TDA7402 datasheet(45/71 Pages) STMICROELECTRONICS

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TDA7402

Functional description of the noise blanker

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Functional description of the noise blanker

In the automotive environment the MPX-signal as well as the AM signal is disturbed by

spikes produced by the ignition and other radiating sources like the wiper motor. The aim of

the noiseblanker part is to cancel the audible influence of the spikes. Therefore the output of

the stereo decoder is held at the actual voltage for a time between 22 and 38µs in FM (370

and 645µs in AM mode). The block diagram of the noise blanker is given in

Figure 34.

Block diagram of the noise blanker

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 trigger stage a

pulse former generates the "blanking"pulse.

7.1

Trigger path FM

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 integrated

(lowpass filtered) to generate a signal called PEAK. The DC-charge/discharge behaviour

can be adjusted as well as the transient behaviour (MP discharge control). 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 the

selected duration.

7.2

Noise controlled threshold adjustment (NCT)

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

the low threshold in 8 steps (bits D1 to D3 of the noiseblanker byte I)

and the noise adjusted threshold in 4 steps (bits D4 and D5 of the noiseblanker byte I,

see

Figure 21).

The low threshold is activ in combination with a good MPX signal without noise; the PEAK

voltage is less than 1V. The sensitivity in this operation is high.

If the MPX signal is noisy (low fieldstrength) 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 gain is programmable in 4 steps (see

Figure 25).

RECTIFIER

AM/FM

INTEGRATOR

DISCHARGE

CONTROL

FM: 22 to 40

μs

AM: 370 to 6400

μs

RECT

THRESHOLD

GENERATOR

VTH

PEAK

ADDITIONAL

THRESHOLD

CONTROL

MONOFLOP

HOLDN

AMIF

D00AU1132

RECTIFIER +

14-56KHz LPF

(1st. order)

140KHz HPF

(1st. order)

140KHz HPF

(2nd. order)

MPX

MPOUT

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