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TDA7407D

20/34

The gain can be programmed through 4 bits in the "Stereodecoder-Adjustment" byte.

The MONO voltage VMO (0dB channel separation) can be choosen selecting VSBLAll necessary internal

reference voltages like REF5V are derived from a bandgap circuit. Therefore they have a temperature co-

efficient near zero. This is useful if the fieldstrength signal is also temperature compensated.

But most IF devices apply a LEVEL voltage with a TC of 3300ppm. The TDA7407D offers this TC for the

reference voltages, too. The TC is selectable with bit D7 of the "stereodecoder adjustment" byte.

6.5.8 Highcut Control

The highcut control setup is similar to the stereoblend control setup : the starting point VHCH can be set

with 2 bits to be 42, 50, 58 or 66% of REF5V whereas the range can be set to be 17, 22, 28 or 33% of

VHCH (see fig. 23).

6.6

FUNCTIONAL DESCRIPTION OF THE NOISEBLANKER

In the automotive environment the MPX signal is disturbed by spikes produced by the ignition and for ex-

ample 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 38

µs

(programmable).

The block diagram of the noiseblanker is given in fig. 24.

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 signalpath the noiseblanker is supplied by his own biasing circuit.

6.6.1 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 frequen-

cy 140kHz increases the PEAK voltage. The resulting voltage can be adjusted by use of the noise rectifier

discharge current.

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-trigger-

able monoflop. The monoflop's output activates the sample-and-hold circuits in the signalpath for selected

duration.

Figure 24. Block Diagram of the Noiseblanker

+

-

RECTIFIER

LOWPASS

RECT

+

+

THRESHOLD

GENERATOR

VTH

PEAK

ADDITIONAL

THRESHOLD

CONTROL

MONOFLOP

HOLDN

MPX

D98AU856

MPX

CONTROL

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