wm8940gefl-v Wolfson Microelectronics plc, wm8940gefl-v Datasheet - Page 59

• Manufacturer Part Number: wm8940gefl-v
• Description: Mono Codec With Speaker Driver
• Manufacturer: Wolfson Microelectronics plc

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Production Data
Table 51 8-bit Companded Word Composition
Figure 31 u-Law Companding
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)
A-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).
SIGN
BIT7
120
100
80
60
40
20
0
0
EXPONENT
0.1
0.2
BIT[6:4]
0.3
-1 ≤ x ≤ 1
u-law Companding
0.4
Normalised Input
} for x ≤ 1/A
} for 1/A ≤ x ≤ 1
0.5
MANTISSA
0.6
0.7
BIT[3:0]
0.8
0.9
PD, Rev 4.2, April 2008
1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
WM8940
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