TDA7406T View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'TDA7406T' PDF : 49 Pages View PDF
TDA7406T
4.9 Highcut Control
The highcut control set-up is similar to the stereoblend control set-up: 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. 26).
Figure 26. Highcut characteristics
5 FUNCTIONAL DESCRIPTION OF THE NOISEBLANKER
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 audi-
ble 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
fig.27.
Figure 27. 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.
5.1 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-dis-
charge 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.
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