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STA326

8

USER PROGRAMMABLE PROCESSING

8.1 EQ - BIQUAD EQUATION

The biquads use the equation that follows. This is diagrammed in Figure 14 below.

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.

Multipliers are 28-bit signed fractional

multipliers, with coefficient values in the range of 800000h (-1) to 7FFFFFh (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 14. - 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 0dBFS. This is a single 28-bit signed multiplier, with 800000h = -1 and 7FFFFFh

= 0.9999998808. By default, all pre-scale factors are set to 7FFFFFh.

8.3 POST-SCALE

The STA326 provides one additional multiplication after the last interpolation stage and before the distor-

tion compensation on each channel. This 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 chan-

nels can use the same settings as channel 1 by setting the post-scale link bit.

8.4 MIX/BASS MANAGEMENT

The STA326 provides a post-EQ mixing block per channel. Each channel has 2 mixing coefficients, which

are each 24-bit signed fractional multipliers, that correspond to the 2 channels of input to the mixing block.

These coefficients are accessible via the User Controlled Coefficient RAM described below. The mix co-

efficients are expressed as 24-bit signed; fractional numbers in the range +1.0 (8388607) to -1.0 (-

8388608) are used used to provide three channels of output from two channels of filtered input.

+

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2

2

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