TDA7406 datasheet(41/64 Pages) STMICROELECTRONICS

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TDA7406

Functional description of the noiseblanker

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9.3

Additional Threshold Control Mechanism

9.3.1

Automatic Threshold Control by the Stereoblend voltage

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

influencing the trigger threshold which depends 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. 20). In

some cases the behavior of the noiseblanker 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 controlled by stereoblend. This increase of the threshold is programmable in 3 steps

or switched off.

9.3.2

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 Figure 21).

9.3.3

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 TDA7406

offers a linear as well as a threshold driven 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 R

Discharge=infinite and VMPout=2.5V

9.3.4

AM mode of the Noiseblanker

The TDA7406 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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