TDA7541 datasheet(36/76 Pages) STMICROELECTRONICS

36 / 76 page

Functional description

TDA7541

36/76

Figure 6.

Stereo blend controlled threshold adjust

Automatic deviation controlled threshold adjustment

If the system is tuned to stations with a high deviation the noise blanker can trigger on the

higher frequencies of the modulation. To avoid this wrong behavior, which causes noise in

the output signal, the noise blanker offers a deviation-dependent threshold adjustment. By

rectifying the MPX signal a further signal representing the actual deviation is obtained. It is

used to increase the PEAK voltage. The gain of this circuit is programmable in 3 steps

(NBDC) of the stereo decoder-byte (the first step turns off the detector).

Figure 7.

Deviation controlled threshold adjust

Multipath influence on noise blanker

To react on high repetitive spikes caused by a Multipath-situation, the discharge-time of the

VPEAK voltage can be decreased depending on the voltage-level at Pin MPTC. There are

two ways to do this.

Switch on the linear influence of the Multipath-Level on the PEAK-signal . In this

case the discharge slew rate is 1V/ms. The slew rate is measured with

RDischarge=infinite and VMPTC=2.5V

Activate a function, which switches to the 18k discharge resistor if the Multipath-

Level is below 2.5V. If multipath influence on noise blanker is switched ON than

MPF bit has to be set to 0.

0.8V

noisy signal

good signal

NOIS E

2.0V (00)

MONO

STEREO

1.6V (01)

1.4V (10)

ATC_SB OFF (11)

VPEAK

≈ 3V

0.8

DEVIATION [KHz]

2.0

1.5

1.0

32.5

VPEAK

Detector off

]

TDA7541 Datasheet(PDF) 36 Page - STMicroelectronics

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