

Table 51 Companding Control

Companding involves using a piecewise linear approximation of the following equations (as set out

by ITU-T G.711 standard) for data compression:

µ-law (where µ=255 for the U.S. and Japan):

F(x) = ln( 1 +

µ|x|) / ln( 1 + µ)

-1 ≤ x ≤ 1

A-law (where A=87.6 for Europe):

F(x) = A|x| / ( 1 + lnA)

} for x ≤ 1/A

F(x) = ( 1 + lnA|x|) / (1 + lnA)

} for 1/A ≤ x ≤ 1

The companded data is also inverted as recommended by the G.711 standard (all 8 bits are inverted

for

µ-law, all even data bits are inverted for A-law). The data will be transmitted as the first 8 MSB’s

of data.

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