Pre-Production
WM8904
GAIN LIMITS
The minimum and maximum gain applied by the DRC is set by register fields DRC_MINGAIN and
DRC_MAXGAIN. These limits can be used to alter the DRC response from that illustrated in Figure
29. If the range between maximum and minimum gain is reduced, then the extent of the dynamic
range control is reduced. The maximum gain prevents quiet signals (or silence) from being
excessively amplified.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
R41 (29h)
Minimum gain the DRC can use
to attenuate audio signals
3:2
DRC_MINGAIN [1:0]
10
DRC Control 1
00 = 0dB (default)
01 = -6dB
10 = -12dB
11 = -18dB
Maximum gain the DRC can use
to boost audio signals
1:0
DRC_MAXGAIN
[1:0]
00
00 = 12dB
01 = 18dB (default)
10 = 24dB
11 = 36dB
Table 17 DRC Gain Limits
DYNAMIC CHARACTERISTICS
The dynamic behaviour determines how quickly the DRC responds to changing signal levels. Note
that the DRC responds to the average (RMS) signal amplitude over a period of time.
DRC_ATK determines how quickly the DRC gain decreases when the signal amplitude is high.
DRC_DCY determines how quickly the DRC gain increases when the signal amplitude is low.
These register fields are described in Table 18. Note that the register defaults are suitable for general
purpose microphone use.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
R41 (29h)
Gain attack rate (seconds/6dB)
15:12
DRC_ATK [3:0]
0011
DRC Control 1
0000 = Reserved
0001 = 182µs
0010 = 363µs
0011 = 726µs (default)
0100 = 1.45ms
0101 = 2.9ms
0110 = 5.8ms
0111 = 11.6ms
1000 = 23.2ms
1001 = 46.4ms
1010 = 92.8ms
1011-1111 = Reserved
PP, Rev 3.3, September 2012
55
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