TDA7411 datasheet(23/43 Pages) STMICROELECTRONICS

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TDA7411

FUNCTIONAL DESCRIPTION OF STERO DECODER

Figure 18. Block diagram of the stereo decoder

The stereo decoder-part of the TDA7411 (see Fig. 18) contains all functions necessary to demodulate the

MPX-signal like pilot tone-dependent MONO/STEREO-switching as well as "stereoblend" and "highcut".

Adaptations like programmable input gain, roll-off compensation, selectable deemphasis time constant

and a programmable field strength input allow using different IF-devices.

InGain + Infilter

The InGain stage allows adjusting the MPX-signal to a magnitude of about 1Vrms internally, which is the

recommended value. The 4.th order input filter has a corner frequency of 80kHz and is used to attenuate

spikes and noise and acts as an anti-aliasing filter for the following switch capacitor filters.

Demodulator

In the demodulator block the left and the right channel are separated from the MPX-signal. In this stage

also the 19-kHz pilot tone is canceled. For reaching a high channel separation the TDA7411 offers an I2C-

bus programmable roll-off adjustment, which is able to compensate the low pass behavior of the tuner sec-

tion. If the tuner's attenuation at 38kHz is in a range from 13.8% to 24.6% the A673 needs no external

network in front of the MPX-pin. Within this range an adjustment to obtain at least 40dB channel separation

is possible. The bits for this adjustment are located together with the field strength adjustment in one byte.

This gives the possibility to perform an optimization step during the production of the car radio where the

channel separation and the field strength control are trimmed. The setup of the stereoblend characteris-

tics, which is programmable in a wide range.

Deemphasis and Highcut

The deemphasis low pass allows to choose between a time constant of 50µs and 75µs (bit D7, stereo de-

coder byte). The highcut control range will be in both cases

τHC = 2x τDeemp. Inside the highcut control

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

pass time constant between

τDeemp...3x τDeemp. Thereby the resolution will remain always 5 bits indepen-

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