TDA7407 datasheet(31/46 Pages) STMICROELECTRONICS

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TDA7407

Multipath detector

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5.3

Automatic threshold control mechanism

5.3.1

Automatic threshold control by the stereoblend voltage

Besides the noise controlled threshold adjustment there is an additional possibility for

influencing 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 trigger adjustment is fixed. In some

cases the behaviour of the noiseblanker can be improved by increasing the threshold even

in a region of higher fieldstrength.

Sometimes a wrong triggering occures 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.

5.3.2

Over deviation detector

If the system is tuned to stations with a high deviation the noiseblanker can trigger on the

higher frequencies of the modulation. To avoid this wrong behaviour, which causes noise in

the output signal, the noiseblanker 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. 18).

5.4

Functional description of the multipath detector

Using the internal multipath detector the audible effects of a multipath condition can be

minimized. A multipath condition is detected by rectifying the 19kHz spectrum in the

fieldstrength signal.An external capacitor is used to define the attack and decay times (see

block diagram Figure 24). the MPOUT pin is used as detector output connected to a

capacitor of about 47nF and additionally the MPIN pin is selected to be the fieldstrength

input.

Using the configuration, an external user requirement adaptation is given in Figure 24.

To keep the old value of the Multipath Detector during an AF-jump, the external capacitor

can be disconnected by the MP Hold switch. This switch can be 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 Figure 24.

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