WM8940GEFLSLASHV datasheet(57/85 Pages) WOLFSON

57 / 85 page

Pre-Production

WM8940

Pre-Production, Rev 3.0, February 2007

LOOPBACK

Setting the ADC_LOOPBACK or DAC_LOOPBACK register bit enables digital loopback. When the

ADC_LOOPBACK bit is set the output data from the ADC audio interface is fed directly into the DAC

data input. When the DAC_LOOPBACK bit is set the output data from the DAC audio interface is fed

directly to the input of the ADC audio interface.

COMPANDING

The WM8940 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 43 Audio Interface Control for setting the output word length.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

DAC_LOOPBACK

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_LOOPBACK

Digital loopback function

0=No ADC loopback

1=Loopback enabled, ADC data output is

fed directly into DAC data input.

Table 50 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.

Companding converts 13 bits (

µ-law) or 12 bits (A-law) to 8 bits using non-linear quantization. The

input data range is separated into 8 levels, allowing low amplitude signals better precision than that

of high amplitude signals. This is to exploit the operation of the human auditory system, where

louder sounds do not require as much resolution as quieter sounds. The companded signal is an 8-

bit word containing sign (1-bit), exponent (3-bits) and mantissa (4-bits).

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