WM8986_07 WOLFSON [Wolfson Microelectronics plc], WM8986_07 Datasheet - Page 57

• Manufacturer Part Number: WM8986_07
• Description: Multimedia DAC With Class D Headphone and Line Out
• Manufacturer: WOLFSON [Wolfson Microelectronics plc]

Pre-Production
COMPANDING
w
The WM8986 supports A-law and µ-law companding. Companding can be enabled on the DAC
audio interface by writing the appropriate value to the DAC_COMP register bit.
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)
law (where A=87.6 for Europe):
F(x) = A|x| / ( 1 + lnA)
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).
Table 50 8-bit Companded Word Composition
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.
R5 (05h)
Companding
Control
SIGN
BIT7
REGISTER
ADDRESS
4:3
5
BIT
EXPONENT
BIT[6:4]
DAC_COMP
WL8
LABEL
-1 ≤ x ≤ 1
} for x ≤ 1/A
} for 1/A ≤ x ≤ 1
0
0
DEFAULT
DAC companding
00 = off
01 = reserved
10 = µ-law
11 = A-law
0 = off
1 = device operates in 8-bit mode.
MANTISSA
BIT[3:0]
DESCRIPTION
PP, Rev 3.1, February 2007
WM8986
57