TDA7406 datasheet(40/64 Pages) STMICROELECTRONICS

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Functional description of the noiseblanker

TDA7406

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Functional description of the noiseblanker

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 stereodecoder 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 noiseblanker is given in Figure 29.

Figure 29.

Block diagram of the noiseblanker

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.

9.1

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 behavior can

be adjusted as well as the transient behavior (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 flip-flop. The

flip-flop's output activates the sample-and-hold circuits in the signal path for the selected

duration.

9.2

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 20).

The low threshold is active 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 22).

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