WM8978 datasheet(57/78 Pages) WOLFSON

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WM8978

PP Rev 1.7 January 2005

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 (linear mode)

01=reserved

10=µ-law

11=A-law

4:3

DAC_COMP

DAC companding

00=off (linear mode)

01=reserved

10=µ-law

11=A-law

Companding

Control

WL8

0 = off

1 = device operates in 8-bit mode.

Table 46 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).

Setting the WL8 register bit allows the device to operate with 8-bit data. In this mode it is possible to

use 8 BCLK’s per LRC frame. When using DSP mode B, this allows 8-bit data words to be output

consecutively every 8 BCLK’s and can be used with 8-bit data words using the A-law and u-law

companding functions.

BIT8

BIT[7:4]

BIT[3:0]

SIGN

EXPONENT

MANTISSA

Table 47 8-bit Companded Word Composition

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