TDA7513 datasheet(25/58 Pages) STMICROELECTRONICS

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TDA7513

compatible analog signal which is used to control the channel separation between 0dB and the maximum

separation. This control range has a fixed upper limit which is the in-ternal reference voltage REF5V. The

lower limit

can be programmed between 29.2% and 58% of REF5V in 4.167% steps (see figs. 14, 15).

To adjust the LEVEL voltage to the proper range two values must be defined: the LEVEL gain LG and

VSBL (see fig. 15). To adjust the voltage where the full channel separation is reached (VST) the LEVEL

gain LG has to be defined. The following equation can be used to estimate the gain:

The gain LG can be programmed with 4 bits. The MONO voltage VMO (0dB channel separation) can be

chosen selecting VSBL. All the necessary internal reference voltages like REF5V are derived from a band-

gap circuit, therefore they have a temperature coefficient which is practically zero.

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. 19).

4.6 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 stereodecoder 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

Trigger Path

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

tified. 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 volt-

age 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 acti-

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

Automatic Noise Controlled Threshold Adjustment (ATC)

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

a programming the so-called "low threshold" in 8 steps;

b 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.

REF5V

Fieldstrengthvoltage STEREO

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