WM8994 datasheet(73/359 Pages) WOLFSON

73 / 359 page

Production Data

WM8994

PD, April 2012, Rev 4.4

The [DRC]_ATK determines how quickly the DRC gain decreases when the signal amplitude is high.

The [DRC]_DCY determines how quickly the DRC gain increases when the signal amplitude is low.

These register fields are described in Table 24, Table 25 and Table 26. Note that the register defaults

are suitable for general purpose microphone use.

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.

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.

Note that the Anti-Clip feature should not be enabled at the same time as the Quick Release feature

(described below) on the same DRC.

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.

Note that the Quick Release feature should not be enabled at the same time as the Anti-Clip feature

(described above) on the same DRC.

SIGNAL ACTIVITY DETECT

The DRC incorporates a configurable signal detect function, allowing the signal level at the DRC input

to be monitored and to be used to trigger other events. This can be used to detect the presence of a

microphone signal on an ADC or digital mic channel, or can be used to detect an audio signal

received over the digital audio interface.

The Peak signal level or the RMS signal level of the DRC input can be selected as the detection

threshold. When the threshold condition is exceeded, an interrupt or GPIO output can be generated.

See “General Purpose Input/Output” for a full description of the applicable control fields.

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