STA381BWS datasheet(59/174 Pages) STMICROELECTRONICS

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STA381BWS

with the original signal. Limiter 1 (DRC1) is then used to control the amplitude of the left/right

high-frequency components, while limiter 2 (DRC2) is used to control the low-frequency

components (see Section 6.23: Dynamic control registers (addr 0x23 - 0x26 / addr 0x43 -

0x46)).

The cutoff frequency of the high-pass filters can be user-defined, XO[3:0] = 0, or selected

from the pre-defined values.

DRC1 and DRC2 are then used to independently limit L/R high frequencies and LFE

channel amplitude (see Section 6.23: Dynamic control registers (addr 0x23 - 0x26 / addr

0x43 - 0x46)) as well as their volume control. To be noted that, in this configuration, the

dedicated channel 3 volume control can actually act as a bass boost enhancer as well (0.5

dB/step resolution).

The processed LFE channel is then recombined with the L and R channels in order to

reconstruct the 2.0 output signal.

Sub-band decomposition

The sub-band decomposition for B2DRC can be configured specifying the cutoff frequency.

The cutoff frequency can be programmed in two ways, using the XO bits in register 0x0C, or

using the “user programmable” mode (coefficients stored in RAM addresses 0x28 to 0x31).

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

where alpha = (1-sin(

0))/cos(0), and 0 is the cutoff frequency.

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

0 the corresponding

low-pass filter obtained as difference (as shown in Figure 26) will have a symmetric (relative

to the 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 filters, it 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 Section 6.23: Dynamic

control registers (addr 0x23 - 0x26 / addr 0x43 - 0x46). 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.

b0 = (1 + alpha) / 2

a0 = 1

b1 = -(1 + alpha) / 2

a1 = -alpha

b2 = 0

a2 = 0

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