Pre-Production
WM8904
DYNAMIC RANGE CONTROL (DRC)
The dynamic range controller (DRC) is a circuit which can be enabled in the digital data path of either
the ADCs or the DACs. The function of the DRC is to adjust the signal gain in conditions where the
input amplitude is unknown or varies over a wide range, e.g. when recording from microphones built
into a handheld system. The DRC can apply Compression and Automatic Level Control to the signal
path. It incorporates ‘anti-clip’ and ‘quick release’ features for handling transients in order to improve
intelligibility in the presence of loud impulsive noises.
The DRC is enabled by DRC_ENA, as shown in Table 15. It can be enabled in the ADC digital path or
in the DAC digital path, under the control of the DRC_DAC_PATH register bit. Note that the DRC can
be active in only one of these paths at any time.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
R40 (28h)
DRC_ENA
DRC enable
15
0
DRC Control 0
0 = disabled
1 = enabled
DRC_DAC_PAT
H
DRC path select
0 = ADC path
1 = DAC path
14
0
Table 15 DRC Enable
COMPRESSION/LIMITING CAPABILITIES
The DRC supports two different compression regions, separated by a “knee” at input amplitude T. For
signals above the knee, the compression slope DRC_HI_COMP applies; for signals below the knee,
the compression slope DRC_LO_COMP applies.
The overall DRC compression characteristic in “steady state” (i.e. where the input amplitude is near-
constant) is illustrated in Figure 29.
DRC Output Amplitude (dB)
(Y0)
“knee”
DRC_KNEE_OP
0dB
DRC_KNEE_IP
DRC Input Amplitude (dB)
Figure 29 DRC Compression Characteristic
The slope of the DRC response is determined by register fields DRC_HI_COMP and
DRC_LO_COMP respectively. A slope of 1 indicates constant gain in this region. A slope less than 1
represents compression (i.e. a change in input amplitude produces only a smaller change in output
amplitude). A slope of 0 indicates that the target output amplitude is the same across a range of input
amplitudes; this is infinite compression.
PP, Rev 3.3, September 2012
53
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
