AD1954YSTZ datasheet(13/36 Pages) AD

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AD1954

–13–

0 dB and 1/0.75 (approximately 3 dB) will produce larger analog

outputs and result in slightly degraded analog performance.This

extra analog range is necessary in order to pass 0 dBFS square

waves through the system, since these square waves cause over-

shoots in the interpolation filters, which would otherwise briefly

clip the digital DAC circuitry.

A separate digital clipper circuit is used in the DSP core to ensure

that any accumulator values that exceed the numeric 3.23 format

range are clipped when taken from the accumulator.

High-Pass Filter

The high-pass filter is a first order double-precision design.The pur-

pose of the high-pass filter is to remove digital dc from the input. If

this dc were allowed to pass, the detectors used in the compressor/

limiter would give an incorrect reading for low signal levels.The

high-pass filter is controlled by a single parameter (alpha_HPF),

which is programmed by writing to SPI location 180 in 2.20 twos

complement format.The following equation can be used to calcu-

late the parameter alpha_HPF from the –3 dB point of the filter:

Alpha_HPF

– EXP

p HPF_Cutoff

× ×









1.0

–2.0

where EXP is the exponential operator,

EXP

HPF_cutoff is the high-

HPF_cutoff

pass cutoff in Hz, and fS

ff is the audio sampling rate.The default

value for the –3 dB cutoff of the high-pass filter is 2.75 Hz at a

sampling rate of 44.1 kHz.

Biquad Filters

Each of the two input channels has seven second order biquad

sections in the signal path. In addition, the left and right channels

have two additional biquad filters that may be used either as

crossover filters or as additional equalization filters.The subchan-

nel has three additional biquad filters that are also to be used

as equalization and/or crossover filters. In a typical scenario, the

first seven biquads would be used for speaker equalization and/or

tone controls, and the remaining filters would be programmed to

function as crossover filters. Note that there is a common equal-

ization section used for both the main and sub channels, followed

by the crossover filters.This arrangement prevents any interaction

from occurring between the crossover filters and the equalization

filters. One section of the biquad IIR filter is shown in Figure 5.

Figure 3. Graphical User Interface

SIGNAL PROCESSING

(3.23 FORMAT)

SERIAL PORT

DAC INTERPOLATION

FILTERS (3.23 FORMAT)

DIGITAL -

MODULATORS

(1.23 FORMAT)

DIGITAL

CLIPPER

DATA IN

2-BIT SIGN EXTENTION

0.75

1.23

3.23

Figure 4. Numeric Precision and Clipping Structure

REV. A

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