TDA7411 Datasheet, PDF (24/43 Pages) STMicroelectronics – ADVANCED CAR SIGNAL PROCESSOR

TDA7411
dently of the absolute voltage range between the VHCH- and VHCL-values.
The highcut function can be switched off by I2C-bus (bit D7, Field strength byte set to "0").
The setup of the highcut characteristics is described in 2.9.
PLL and Pilot tone-Detector
The PLL has the task to lock on the 19kHz pilot tone during a stereo-transmission to allow a correct de-
modulation. The included pilot tone-detector enables the demodulation if the pilot tone reaches the select-
ed pilot tone threshold V_PTHST. Two different thresholds are available. By reading the status byte of the
A673 via I²C-bus the detector output (signal STEREO, see block diagram) can be checked.
Field Strength Control
The field strength input is used to control the highcut- and the stereoblend-function. In addition the signal
can be also used to control the noise blanker thresholds and as input for the multipath detector.
LEVEL-Input and -Gain
To suppress undesired high frequency modulation on the highcut- and stereoblend-function the LEVEL
signal is low pass filtered firstly. The filter is a combination of a 1.st-order RC-low pass at 53kHz (working
as anti-aliasing filter) and a 1.st-order switched capacitor low pass at 2.2kHz. The second stage is a pro-
grammable gain stage to adapt the LEVEL signal internally to different IF-devices (see test mode section
5: LEVELINTERN). The gain is widely programmable in 16 steps from 0dB to 10dB (step=0.67dB). These
4 bits are located together with the Roll-Off bits in the "Stereo decoder-Adjustment"-byte to simplify a pos-
sible adaptation during the production of the car radio.
Stereoblend Control
The stereoblend control block converts the internal LEVEL-voltage (LEVELINTERN) into a demodulator
compatible analog signal, which is used to control the channel separation between 0dB and the maximum
separation. Internally this control range has a fixed upper limit, which is the internal reference voltage
REF5V. The lower limit can be programmed between 29.2 and 58% of REF5V in 4.167% steps (see
figs.19, 20).
To adjust the external LEVEL-voltage to the internal range two values must be defined: the LEVEL gain
L_G and VSBL (see fig. 20). To adjust the voltage where the full channel separation is reached (VST) the
LEVEL gain LG has to be defined. The following equation can be used to estimate the gain:
L_G
=
---------------------------------^R-----^E-----^F----⁵----^V-----------------------------------
Field strenght voltage [STEREO]
Figure 19. Internal stereo blend characteristics
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