WM8904
Pre-Production
DESCRIPTION
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
Gain decay rate (seconds/6dB)
0000 = 186ms
11:8
DRC_DCY [3:0]
0010
0001 = 372ms
0010 = 743ms (default)
0011 = 1.49s
0100 = 2.97s
0101 = 5.94s
0110 = 11.89s
0111 = 23.78s
1000 = 47.56s
1001-1111 = Reserved
Table 18 DRC Attack and Decay Rates
Note:
For detailed information about DRC attack and decay rates, please see Wolfson application note
WAN0247.
ANTI-CLIP CONTROL
The DRC includes an Anti-Clip feature to avoid signal clipping when the input amplitude rises very
quickly. This feature uses a feed-forward technique for early detection of a rising signal level. Signal
clipping is avoided by dynamically increasing the gain attack rate when required. The Anti-Clip feature
is enabled using the DRC_ANTICLIP bit.
Note that the feed-forward processing increases the latency in the input signal path. For low-latency
applications (e.g. telephony), it may be desirable to reduce the delay, although this will also reduce
the effectiveness of the anti-clip feature. The latency is determined by the DRC_FF_DELAY bit. If
necessary, the latency can be minimised by disabling the anti-clip feature altogether.
The DRC Anti-Clip control bits are described in Table 19.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
R40 (28h)
DRC Control 0
Feed-forward delay for anti-clip
feature
5
DRC_FF_DELAY
1
0 = 5 samples
1 = 9 samples
Time delay can be calculated as 5/fs
or 9/ fs, where fs is the sample rate.
Anti-clip enable
0 = disabled
1 = enabled
1
DRC_ANTICLIP
1
Table 19 DRC Anti-Clip Control
Note that the Anti-Clip feature operates entirely in the digital domain. It cannot be used to prevent
signal clipping in the analogue domain nor in the source signal. Analogue clipping can only be
prevented by reducing the analogue signal gain or by adjusting the source signal.
PP, Rev 3.3, September 2012
56
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
