TDA7405 datasheet(34/57 Pages) STMICROELECTRONICS

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TDA7405

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Figure 34. Highcut characteristics

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 audi-

ble 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 blockdiagram of the noiseblanker is given in

Figure 34.

Figure 35. 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 triggerstage a pulse former generates the "blanking"-pulse.

11.1 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-dis-

charge 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 acti-

vates the sample-and-hold circuits in the signalpath for the selected duration

11.2 Noise Controlled Threshold Adjustment (NCT)

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

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

2. and the noise adjusted threshold in 4 steps (bits D4 and D5 of the noiseblanker-byte I, see Figure 22).

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