AS3675 datasheet(61/81 Pages) AMSCO

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AS3675

Datasheet - D e ta i l e d D e s c r i p t i o n

8.8.2

Frequency Mode

This mode is selected by freq_mode=1.

The input signal is frequency filtered by three digital filters. The filters are 2nd order biquad IIR (infinite impulse

response filters). Each of these filters has the following structure (sampling frequency = 45.4kHz / internal clock of

1MHz divided by 22):

Figure 29. Audio controlled LED frequency filter

The mathematical formula for these filters is:

y(n) = A0(x(n) + b1 x(n-1) + b2 x(n-2)) – a1 y(n-1) – a2 y(n-2)

(EQ 6)

The internal calculation is preformed using 12bits coefficients (state variables (z

-1) are rounded to 12bit, the output

uses 8bits). All coefficients can be set individually for each of the three filters (filter1, filter2, filter3). (see Audio Con-

trolled LED Registers on page 62), registers filt_type (70h) and registers 71h to 82h. It is recommended to use austri-

amicrosystems ‘Demoboard Software’ for simple control of the filter cutoff frequencies and filter type.

Note: Do not set filter cutoff frequencies below 500Hz.

8.8.3 AGC

The AGC (available in amplitude and frequency mode) is used to ‘compress’ the input signal and to attenuate very low

input amplitude signals (this is performed to ensure no light output for low signals especially for noisy input signals).

The AGC monitors the input signal amplitude and filters this amplitude with a filter with a short attack time, but a long

decay time (decay time depends on the register agc_ctrl). This amplitude measurement (represented by an integer

value from 0 to 15) is then used to amplify or attenuate the input signal with one of the following amplification ratios

(output to input ratio) – the curve A, B, or C is selected depending on the register agc_ctrl:

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