TDA7402 Datasheet, PDF (37/59 Pages) STMicroelectronics – CARRADIO SIGNAL PROCESSOR

TDA7402
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 A619 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 A619 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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