STA381BWS datasheet(142/174 Pages) STMICROELECTRONICS

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STA381BWS

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DocID018937 Rev 7

7.12.18

User-defined EQ

The STA381BWS can be programmed for four EQ filters (biquads) per each of the two input

channels. The biquads use the following equation:

Y[n] = 2 * (b0 / 2) * X[n] + 2 * (b1 / 2) * X[n-1] + b2 * X[n-2] - 2 * (a1 / 2) * Y[n-1] - a2 * Y[n-2]

= b0 * X[n] + b1 * X[n-1] + b2 * X[n-2] - a1 * Y[n-1] - a2 * Y[n-2]

where Y[n] represents the output and X[n] represents the input. Multipliers are 24-bit signed

fractional multipliers, with coefficient values in the range of 0x800000 (-1) to 0x7FFFFF

(0.9999998808).

Coefficients stored in the user-defined coefficient RAM are referenced in the following

manner:

CxHy0 = b1 / 2

CxHy1 = b2

CxHy2 = -a1 / 2

CxHy3 = -a2

CxHy4 = b0 / 2

where x represents the channel and y the biquad number. For example, C2H41 is the b2

coefficient in the fourth biquad for channel 2.

Additionally, the STA381BWS can be programmed for a high-pass filter (processing

channels 1 and 2) and a low-pass filter (processing channel 3) to be used for bass

management crossover when the XO setting is 000 (user-defined). Both of these filters,

when defined by the user (rather than using the preset crossover filters), are second order

filters that use the biquad equation given above. They are loaded into the C12H0-4 and

C3Hy0-4 areas of RAM noted in Table 150.

Channel 1 and channel 2 biquads use by default the extended coefficient range (-4, +4);

Xover filters use only the standard coefficients range (-1, +1).

By default, all user-defined filters are pass-through where all coefficients are set to 0, except

the channel 1 and 2 b0/2 coefficient which is set to 0x100000 (representing 0.5) and xover

b0/2 coefficient which is set to 0x400000 (representing 0.5).

7.12.19

Pre-scale

The STA381BWS provides a multiplication for each input channel for the purpose of scaling

the input prior to EQ. This pre-EQ scaling is accomplished by using a 24-bit signed

fractional multiplier, with 0x800000 = -1 and 0x7FFFFF = 0.9999998808. The scale factor

for this multiplication is loaded into RAM using the same I2C registers as the biquad

coefficients and the bass management. All channels can use the channel-1 pre-scale factor

by setting the biquad link bit. By default, all pre-scale factors are set to 0x7FFFFF.

7.12.20

Post-scale

The STA381BWS provides one additional multiplication after the last interpolation stage and

the distortion compensation on each channel. This post-scaling is accomplished by using a

24-bit signed fractional multiplier, with 0x800000 = -1 and 0x7FFFFF = 0.9999998808. The

scale factor for this multiplication is loaded into RAM using the same I2C registers as the

biquad coefficients and the bass management. This post-scale factor can be used in

conjunction with an ADC-equipped microcontroller to perform power-supply error correction.

All channels can use the channel-1 post-scale factor by setting the post-scale link bit. By

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