TDA7412 datasheet(36/56 Pages) STMICROELECTRONICS

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TDA7412

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5.0 FUNCTIONAL DESCRIPTION OF THE MULTIPATH-DETECTOR

Using the Multipath-Detector the audible effects of a multipath condition can be minimized. A multipath-condi-

tion is detected by rectifying the spectrum around 19kHz in the fieldstrength signal. An external capacitor is used

to define the attack- and decay-times for the Stereoblend (see blockdiagram, fig. 32). Due to the very small

charge currents this capacitor should be a low leakage current type (e.g ceramic). Using this configuration an

adaptation to the user's requirement is possible without effecting the "normal" fieldstrength input (LEVEL) for

the stereodecoder. This application is given in fig. 32. Another (internal) time constant is used to control the

Highcut through the multipath detector

Selecting the "internal influence" in the configuration byte the Stereo-Blend and/or the Highcut is automatically

invoked during a multipath condition according to the voltage appearing at the MP_OUT-pin.

Figure 30. Blockdiagram of the Multipath-Detector

6.0 QUALITY DETECTOR

The A679 offers a quality detector output which gives a voltage representing the FM-reception conditions. To

calculate this voltage the MPX-noise and the multipath-detector output are summed according to the following

formula :

VQual = 1.6 (VNoise-0.8 V)+ a (REF5V-VMpout).

The noise-signal is the PEAK-signal without additional influences (see noiseblanker description). The factor 'a'

can by programmed to 0.7 .... 1.15. The output is a low impedance output able to drive external circuitry as well

as simply fed to an AD-converter for RDS applications.

7.0 TESTMODE

During the Testmode, which can be activated by setting bit D0 and bit D1 of the stereodecoder testing-byte, sev-

eral internal signals are available at the FD2R+ pin. During this mode the input resistance of 100kOhm is dis-

connected from the pin. The internal signals available are shown in the Data-byte specification.

INT.

INFLUENCE

CHARGE

0.25

µA/0.5µA

FAST

CHARGE

300

µA

LEVEL

D00AU1166

LOWPASS

2.2KHz

BANDPASS

19KHz

RECTIFIER

SHIFT

0 ... 1.5V

LOWPASS

100Hz

LOWPASS

53KHz

INT.

INFLUENCE

MP HOLD

GAIN 2 bits

MP_IN

to HCC

to SB

470nF

MP_OUT

VDD

High-Cut-Infl.

Attach<500

µs

Decay=2/10ms

Stereoblend-Infl.

Attach=1ms

Decay=4/8s

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