TDA7407 Datasheet, PDF (18/30 Pages) STMicroelectronics – ADVANCED CAR SIGNAL PROCESSOR

TDA7407
FUNCTIONAL DESCRIPTION OF THE NOISE-
BLANKER
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 influ-
ence of the spikes.
Therefore the output of the stereodecoder is held
at the actual voltage for a time between 22 and
38µs (programmable).
The block diagram of the noiseblanker is given in
fig.20.
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 imple-
mented. Behind the triggerstage a pulse former
generates the "blanking" pulse. To avoid any
crosstalk to the signalpath the noiseblanker is
supplied by his own biasing circuit.
Trigger Path
The incoming MPX signal is highpass filtered,
amplified and rectified. This second order high-
pass-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 resulting voltage can be adjusted by use of
the noise rectifier discharge current.
The PEAK voltage is fed to a threshold generator,
which adds to the PEAK voltage a DC depend-
ent threshold VTH. Both signals, RECT and
PEAK+VTH are fed to a comparator which trig-
gers a re-triggerable monoflop. The monoflop’s
output activates the sample-and-hold circuits in
the signalpath for selected duration.
Figure 20. Block Diagram of the Noiseblanker
Automatic Noise Controlled Threshold Adjust-
ment (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 fig. 17).
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 op-
eration is high.
If the MPX signal is noisy the PEAK voltage in-
creases due to the higher noise, which is also
rectified. With increasing of the PEAK voltage the
trigger threshold increases, too. This particular
gain is programmable in 4 steps.
AUTOMATIC THRESHOLD CONTROL MECHA-
NISM
Automatic Threshold Control by the
Stereoblend Voltage
Besides the noise controlled threshold adjust-
ment there is an additional possibility for influenc-
ing the trigger threshold. It is depending on the
stereoblend control.
The point where the MPX signal starts to become
noisy is fixed by the RF part. Therefore also the
starting point of the normal noise-controlled trig-
ger adjustment is fixed (fig. 14). In some cases
the behaviour of the noiseblanker can be im-
proved by increasing the threshold even in a re-
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MPX
MPX
RECTIFIER
RECT
+
-
VTH
+
LOWPASS
CONTROL
PEAK
+
D98AU856
MONOFLOP
THRESHOLD
GENERATOR
ADDITIONAL
THRESHOLD
CONTROL
HOLDN