TDA7412 datasheet(32/56 Pages) STMICROELECTRONICS

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optimization step during the production of the carradio where the channel separation and the fieldstrength con-

trol are trimmed. The setup of the Stereoblend characteristics which is programmable in a wide range is de-

scribed in 2.8.

3.4 Deemphasis and Highcut

The deemphasis-lowpass allows to choose a time constant between 37.5 and 100µs. The highcut control range

will be 2 x

τDeemp or 2.7 x τDeemp dependent on the selected time constant (see programming section). The bit

D7 of the hightcut-byte will shift timeconstant and range.

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

τDeemp...3 (3.7) x τDeemp. Thereby the resolution will remain

always 5 bits independently of the absolute voltage range between the VHCH- and VHCL-values. In addition

the maximum attenuation can be fixed between 2 and 10dB.

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

The setup of the highcut characteristics is described in 2.9.

3.5 PLL and Pilottone-Detector

The PLL has the task to lock on the 19kHz pilottone during a stereo-transmission to allow a correct demodula-

tion. The included pilottone-detector enables the demodulation if the pilottone reaches the selected pilottone

threshold VPTHST. Two different thresholds are available. The detector output (signal STEREO, see Blockdia-

gram) can be checked by reading the status byte of the A679 via I2C-bus. During a Softmute the PLL can be

set into "Hold"-mode which freezes the PLL's state (bit D4, Softmute byte). After releasing the Softmute the PLL

will again follow the input signal only by correcting the phase error.

3.6 Fieldstrength Control

The fieldstrength input is used to control the highcut- and the stereoblend-function. In addition the signal can be

also used to control the noiseblanker thresholds and as input for the multipath detector. These additional func-

tions are described in sections 3.3 and 4.

3.7 EVEL-Input and -Gain

To suppress undesired high frequency modulation on the highcut- and stereoblend-control signal the LEVEL

signal is lowpass filtered firstly. The filter is a combination of a 1.st-order RC-lowpass at 53kHz (working as anti-

aliasing filter) and a 1.st-order switched-capacitor-lowpass at 2.2kHz. The second stage is a programmable gain

stage to adapt the LEVEL signal internally to different IF-devices (see Testmode section 5: LEVELHCC). The

gain is widely programmable in 16 steps from 0dB to 6dB (step=0.4dB). These 4 bits are located together with

the Roll-Off bits in the "Stereodecoder-Adjustment"-byte to simplify a possible adjustment during the production

of the carradio. This signal controls directly the Highcut stage whereas the signal is filtered again (fc=100Hz)

before the stereoblend stage (see fig. 32).

3.8 Stereoblend Control

The stereoblend control block converts the internal LEVEL-voltage (LEVELSB) into an demodulator compatible

analog signal which is used to control the channel separation between 0dB and the maximum separation. Inter-

nally this control range has a fixed upper limit which is the internal reference voltage REF5V. The lower limit can

be programmed between 20 and 70% of REF5V in 3.3% steps (see figs.28, 29).

To adjust the external LEVEL-voltage to the internal range two values must be defined: the LEVEL gain LG and

VSBL (see fig. 29). At the point of full channel separation the external level signal has to be amplified that inter-

nally it becomes equal to REF5V. The second point (e.g. 10dB channel sep.) is then adjusted with the VSBL

voltage.

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