wm8351 Wolfson Microelectronics plc, wm8351 Datasheet - Page 96

• Manufacturer Part Number: wm8351
• Description: Wolfson Audioplus? Stereo Codec With Power Management
• Manufacturer: Wolfson Microelectronics plc

WM8351
13.11 COMPANDING
w
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.
R113 (71h)
ADC Control
Table 46 Enabling loopback
The WM8351 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.
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 MSBs
of data.
Companding converts 13 bits (μ-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).
ADDRESS
R113 (71h)
Companding
Control
REGISTER
ADDRESS
BIT
0
BIT
4
5
6
7
LOOPBACK
ADC_COMPM
ODE
ADC_COMP
DAC_COMPM
ODE
DAC_COMP
LABEL
LABEL
-1 ≤ x ≤ 1
} for x ≤ 1/A
} for 1/A ≤ x ≤ 1
DEFAULT
DEFAULT
0
0
0
0
0
Digital Loopback Function
0 = No loopback.
1 = Loopback enabled, ADC data output is
fed directly into DAC data input.
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 Companding enable
0 = off
1 = on
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 Companding enable
0 = off
1 = on
DESCRIPTION
DESCRIPTION
PD, June 2009, Rev 4.1
Production Data
96