TDA7410ND Datasheet, PDF (22/34 Pages) STMicroelectronics – Signal processor for car radio applications

Functional description of the noiseblanker
TDA7410ND
7
Functional description of the noiseblanker
In the automotive environment the MPX signal is disturbed by spikes produced by the
ignition and for example the wiper motor. The aim of the noiseblanker part is to cancel the
audible influence of the spikes. Therefore the output of the stereodecoder is held at the
actual voltage for 40μs. In a first stage the spikes must be detected but to avoid a wrong
triggering on high frequency (white) noise a complex trigger control is implemented. Behind
the triggerstage a pulse former generates the "blanking" pulse. To avoid any crosstalk to the
signal path the noiseblanker is supplied by its own biasing circuit.
7.1
Trigger path
The incoming MPX signal is highpass filtered, amplified and rectified. This second order
highpass-filter has a corner frequency of 140kHz. The rectified signal, RECT, is lowpass
filtered to generate a signal called PEAK. Also noise with a frequency 140kHz increases the
PEAK voltage. The PEAK voltage is fed to a threshold generator, which adds to the PEAK
voltage a DC dependent threshold VTH. Both signals, RECT and PEAK+VTH are fed to a
comparator which triggers a re-triggerable monoflop. The monoflop's output activates the
sample-and-hold circuits in the signalpath for 40μs. The block diagram of the noiseblanker is
given in Figure 16.
Figure 16. Block diagram of the noiseblander
MPX
HIGH PASS
RECTIFIER
RECT
+
-
VTH
+
PEAK
LOWPASS
+
D98AU861
MONOFLOP
THRESHOLD
GENERATOR
ADDITIONAL
THRESHOLD
CONTROL
HOLDN
7.2
Automatic noise controlled threshold adjustment (ATC)
There are mainly two independent possibilities for programming the trigger threshold:
a) the low threshold in 8 steps (bits D0 to D2 of the noiseblanker byte)
b) the noise adjusted threshold in 4 steps (bits D3 and D4 of the noiseblanker byte,
(see Figure 9).
The low threshold is active in combination with a good MPX signal without any noise; the
PEAK voltage is less than 1V. The sensitivity in this operation is high. If the MPX signal is
noisy the PEAK voltage increases
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