TDA7411 ST Microelectronics, TDA7411 Datasheet - Page 27

• Manufacturer Part Number: TDA7411
• Description: ADVANCED CAR SIGNAL PROCESSOR
• Manufacturer: ST Microelectronics

TDA7411
ADDITIONAL THRESHOLD CONTROL MECHANISM
Automatic Threshold Control by the Stereoblend voltage
Besides the noise controlled threshold adjustment there is an additional possibility for influencing the trig-
ger 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 trigger adjustment is fixed (fig. 17). In some cases the behavior of the
noise blanker can be improved by increasing the threshold even in a region of higher fieldstrength. Some-
times a wrong triggering occurs for the MPX signal often shows distortion in this range, which can be
avoided even if using a low threshold. Because of the overlap of this range and the range of the stereo/
mono transition it can be controlled by stereoblend. This threshold increase is programmable in 3 steps
or switched off with bits D
and D
of the fieldstrength control-byte.
0
1
Over Deviation Detector
If the system is tuned to stations with a high deviation the noise blanker can trigger on the higher frequen-
cies of the modulation. To avoid this wrong behavior, which causes noise in the output signal, the noise
blanker offers a deviation-dependent threshold adjustment. By rectifying the MPX signal a further signal
representing the actual deviation is obtained. It is used to increase the PEAK voltage. Offset and gain of
this circuit are programmable in 3 steps with the bits D
and D
of the stereo decoder-byte (the first step
6
7
turns off the detector, see fig. 16).
Multipath-Level
To react on high repetitive spikes caused by a Multipath-situation, the discharge-time of the PEAK voltage
can be decreased depending on the voltage-level at Pin MPout. There are two ways to do this. One way
is to switch on the linear influence of the Multipath-Level on the PEAK-signal (D7 of field strength control
1
byte). In this case the discharge slew rate is 1V/ms
. The second possibility is to activate a function, which
switches to the 18k discharge if the Multipath-Level is below 2.5V (D7 of High-Cut-Control-Byte).
1
The slew rate is measured with R
= infinite and V
= 2.5V
Discharge
MPout
Functional Description of the Multipath-Detector
Using the internal Multipath-Detector the audible effects of a multipath condition can be minimized. A mul-
tipath-condition is detected by rectifying the 19kHz spectrum in the fieldstrength signal. An external ca-
pacitor is used to define the attack- and decay-times (see block diagram, fig. 23). The MP_OUT-pin is
used as detector-output connected to a capacitor of about 47nF and additionally the MP_IN-pin is selected
to be the fieldstrength input. Using this configuration an external adaptation to the user's requirement is
possible without affecting the "normal" fieldstrength input (LEVEL) for the stereo decoder. This application
is given in fig. 29.
To keep the old value of the Multipath Detector during an AF-jump, the MP-Hold switch can disconnect
the external capacitor. This switch is controlled directly by the AFS-Pin.
Selecting the "internal influence" in the configuration byte the channel separation is automatically reduced
during a multipath condition according to the voltage appearing at the MP_OUT-pin. A possible application
is shown in fig. 29.
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