STA369BWS datasheet(68/89 Pages) STMICROELECTRONICS

68 / 89 page

STA369BWS

68/89

Doc ID 16861 Rev 4

For the user programmable mode, use the formulae below to compute the high pass filters:

where alpha = (1-sin(

ω0)) / cos(ω0), and ω0 is the cut-off frequency.

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

ω0 the corresponding low-pass

filter obtained as difference (as shown in Figure 19) has 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

chosen 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 6.6. 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.

6.13.2

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 bass boost of +2 dB is applied (using a low-shelf

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

the selected attack threshold.

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

compressed sine wave at the output is 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.

b0 = (1 + alpha) / 2

a0 = 1

b1 = -(1 + alpha) / 2

a1 = -alpha

b2 = 0

a2 = 0

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