WM8952ECSSLASHRV datasheet(47/72 Pages) WOLFSON

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Pre Production

WM8952

PP, Rev 3.1, June 2011

COMPANDING

The WM8952 supports A-law and

μ-law companding. This can be enabled by writing the appropriate

value to the ADC_COMP register bit. 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 29 Audio Interface Control for setting the output word length.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

Companding

control

2:1

ADC_COMP

ADC companding

00=off

01=reserved

10=μ-law

11=A-law

Table 36 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).

BIT7

BIT[6:4]

BIT[3:0]

SIGN

EXPONENT

MANTISSA

Table 37 8-bit Companded Word Composition

u-law Companding

100

120

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Normalised Input

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Figure 25 u-Law Companding

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