E-TDA7461ND View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'E-TDA7461ND' PDF : 48 Pages View PDF
Stereo decoder part
TDA7461
4.6.1
4.6.2
Trigger path
The incoming MPX signal is highpass filtered, amplified and rectified. This second order
highpass-filter has a corner frequency of 140 kHz. The rectified signal, RECT, is lowpass
filtered to generate a signal called PEAK. Also noise with a frequency 140 kHz 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 40 µs.
The block diagram of the noiseblanker is given in Figure 25.
Automatic noise controlled threshold adjustment (ATC)
There are mainly two independent possibilities for programming the trigger threshold:
the low threshold in 8 steps (bits D0 to D2 of the noiseblanker byte)
the noise adjusted threshold in 4 steps (bits D3 and D4 of the noiseblanker byte, see fig.
18).
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 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 Figure 17).
Figure 25. Block diagram of the noiseblankert
MPX
HIGH PASS
RECTIFIER
RECT
+
-
VTH
+
PEAK
LOWPASS
+
D98AU861
MONOFLOP
THRESHOLD
GENERATOR
ADDITIONAL
THRESHOLD
CONTROL
HOLDN
4.6.3
Automatic threshold control
Besides the noise controlled threshold adjustment there is an additional possibility for
influencing the trigger threshold. It is depending on the stereo blend 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 trigger adjustment is fixed (Figure 19).
In some cases the behavior of the noise blanker can be improved by increasing the
threshold even in a region of higher fieldstrength. Sometimes a wrong triggering occurs for
the MPX signal often shows distortion in this range which can be avoided even if using a low
threshold.
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