TDA7406 Datasheet, PDF (40/64 Pages) STMicroelectronics – Car radio signal processor

Functional description of the noiseblanker
9
Functional description of the noiseblanker
TDA7406
In the automotive environment the MPX-signal as well as the AM-signal is disturbed by
spikes produced by the ignition and other radiating sources like 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 a time between 22 and 38µs in FM (370
and 645µs in AM-mode). The block diagram of the noiseblanker is given in Figure 29.
Figure 29. Block diagram of the noiseblanker
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 trigger stage a
pulse former generates the “blanking”-pulse.
9.1
Trigger Path FM
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 integrated
(lowpass filtered) to generate a signal called PEAK. The DC-charge/discharge behavior can
be adjusted as well as the transient behavior (MP-discharge control). 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 flip-flop. The
flip-flop's output activates the sample-and-hold circuits in the signal path for the selected
duration.
9.2
Noise Controlled Threshold Adjustment (NCT)
There are mainly two independent possibilities for programming the trigger threshold:
1. the low threshold in 8 steps (bits D1 to D3 of the noiseblanker-byte I)
2. and the noise adjusted threshold in 4 steps (bits D4 and D5 of the noiseblanker-byte I,
see Figure 20).
The low threshold is active in combination with a good MPX signal without noise; the PEAK
voltage is less than 1V. The sensitivity in this operation is high.
If the MPX signal is noisy (low fieldstrength) the PEAK voltage increases due to the higher
noise, which is also rectified. With increasing of the PEAK voltage the trigger threshold
increases, too. This gain is programmable in 4 steps (see Figure 22).
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