TDA7407D Datasheet, PDF (20/34 Pages) STMicroelectronics – ADVANCED CAR SIGNAL PROCESSOR

TDA7407D
The gain can be programmed through 4 bits in the "Stereodecoder-Adjustment" byte.
The MONO voltage VMO (0dB channel separation) can be choosen selecting VSBLAll necessary internal
reference voltages like REF5V are derived from a bandgap circuit. Therefore they have a temperature co-
efficient near zero. This is useful if the fieldstrength signal is also temperature compensated.
But most IF devices apply a LEVEL voltage with a TC of 3300ppm. The TDA7407D offers this TC for the
reference voltages, too. The TC is selectable with bit D7 of the "stereodecoder adjustment" byte.
6.5.8 Highcut Control
The highcut control setup is similar to the stereoblend control setup : the starting point VHCH can be set
with 2 bits to be 42, 50, 58 or 66% of REF5V whereas the range can be set to be 17, 22, 28 or 33% of
VHCH (see fig. 23).
) 6.6 FUNCTIONAL DESCRIPTION OF THE NOISEBLANKER
t(s ^{In the automotive environment the MPX signal is disturbed by spikes produced by the ignition and for ex-}
ample the wiper motor. The aim of the noiseblanker part is to cancel the audible influence of the spikes.
uc _{Therefore the output of the stereodecoder is held at the actual voltage for a time between 22 and 38µs}
d _{(programmable).}
Pro _t(s) ^{The block diagram of the noiseblanker is given in fig. 24.}
In a first stage the spikes must be detected but to avoid a wrong triggering on high frequency (white) noise
te ^c a complex trigger control is implemented. Behind the triggerstage a pulse former generates the "blanking"
le du ^{pulse. To avoid any crosstalk to the signalpath the noiseblanker is supplied by his own biasing circuit.}
so ^ro _{6.6.1 Trigger Path}
b ^P _{The incoming MPX signal is highpass filtered, amplified and rectified. This second order highpass-filter}
- ^{O te has a corner frequency of 140kHz.}
) ^{le The rectified signal, RECT, is lowpass filtered to generate a signal called PEAK. Also noise with a frequen-}
t(s ^{o cy 140kHz increases the PEAK voltage. The resulting voltage can be adjusted by use of the noise rectifier}
s _{discharge current.}
uc Ob ^{The PEAK voltage is fed to a threshold generator, which adds to the PEAK voltage a DC dependent}
d _- threshold VTH. Both signals, RECT and PEAK+VTH are fed to a comparator which triggers a re-trigger-
ro ^{) able monoflop. The monoflop's output activates the sample-and-hold circuits in the signalpath for selected}
P _t(s ^duration.
te ^c Figure 24. Block Diagram of the Noiseblanker
Obs^{OoblestoeleProdu MPX}
RECTIFIER
RECT
+
-
VTH
+
PEAK
MONOFLOP
THRESHOLD
GENERATOR
HOLDN
LOWPASS
+
MPX
CONTROL
ADDITIONAL
THRESHOLD
CONTROL
D98AU856
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