TDA7410ND datasheet(22/34 Pages) STMICROELECTRONICS

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Functional description of the noiseblanker

TDA7410ND

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Functional description of the noiseblanker

In the automotive environment the MPX signal is disturbed by spikes produced by the

ignition and for example the wiper motor. The aim of the noiseblanker part is to cancel the

audible influence of the spikes. Therefore the output of the stereodecoder is held at the

actual voltage for 40

μs. In a first stage the spikes must be detected but to avoid a wrong

triggering on high frequency (white) noise a complex trigger control is implemented. Behind

the triggerstage a pulse former generates the "blanking" pulse. To avoid any crosstalk to the

signal path the noiseblanker is supplied by its own biasing circuit.

7.1

Trigger path

The incoming MPX signal is highpass filtered, amplified and rectified. This second order

highpass-filter has a corner frequency of 140kHz. The rectified signal, RECT, is lowpass

filtered to generate a signal called PEAK. Also noise with a frequency 140kHz increases the

PEAK voltage. The PEAK voltage is fed to a threshold generator, which adds to the PEAK

voltage a DC dependent threshold VTH. Both signals, RECT and PEAK+VTH are fed to a

comparator which triggers a re-triggerable monoflop. The monoflop's output activates the

sample-and-hold circuits in the signalpath for 40

μs. The block diagram of the noiseblanker is

given in Figure 16.

Figure 16.

Block diagram of the noiseblander

7.2

Automatic noise controlled threshold adjustment (ATC)

There are mainly two independent possibilities for programming the trigger threshold:

the low threshold in 8 steps (bits D0 to D2 of the noiseblanker byte)

the noise adjusted threshold in 4 steps (bits D3 and D4 of the noiseblanker byte,

(see Figure 9).

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

RECTIFIER

LOWPASS

RECT

THRESHOLD

GENERATOR

VTH

PEAK

ADDITIONAL

THRESHOLD

CONTROL

MONOFLOP

HOLDN

MPX

D98AU861

HIGH PASS

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