Pre-Production
WM8904
QUICK RELEASE CONTROL
The DRC includes a Quick-Release feature to handle short transient peaks that are not related to the
intended source signal. For example, in handheld microphone recording, transient signal peaks
sometimes occur due to user handling, key presses or accidental tapping against the microphone.
The Quick Release feature ensures that these transients do not cause the intended signal to be
masked by the longer time constants of DRC_DCY.
The Quick-Release feature is enabled by setting the DRC_QR bit. When this bit is enabled, the DRC
measures the crest factor (peak to RMS ratio) of the input signal. A high crest factor is indicative of a
transient peak that may not be related to the intended source signal. If the crest factor exceeds the
level set by DRC_QR_THR, then the normal decay rate (DRC_DCY) is ignored and a faster decay
rate (DRC _QR_DCY) is used instead.
The DRC Quick-Release control bits are described in Table 20.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
Quick release enable
R40 (28h)
2
DRC_QR
1
DRC Control 0
0 = disabled
1 = enabled
R41 (29h)
Quick release crest factor threshold
00 = 12dB
7:6
5:4
DRC_QR_THR
[1:0]
01
00
DRC Control 1
01 = 18dB (default)
10 = 24dB
11 = 30dB
Quick release decay rate
(seconds/6dB)
DRC_QR_DCY
[1:0]
00 = 0.725ms (default)
01 = 1.45ms
10 = 5.8ms
11 = Reserved
Table 20 DRC Quick-Release Control
GAIN SMOOTHING
The DRC includes a gain smoothing filter in order to prevent gain ripples. A programmable level of
hysteresis is also used to control the DRC gain. This improves the handling of very low frequency
input signals whose period is close to the DRC attack/decay time. DRC Gain Smoothing is enabled by
default and it is recommended to use the default register settings.
The extent of the gain smoothing filter may be adjusted or disabled using the control fields described
in Table 21.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
R40 (28h)
Gain smoothing hysteresis
threshold
12:11
DRC_GS_HYST
_LVL [1:0]
00
DRC Control 0
00 = Low
01 = Medium (recommended)
10 = High
11 = Reserved
Gain smoothing enable
0 = disabled
3
0
DRC_GS_ENA
DRC_GS_HYST
1
1
1 = enabled
Gain smoothing hysteresis enable
0 = disabled
1 = enabled
Table 21 DRC Gain Smoothing
PP, Rev 3.3, September 2012
57
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
