WM8940
Pre-Production
DAC OUTPUT LIMITER
The WM8940 has a digital output limiter function. The operation of this is shown in Figure 20. In
this diagram the upper graph shows the envelope of the input/output signals and the lower graph
shows the gain characteristic.
Figure 20 DAC Digital Limiter Operation
The limiter has a programmable upper threshold which is close to 0dB. Referring to Table 33, in
normal operation (LIMBOOST=000 => limit only) signals below this threshold are unaffected by the
limiter. Signals above the upper threshold are attenuated at a specific attack rate (set by the
LIMATK register bits) until the signal falls below the threshold. The limiter also has a lower threshold
1dB below the upper threshold. When the signal falls below the lower threshold the signal is
amplified at a specific decay rate (controlled by LIMDCY register bits) until a gain of 0dB is reached.
Both threshold levels are controlled by the LIMLVL register bits. The upper threshold is 0.5dB above
the value programmed by LIMLVL and the lower threshold is 0.5dB below the LIMLVL value.
VOLUME BOOST
The limiter has programmable upper gain which boosts signals below the threshold to compress the
dynamic range of the signal and increase its perceived loudness. This operates as an ALC function
with limited boost capability. The volume boost is from 0dB to +12dB in 1dB steps, controlled by the
LIMBOOST register bits.
The output limiter volume boost can also be used as a stand alone digital gain boost when the limiter
is disabled.
Pre-Production, Rev 3.0, February 2007
42
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
