WM8351 datasheet(96/330 Pages) WOLFSON

96 / 330 page

WM8351

Production Data

PD, April 2012, Rev 4.5

13.10.4 LOOPBACK

When the loopback feature is enabled, the audio ADC’s digital output data is looped back to the audio

DAC and converted back into an analogue signal. This is often useful for test and evaluation

purposes.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R113 (71h)

ADC Control

LOOPBACK

Digital Loopback Function

0 = No loopback.

1 = Loopback enabled, ADC data output is

fed directly into DAC data input.

Table 47 Enabling loopback

13.11 COMPANDING

The WM8351 supports A-law and

-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.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R113 (71h)

Companding

Control

ADC_COMPM

ODE

ADC Companding mode select:

0 = µ-law

1 = A-law

(Note: Setting ADC_COMPMODE=1

selects 8-bit mode when DAC_COMP=0

and ADC_COMP=0)

ADC_COMP

ADC Companding enable

0 = off

1 = on

DAC_COMPM

ODE

DAC Companding mode select:

0 = µ-law

1 = A-law

(Note: Setting DAC_COMPMODE=1

selects 8-bit mode when DAC_COMP=0

and ADC_COMP=0)

DAC_COMP

DAC Companding enable

0 = off

1 = on

Table 49 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:

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

F(x) = ln( 1 +

|x|) / ln( 1 + )

-1 ≤ x ≤ 1

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

-law, all even data bits are inverted for A-law). The data will be transmitted as the first 8 MSBs of

data.

Companding converts 13 bits (

-law) or 12 bits (A-law) to 8 bits using non-linear quantization. This

provides greater precision for low amplitude signals than for high amplitude signals, resulting in a

greater usable dynamic range than 8 bit linear quantization. The companded signal is an 8-bit word

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

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