TDA7412 ST Microelectronics, TDA7412 Datasheet - Page 32

• Manufacturer Part Number: TDA7412
• Description: CARRADIO SIGNAL PROCESSOR
• Manufacturer: ST Microelectronics

TDA7412
optimization step during the production of the carradio where the channel separation and the fieldstrength con-
trol are trimmed. The setup of the Stereoblend characteristics which is programmable in a wide range is de-
scribed in 2.8.
3.4 Deemphasis and Highcut
The deemphasis-lowpass allows to choose a time constant between 37.5 and 100µs. The highcut control range
will be 2 x
or 2.7 x
dependent on the selected time constant (see programming section). The bit
Deemp
Deemp
D7 of the hightcut-byte will shift timeconstant and range.
Inside the highcut control range (between VHCH and VHCL) the LEVEL signal is converted into a 5 bit word
which controls the lowpass time constant between
...3 (3.7) x
. Thereby the resolution will remain
Deemp
Deemp
always 5 bits independently of the absolute voltage range between the VHCH- and VHCL-values. In addition
the maximum attenuation can be fixed between 2 and 10dB.
2
The highcut function can be switched off by I
C-bus (bit D
, Highcut byte set to "0").
7
The setup of the highcut characteristics is described in 2.9.
3.5 PLL and Pilottone-Detector
The PLL has the task to lock on the 19kHz pilottone during a stereo-transmission to allow a correct demodula-
tion. The included pilottone-detector enables the demodulation if the pilottone reaches the selected pilottone
threshold V
. Two different thresholds are available. The detector output (signal STEREO, see Blockdia-
PTHST
2
gram) can be checked by reading the status byte of the A679 via I
C-bus. During a Softmute the PLL can be
set into "Hold"-mode which freezes the PLL's state (bit D
, Softmute byte). After releasing the Softmute the PLL
4
will again follow the input signal only by correcting the phase error.
3.6 Fieldstrength Control
The fieldstrength input is used to control the highcut- and the stereoblend-function. In addition the signal can be
also used to control the noiseblanker thresholds and as input for the multipath detector. These additional func-
tions are described in sections 3.3 and 4.
3.7 EVEL-Input and -Gain
To suppress undesired high frequency modulation on the highcut- and stereoblend-control signal the LEVEL
signal is lowpass filtered firstly. The filter is a combination of a 1.st-order RC-lowpass at 53kHz (working as anti-
aliasing filter) and a 1.st-order switched-capacitor-lowpass at 2.2kHz. The second stage is a programmable gain
stage to adapt the LEVEL signal internally to different IF-devices (see Testmode section 5: LEVELHCC). The
gain is widely programmable in 16 steps from 0dB to 6dB (step=0.4dB). These 4 bits are located together with
the Roll-Off bits in the "Stereodecoder-Adjustment"-byte to simplify a possible adjustment during the production
of the carradio. This signal controls directly the Highcut stage whereas the signal is filtered again (fc=100Hz)
before the stereoblend stage (see fig. 32).
3.8 Stereoblend Control
The stereoblend control block converts the internal LEVEL-voltage (LEVELSB) into an demodulator compatible
analog signal which is used to control the channel separation between 0dB and the maximum separation. Inter-
nally this control range has a fixed upper limit which is the internal reference voltage REF5V. The lower limit can
be programmed between 20 and 70% of REF5V in 3.3% steps (see figs.28, 29).
To adjust the external LEVEL-voltage to the internal range two values must be defined: the LEVEL gain L
and
G
VSBL (see fig. 29). At the point of full channel separation the external level signal has to be amplified that inter-
nally it becomes equal to REF5V. The second point (e.g. 10dB channel sep.) is then adjusted with the VSBL
voltage.
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