STA323W datasheet(63/77 Pages) STMICROELECTRONICS

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STA323W

User-programmable settings

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User-programmable settings

8.1

EQ - biquad equation

The biquads use the equation that follows. This is shown in Figure 49.

Y[n] = 2(b0/2)X[n] + 2(b1/2)X[n-1] + b2X[n-2] - 2(a1/2)Y[n-1] - a2Y[n-2]

= b0X[n] + b1X[n-1] + b2X[n-2] - a1Y[n-1] - a2Y[n-2]

where Y[n] represents the output and X[n] represents the input. Signed, fractional 28-bit

multipliers are used, 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

The x represents the channel and the y the biquad number. For example C3H41 is the b0/2

coefficient in the fourth biquad for channel 3.

Figure 49.

Biquad filter

8.2

Pre-scale

The pre-scale block, which precedes the first biquad, is used for attenuation when filters are

designed that boost frequencies above 0 dBFS. The Pre-Scale block is a single 28-bit

signed multiplier, with 0x800000 = -1 and 0x7FFFFF = 0.9999998808. By default, all

pre-scale factors are set to 0x7FFFFF.

8.3

Post-scale

The STA323W provides one additional multiplication after the last interpolation stage and

before the distortion compensation on each channel. The post-scale block is a 24-bit signed

fractional multiplier. The scale factor for this multiplier is loaded into RAM using the same

I2C registers as the biquad coefficients and the mix. All channels can use the same settings

as channel 1 by setting the post-scale link bit.

b1/2

b0/2

Z -1

-a2

-a1/2

Z -1

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