TDA7407D Datasheet, PDF (18/34 Pages) STMicroelectronics – ADVANCED CAR SIGNAL PROCESSOR

TDA7407D
Figure 21. Internal Stereoblend Characteristics
t(s) _{If any other source than the stereodecoder is selected the decoder remains muted and the MPX pin is}
c _{connected to Vref to avoid any discharge of the coupling capacitor through leakage currents.}
rodu ^{6.5.2 Ingain + Infilter}
P _t(s) ^{The Ingain stage allows to adjust the MPX signal to a magnitude of about 1Vrms internally which is the}
recommended value. The 4th order input filter has a corner frequency of 80KHz and is used to attenuate
te ^{c spikes and nose and acts as an anti allasing filter for the following switch capacitor filters.}
sole ^{rodu 6.5.3 Demodulator}
In the demodulator block the left and the right channel are separated from the MPX signal. In this stage
b ^P _{also the 19 kHz pilot tone is cancelled. For reaching a high channel separation the TDA7407D offers an}
- ^{O te I2C bus programmable roll-off adjustment which is able to compensate the lowpass behaviour of the tuner}
) ^{le section. If the tuner attenuation at 38kHz is in a range from 7.2% to 31.0% the TDA7407D needs no ex-}
ternal network in front of the MPX pin. Within this range an adjustment to obtain at least 40dB channel
t(s ^{so separation is possible.}
c ^b _{The bits for this adjustment are located together with the fieldstrength adjustment in one byte. This gives}
u O _{the possibility to perform an optimization step during the production of the carradio where the channel sep-}
d _{- aration and the fieldstrength control are trimmed.}
Pro _t(s) 6.5.4 Deemphasis and Highcut.
te ^{c The lowpass filter for the deemphasis allows to choose between a time constant of 50µs and 75µs (bit D7,}
le ^{u Stereodecoder byte).}
so ^{rod The highcut control range will be in both cases τHC = 2 · τDeemp. Inside the highcut control range (between}
VHCH and VHCL) the LEVEL signal is converted into a 5 bit word which controls the lowpass time con-
b P _{stant between τDeemp...3 · τDeemp. There by the resolution will remain always 5 bits independently of the}
O _te absolute voltage range between the VHCH and VHCL values.
le ^{The highcut function can be switched off by I2C bus (bit D7, Fieldstrength byte set to "0").}
so 6.5.5 PLL and Pilot Tone Detector
b_{The PLL has the task to lock on the 19kHz pilotone during a stereo transmission to allow a correct demod-}
O_{ulation. The included detector enables the demodulation if the pilot tone reaches the selected pilot tone}
threshold V_PTHST. Two different thresholds are available. The detector output (signal STEREO, see block
diagram) can be checked by reading the status byte of the TDA7407D via I²C bus.
Fieldstrength Control
The fieldstrength input is used to control the high cut and the stereoblend function. In addition the signal
can be also used to control the noiseblanker thresholds and as input for the multipath detector.
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