WM8904ECSSLASHR datasheet(53/188 Pages) WOLFSON

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Pre-Production

WM8904

PP, Rev 3.3, September 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 ADCs or the DACs. 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 15. It can be enabled in the ADC digital 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 = ADC path

1 = DAC path

Table 15 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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