WM8602SEDSSLASHV datasheet(31/46 Pages) WOLFSON

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WM8602

PP Rev 1.5 May 2004

DYNAMIC PEAK COMPRESSOR

The WM8602 includes a Dynamic Peak Compressor for each channel, which prevents the

occurrence of digital clipping when gains in excess of 0dB are applied. The compressor automatically

adjusts the signal amplitude to allow headroom for the LFE boost, sub-woofer mixing, tone controls,

loudspeaker equalisation and digital volume control. The compressor has a programmable limit

threshold, programmable attack and decay time-constants, a frequency-dependent decay mode, and

built-in zero-cross detect. The compressor can be configured either to operate independently on all

channels (DUAL MONO), or on linked channel-pairs (STEREO).

COMPRESSOR THRESHOLD

The compressor has a digital peak detector which tracks the maximum input signal level at the

output of the volume control. With reference to Figure 21, if this signal is below the threshold set by

control bit THRESH, the compressor operates transparently with no change to the signal level.

However, if peak signal rises above the threshold, the gain through the compressor is modified so

that the upper part of the curve is followed. This ensures that the output signal does not exceed 0dB.

ATTACK AND DECAY TIMES

The attack time-constant ATK controls how fast the gain is reduced when the signal goes above the

threshold. It is defined as the time taken for the gain to reduce by 6dB. Normally a short attack time-

constant is used to prevent the signal clipping when a high-amplitude transient occurs.

The decay time-constant DCY controls how fast the gain is increased when the signal begins to fall

again. It is defined as the time taken for the gain to increase by 6dB. Normally, the decay time-

constant is much longer than the attack time-constant, to prevent the input signal from entering

repeated limiting cycles.

The frequency-dependent decay feature automatically detects the input frequency and sets the

decay time to decay slower for low frequency signals. This reduces low-frequency signal distortion by

preserving the waveform of each input cycle, whilst allowing the compressor to respond quickly to

high frequency transients. This feature is enabled via the FDEP control bit.

0dB

Output Level

(dB)

Peak Input Level [dB]

THRESH

[dB]

-6

-12

+12

-18

-24

-30

-36

-12

-18

-24

-30

-36

0dB

-6dB

-12dB

+6dB

+12dB

typical

Volume Control

Setting

Figure 21 Dynamic Peak Compressor Characteristics

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