TDA7407 datasheet(28/46 Pages) STMICROELECTRONICS

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Multipath detector

TDA7407

28/46

5.2.4

De-emphasis and Highcut.

The lowpass filter for the de-emphasis allows to choose between a time constant of 50

μs

and 75

μs (bit D7, stereo decoder byte).

The highcut control range will be in both cases

HC = 2 · τDeemp. Inside the highcut control

range (between VHCH and VHCL) the LEVEL signal is converted into a 5 bit word which

controls the lowpass time constant between

Deemp...3 · τDeemp. There by the resolution will

remain always 5 bits independently of the absolute voltage range between the VHCH and

VHCL values.

The highcut function can be switched off by I2C bus (bit D7, fieldstrength byte set to "0").

5.2.5

PLL and pilot tone detector

The PLL has the task to lock on the 19kHz pilotone during a stereo transmission to allow a

correct demodulation. The included detector enables the demodulation if the pilot tone

reaches the selected pilot tone threshold VPTHST. Two different thresholds are available. The

detector output (signal STEREO, see block diagram) can be checked by reading the status

byte of the TDA7407 via I2C bus.

5.2.6

Field strength control

The fieldstrength input is used to control the high cut and the stereoblend function. In

addition the signal can be also used to control the noiseblanker thresholds and as input for

the multipath detector.

Figure 21.

Relation between internal and external LEVEL voltage and setup of

stereoblend

Figure 22.

High cut characteristics

INTERNAL

VOLTAGES

D97AU639

VSBL

REF 5V

SETUP OF VST

INTERNAL

VOLTAGES

33%

REF 5V

SETUP OF VMO

LEVEL

LEVEL INTERN

FIELDSTRENGHT VOLTAGE

VST

VMO

LEVEL INTERN

42%

50%

58%

VSBL

VST

VMO

FIELDSTRENGHT VOLTAGE

LOWPASS

TIME CONSTANT

D97AU640

τDeemp

FIELDSTRENGHT

VHCH

VHCL

•

τDeemp

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