WM8904ECSSLASHR datasheet(57/188 Pages) WOLFSON

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WM8904

PP, Rev 3.3, September 2012

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.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R40 (28h)

DRC Control 0

DRC_QR

Quick release enable

0 = disabled

1 = enabled

R41 (29h)

DRC Control 1

7:6

DRC_QR_THR

[1:0]

Quick release crest factor threshold

00 = 12dB

01 = 18dB (default)

10 = 24dB

11 = 30dB

5:4

DRC_QR_DCY

[1:0]

Quick release decay rate

(seconds/6dB)

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.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R40 (28h)

DRC Control 0

12:11

DRC_GS_HYST

_LVL [1:0]

Gain smoothing hysteresis

threshold

00 = Low

01 = Medium (recommended)

10 = High

11 = Reserved

DRC_GS_ENA

Gain smoothing enable

0 = disabled

1 = enabled

DRC_GS_HYST

Gain smoothing hysteresis enable

0 = disabled

1 = enabled

Table 21 DRC Gain Smoothing

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