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TDA7407D

6.6.2 Automatic Noise Controlled ThresholdAdjustment (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).

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.

6.7

AUTOMATIC THRESHOLD CONTROL MECHANISM

6.7.1 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. ). In some cases the behaviour of the

noiseblanker can be improved by increasing the threshold even in a region of higher fieldstrength. Some-

times 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.

Figure 25. Block Diagram of the Multipath Detector

6.7.2 Over Deviation Detector

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

cies of the modulation. To avoid this wrong behaviour, 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 in-

crease the PEAK voltage. Offset and gain of this circuit are programmable in 3 steps with the bits D6 and

D7 of the stereodecoder byte (the first step turns off the detector, see fig. 25).

6.8

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 capac-

itor is used to define the attack and decay times. 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

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