TDA7411 datasheet(27/43 Pages) STMICROELECTRONICS

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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 D0 and D1 of the fieldstrength control-byte.

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 D6 and D7 of the stereo decoder-byte (the first step

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

byte). In this case the discharge slew rate is 1V/ms1. 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 RDischarge = infinite and VMPout = 2.5V

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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