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19/34

TDA7407D

Figure 22. Relation Between Internal and External LEVEL Voltage and Setup of Stereoblend

Figure 23. High cut Characteristics

6.5.6 LEVEL Input and Gain

To suppress undesired high frequency modulation on the highcut and stereoblend function the LEVEL sig-

nal is lowpass filtered firstly.

The filter is a combination of a 1st order RC lowpass at 53kHz (working as anti-aliasing filter) and a 1st-

order switched capacitor lowpass at 2.2kHz. The second stage is a programmable gain stage to adapt the

LEVEL signal internally to different IF device (see Testmode section 5 LEVELINTERN).

The gain is widely programmable in 16 steps from 0dB to 10dB (step = 0.67dB). These 4 bits are located

together with the Roll-Off bits in the "Stereodecoder Adjustment" byte to simplify a possible adaptation dur-

ing the production of the carradio.

6.5.7 Stereoblend Control

The stereoblend control block converts the internal LEVEL voltage (LEVEL INTERN) into an demodulator

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

separation. Internally this control range has a fixed upper limit which is the internal reference voltage

REF5V. The lower limit can be programmed between 29.2% and 58%, of REF5V in 4.167% steps (see

figs. 22, 23). To adjust the external LEVEL voltage to the internal 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:

INTERNAL

VOLTAGES

t

D97AU639

VSBL

REF 5V

SETUP OF VST

INTERNAL

VOLTAGES

t

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

FIELDSTRENGHT

VHCH

VHCL

3

•

L

G

REF5V

Field strenght voltage [STEREO]

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=

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