WM8918 datasheet(45/176 Pages) CIRRUS

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WM8918

PD, Rev 4.1, January 2012

DYNAMIC RANGE CONTROL (DRC)

The dynamic range controller (DRC) is a circuit which can be enabled in the digital data path of either

the Digital Microphone input or the DAC playback. The function of the DRC is to adjust the signal gain

in conditions where the input amplitude is unknown or varies over a wide range, e.g. when recording

from microphones built into a handheld system. The DRC can apply Compression and Automatic

Level Control to the signal path. It incorporates ‘anti-clip’ and ‘quick release’ features for handling

transients in order to improve intelligibility in the presence of loud impulsive noises.

The DRC is enabled by DRC_ENA, as shown in Table 13. It can be enabled in the Digital Microphone

path or in the DAC digital path, under the control of the DRC_DAC_PATH register bit. Note that the

DRC can be active in only one of these paths at any time.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R40 (28h)

DRC Control 0

DRC_ENA

DRC enable

0 = Disabled

1 = Enabled

DRC_DAC_PAT

DRC path select

0 = Digital Microphone path

1 = DAC path

Table 13 DRC Enable

COMPRESSION/LIMITING CAPABILITIES

The DRC supports two different compression regions, separated by a “knee” at input amplitude T. For

signals above the knee, the compression slope DRC_HI_COMP applies; for signals below the knee,

the compression slope DRC_LO_COMP applies.

The overall DRC compression characteristic in “steady state” (i.e. where the input amplitude is near-

constant) is illustrated in Figure 29.

DRC_KNEE_IP

(Y0)

0dB

DRC

_HI_C

OMP

DRC Input Amplitude (dB)

DRC Output Amplitude (dB)

DRC_KNEE_OP

“knee”

Figure 29 DRC Compression Characteristic

The slope of the DRC response is determined by register fields DRC_HI_COMP and

DRC_LO_COMP respectively. A slope of 1 indicates constant gain in this region. A slope less than 1

represents compression (i.e. a change in input amplitude produces only a smaller change in output

amplitude). A slope of 0 indicates that the target output amplitude is the same across a range of input

amplitudes; this is infinite compression.

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