STA3398 datasheet(67/77 Pages) STMICROELECTRONICS

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STA339BW

Doc ID 15251 Rev 5

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A first-order filter is suggested to guarantee that for every

ω0 the corresponding low-pass

filter obtained as difference (as shown in Figure 20) will have a symmetric (relative to HP

filter) frequency response, and the corresponding recombination after the DRC has low

ripple. Second-order filters can be used as well, but in this case the filter shape must be

carefully choosen to provide good low pass response and minimum ripple recombination.

For second-order is not possible to give a closed formula to get the best coefficients, but

empirical adjustment should be done.

DRC settings

The DRC blocks used by B2DRC are the same as those described in Chapter 6.11. B2DRC

configure automatically the DRC blocks in anticlipping mode. Attack and release thresholds

can be selected using registers 0x32, 0x33, 0x34, 0x35, while attack and release rates are

configured by registers 0x12 and 0x14.

Band downmixing

The low-frequency band is down-mixed to the left and right channels at the B2DRC output.

Channel volume can be used to weight the bands recombination to fine tune the overall

frequency response.

6.18.2

EQ DRC mode

Setting MDRC = 01, it is possible to add a programmable biquad (the XO biquad at RAM

addresses 0x28 to 0x2C is used for this purpose) to the Limiter/compressor measure path

(side chain). Using EQDRC the peak detector input can be shaped in frequency using the

programmable biquad. For example if a +2 dB bass boost is applied (using a low shelf filter

for example), the effect is that the EQDRC out will limit bass frequencies to -2 dB below the

selected attack treshold.

Generally speaking, if the biquad boosts frequency f with an amount of X dB, the level of a

compressed sinusoid at the output will be TH - X, where TH is the selected attack threshold.

Note:

EQDRC works only if the biquad frequency response magnitude is >= 0 dB for every

frequency.

Figure 21.

EQDRC scheme

Channel in

Peak

detector

Attenuation

calculator

Atten

Channel in

BIQUAD

Peak

detector

Attenuation

calculator

Atten

EQDRC

Standard DRC

Channel in

Peak

detector

Attenuation

calculator

Atten

Channel in

BIQUAD

Peak

detector

Attenuation

calculator

Atten

EQDRC

Standard DRC

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