Pre-Production
WM8985
LOOPBACK
Setting the LOOPBACK register bit enables digital loopback. When this bit is set the output data
from the ADC audio interface is fed directly into the DAC data input.
COMPANDING
The WM8985 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.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
R5 (05h)
2:1
ADC_COMP
0
ADC companding
Companding
Control
00 = off
01 = reserved
10 = µ-law
11 = A-law
4:3
DAC_COMP
0
0
DAC companding
00 = off
01 = reserved
10 = µ-law
11 = A-law
5
WL8
0 = off
1 = device operates in 8-bit mode.
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:
µ-law (where µ=255 for the U.S. and Japan):
F(x) = ln( 1 + µ|x|) / ln( 1 + µ)
-1 ≤ x ≤ 1
Table 58 law (where A=87.6 for Europe):
F(x) = A|x| / ( 1 + lnA)
} for x ≤ 1/A
} for 1/A ≤ x ≤ 1
F(x) = ( 1 + lnA|x|) / (1 + lnA)
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.
BIT7
BIT[6:4]
BIT[3:0]
SIGN
EXPONENT
MANTISSA
Table 50 8-bit Companded Word Composition
PP, Rev 3.4, October 2006
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
