TDA7406 datasheet(37/64 Pages) STMICROELECTRONICS

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TDA7406

Functional Description of Stereodecoder

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8.4

De-emphasis and Highcut

The de-emphasis-lowpass allows to choose a time constant between 37.5 and 100µs. The

highcut control range will be 2 x

Deemp or 2.7 x τDeemp dependent on the selected time

constant (see programming section). The bit D7 of the hightcut-byte will shift time constant

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

Deemp...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 Section 8.9.

8.5

PLL and Pilot tone-Detector

The PLL has the task to lock on the 19kHz pilot tone during a stereo-transmission to allow a

correct demodulation. The included pilot tone-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 TDA7406 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.

8.6

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 functions are described in sections 5.3 and 6.

8.7

LEVEL-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 car radio. This signal controls directly the Highcut

stage whereas the signal is filtered again (fc=100Hz) before the stereoblend stage (see

Figure 26).

8.8

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

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