TDA7402_07 datasheet(42/71 Pages) STMICROELECTRONICS

42 / 71 page

Functional description of stereo decoder

TDA7402

42/71

channel separation and the fieldstrength control are trimmed. The setup of the stereo blend

characteristics which is programmable in a wide range is described in

Chapter 6.8.

6.4

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 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

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

Chapter 6.9.

6.5

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 the Block diagram) can be checked by

reading the status byte of the A619 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.

6.6

Field strength control

The fieldstrength input is used to control the highcut and the stereo blend 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

6.3 and 7.

6.7

EVEL input and gain

To suppress undesired high frequency modulation on the highcut- and stereo blend-control

signal the LEVEL signal 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 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 "Stereo decoder 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 stereo blend stage

(see

Figure 35).

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