TDA7461N Datasheet, PDF (19/31 Pages) STMicroelectronics – CAR RADIO SIGNAL PROCESSOR

TDA7461N
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 or 33% of VHCH (see fig. 21).
Figure 21. Highcut characteristics
LOWPASS
TIME CONSTANT
3•τ_Deemp
τ_Deemp
VHCL
D97AU640
VHCH FIELDSTRENGHT
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
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 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 gen-
erate a signal called PEAK. Also noise with a fre-
quency 140kHz increases the PEAK voltage. 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 40µs.
The block diagram of the noiseblanker is given in
fig.22.
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. 14).
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 (see fig. 14).
Figure 22. Block diagram of the noiseblanker
MPX
HIGH PASS
RECTIFIER
RECT
+
-
VTH
+
PEAK
LOWPASS
+
D98AU861
MONOFLOP
THRESHOLD
GENERATOR
ADDITIONAL
THRESHOLD
CONTROL
HOLDN
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