TDA7513T datasheet(37/84 Pages) STMICROELECTRONICS

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TDA7513T

Functional description

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like REF5V are derived from a bandgap circuit, therefore they have a temperature coefficient

which is practically zero.

3.5.9

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

Figure 18).

3.5.10

Noise blanker

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

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

audible influence of these spikes. To perform this function the output of the stereo decoder is

held at the curent voltage for a time between 22 and 38

μs (programmable). The block

diagram of the noiseblanker is shown in fig.20. In the first stage the spikes are detected but

to avoid a wrong triggering on high frequency (white) noise a complex trigger desensitization

control is implemented. Behind the trigger stage a pulse former generates the "blanking"

pulse

3.5.11

Trigger path

The incoming MPX signal is highpassed by a filter with a corner frequency of 140kHz,

amplified and rectified. The rectified signal (RECT) is lowpassed to generate the signal

PEAK. Also noise at a frequency higher than 140kHz increases PEAK. The lowpass output

voltage can be adjusted by changing the noise rectifier discharge current. The PEAK

voltage is fed to a threshold generator which adds to the PEAK voltage a constant voltage

VTH, thus producing the trigger threshold PEAK+VTH. Both RECT and PEAK+VTH are fed

to a comparator which trig-gers a re-triggerable monoflop. The monoflop output activates

the sample-and-hold circuits in the signalpath for a selectable duration.

3.5.12

Automatic noise controlled threshold adjustment (ATC)

There are mainly two independent possibilities to program the trigger threshold:

Programming the so-called "low threshold" in 8 steps;

Programming the so-called "noise adjusted threshold" in 4 steps

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 operating mode is high.

If the MPX signal is noisy 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

particular mechanism ("noise adjusted threshold") is programmable in 4 steps.

3.6

Automatic threshold control mechanism

3.6.1

Automatic threshold control by the stereoblend voltage

Besides the noise controlled threshold adjustment there is an additional possibility to

influence the trigger threshold which depends on the stereoblend control.

The point where the MPX signal starts to become noisy is fixed by the RF part. Therefore

also the starting point of the normal noise controlled trigger adjustment is fixed. In some

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