STA369BWS datasheet(63/89 Pages) STMICROELECTRONICS

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STA369BWS

Doc ID 16861 Rev 4

63/89

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 the y the biquad number. For example, C2H41 is the b2

coefficient in the fourth biquad for channel 2.

Crossover and biquad #8

Additionally, the STA369BWS 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 71, addresses 0x28 to 0x31.

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

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

Prescale

The STA369BWS 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 multiplier is loaded into RAM. All channels can use the channel-1 prescale factor by

setting the Biquad link bit. By default, all prescale factors (RAM addresses 0x32 to 0x33) are

set to 0x7FFFFF.

Postscale

The STA369BWS provides one additional multiplication after the last interpolation stage and

the distortion compensation on each channel. This postscaling is accomplished by using a

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

scale factor for this multiplier is loaded into RAM. This postscale 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 postscale factor by setting the postscale link

bit. By default, all postscale factors (RAM addresses 0x34 to 0x36) are set to 0x7FFFFF.

When line output is being used, channel-3 postscale affects both channels 3 and 4.

Thermal warning and overcurrent adjustment (TWOCL)

The STA369BWS provides a simple mechanism for reacting to overcurrent or thermal

warning detection in the power block. When the warning occurs, the TWOCL value is used

to provide output attenuation clipping on all channels.

The amount of attenuation to be applied in this situation can be adjusted by modifying the

overcurrent and thermal warning limiting value (RAM addr 0x37). By default, the overcurrent

postscale adjustment factor is set to 0x5A9DF7 (that is, -3 dB). Once the limiting is applied it

remains until the device is either reset or according to the TWRB and OCRB settings.

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