TDA7513T datasheet(38/84 Pages) STMICROELECTRONICS

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Functional description

TDA7513T

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cases the behavior of the noiseblanker can be improved by increasing the threshold even in

a region of higher fieldstrength. Sometimes a wrong triggering occures for the MPX signal

often shows distortion in this range which can be avoided even if using a low threshold.

Because of the overlap of this range and the range of the stereo/mono transition it can be

controlled by stereoblend.

This threshold increase is programmable in 3 steps or switched off.

3.6.2

Over deviation detector

If the system is tuned to stations with a high de-viation the noiseblanker might be

erroneously triggered on the higher frequencies of the modulation. To avoid this

unnecessary muting of the signal, the noiseblanker offers a deviation-dependent threshold

adjustment.

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

This is used to increase the PEAK voltage. The circuit offset, gain (and enabling) are

programmable in 3 steps.

3.7

Multipath detector

Using the stereo decoder multipath detector the audible effects of a multipath condition can

be minimized. A multipath condition is detected by rectifying the 19kHz spectrum in the

fieldstrength signal. An external capacitor is used to define the attack and decay times (see

Figure 20). The pin #MULTIPATHTC is externally connected to a capacitor of about 47nF

and the MPIN signal is internally connected to the unfiltered field strength. To avoid losing

the information stored in the external capacitor during AF checks but at the same time to

allow some fast multipath detection capability during the same AF check period, the external

capacitor is disconnected by the MP-Hold switch. This switch is controlled directly by the pin

#RDSMUTE.

Moreover, selecting the "internal influence" in the configuration byte, the channel separation

is automatically reduced during a multipath condition according to the voltage appearing at

the pin #MULTIPATHTC.

3.7.1

Programming

To obtain a good multipath performance an adaptation is necessary. Therefore tha gain of

the 19kHz bandpass is programmable in four steps as well as the rectifier gain. The attack

and decay times can be set by properly choosing the value of the external capacitor.

3.8

Quality detector

The device offers a quality detector output voltage representing the quality of the FM

reception conditions. This voltage is derived from MPX noise information and multipath

information according to the following formula:

Quality = 1.6 (Vnoise -0.8V)+ a (REF5V- VMPOUT)

The noise signal is the PEAK signal of the noise blanker without additional influences. The

multipath information weight "a" can be programmed between 0.7 and 1.15. The circuit

output pin #QUALITY is a low impedance output able to drive external circuitry as well as

suitable to be simply fed to an A/D converter for RDS applications.

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