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TDA7412

of the normal noise-controlled trigger adjustment is fixed (fig. 24). In some cases the behavior of the noiseblank-

er 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. Because of the overlap of this range and the range of the stereo/mono transition it can be con-

trolled by stereoblend. This increase of the threshold is programmable in 3 steps or switched off.

4.3.2 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 or distortion. To avoid this behavior, which causes audible 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 noiseblanker-byte I (bit combination '00' turns

off the detector, see fig. 23).

4.3.3 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. The A679 offers a linear as well as a threshold driv-

en control. The linear influence of the Multipath-Level on the PEAK-signal (D7 of Multipath-Control-Byte) gives

a discharge slewrate of 1V/ms1 . The second possibility is to activate the threshold driven discharge which

switches on the 18kOhm discharge if the Multipath-Level is below 2.5V (D7 of noiseblanker-byte II-byte).

1 The slewrate is measured with RDischarge=infinite and VMPout=2.5V

AM mode of the Noiseblanker

The A679 noiseblanker is also suitable for AM noise cancelling. The detector uses in AM mode the 450kHz

unfiltered IF-output of the tuner for spike detection. A combination of programmable gain-stage and lowpass-

filter forms an envelope detector which drives the noiseblanker's input via a 120kHz 1.st order highpass. In order

to blank the whole spike in AM mode the hold-times of the sample and hold circuit are much longer then in FM

(640µs to 1.2ms). All threshold controls can be used like in FM mode.

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