sta381bw STMicroelectronics, sta381bw Datasheet - Page 138

• Manufacturer Part Number: sta381bw
• Description: Sound Terminal 2.1-channel High-efficiency Digital Audio System
• Manufacturer: STMicroelectronics

Register description: Sound Terminal compatibility
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User-defined EQ
The STA381BW can be programmed for four EQ filters (biquads) per each of the two input
channels. The biquads use the following equation:
Y[n] = 2 * (b
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:
where x represents the channel and y the biquad number. For example, C2H41 is the b
coefficient in the fourth biquad for channel 2.
Additionally, the STA381BW 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
Channel 1 and channel 2 biquads use by default the extended coefficient range (-4, +4);
Xover filters use only 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 b
b
Pre-scale
The STA381BW 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 I
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.
Post-scale
The STA381BW 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 I
biquad coefficients and the bass management. This post-scale factor can be used in
conjunction with an ADC-equipped micro-controller to perform power supply error
correction. All channels can use the channel-1 post-scale factor by setting the post-scale
0
/2 coefficient which is set to 0x400000 (representing 0.5)
= b
CxHy0 = b
CxHy1 = b
CxHy2 = -a
CxHy3 = -a
CxHy4 = b
0
* X[n] + b
0
/ 2) * X[n] + 2 * (b
1
2
0
1
2
/ 2
/ 2
/ 2
1
* X[n-1] + b
0
/2 coefficient which is set to 0x100000 (representing 0.5) and xover
Doc ID 018835 Rev 2
1
2
/ 2) * X[n-1] + b
Table
* X[n-2] - a
150.
1
* Y[n-1] - a
2
* X[n-2] - 2 * (a
2
* Y[n-2]
2
C registers as the biquad
1
/ 2) * Y[n-1] - a
2
C registers as the
STA381BW
2
* Y[n-2]
2