TDA7402 Datasheet, PDF (34/59 Pages) STMicroelectronics – CARRADIO SIGNAL PROCESSOR

TDA7402
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 τ_Deempor 2.7 x τ_Deemp dependent on the selected time constant (see programming section). The bit
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 τ_Deemp...3 (3.7) x τ_Deemp. Thereby the resolution will remain
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.
The highcut function can be switched off by I²C-bus (bit D₇, Highcut byte set to ”0”).
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_PTHST. Two different thresholds are available. The detector output (signal STEREO, see Blockdia-
gram) can be checked by reading the status byte of the A619 via I²C-bus. During a Softmute the PLL can be
set into ”Hold”-mode which freezes the PLL’s state (bit D₄, Softmute byte). After releasing the Softmute the PLL
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_G and
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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