WM8974 datasheet(45/63 Pages) WOLFSON

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WM8974

PP Rev 1.1 September 2004

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

LOOPBACK

Digital loopback function

0=No loopback

1=Loopback enabled, ADC data output is

ded directly into DAC data input.

2:1

ADC_COMP

ADC companding

00=off

01=reserved

10=µ-law

11=A-law

Companding

control

4:3

DAC_COMP

DAC companding

00=off

01=reserved

10=µ-law

11=A-law

Table 32 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

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).

BIT8

BIT[7:4]

BIT[3:0]

SIGN

EXPONENT

MANTISSA

Table 33 8-bit Companded Word Composition

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