TDA7410ND datasheet(19/34 Pages) STMICROELECTRONICS

19 / 34 page

TDA7410ND

Description of stereodecoder

19/34

Description of stereodecoder

The stereodecoder part of the TDA7410ND (see Figure 12) contains all functions necessary

to demodulate the MPX signal like pilot tone dependent MONO/STEREO switching as well

as "stereoblend" and "highcut" functions. Adaptations like programmable input gain,

selectable deemphasis time constant and a programmable fieldstrength input allow to use

different IF devices.

Figure 12.

Block diagram of the stereodecoder

6.1

Input stages

The Ingain stage allows to adjust the MPX signal to a magnitude of about 1Vrms internally

which is the recommended value. The 4th order input filter has a corner frequency of 80kHz

and is used to attenuate spikes and noise and acts as an antialiasing filter for the following

switch capacitor filters.

6.2

Demodulator

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

In this stage also the 19kHz pilot tone is cancelled.

6.3

Deemphasis and highcut

The lowpass filter for the deemphasis allows to choose between a time constant of 50

μs and

μs (bit D5, Stereodecoder Adjustment byte).

The highcut control range will be in both cases tHC = 2·tDeemp. 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 tDeemp...3·tDeemp. There by the resolution will

remain always 5 bits independently of the absolute voltage range between the VHCH and

VHCL values. The highcut function can be switched off by I2C bus (bit D7, Fieldstrength

Control byte set to "0").

INGAIN

3.5 ... 11dB

STEP 2.5dB

INFILTER

LP 80KHz

4.th ORDER

DEMODULATOR

- PLOT CANC

- ROLL-OFF COMP.

- LP 25KHz

PLL +

PILOT-DET.

F19

NOISE BLANKER

F38

STEREO

HOLDN

SB CONTROL

DEEMPHASIS

+ HIGHCUT

t=50 or 75

μs

REF 5V

VSBL

LEVEL INTERN

CONTROL

VHCCH

VHCCL

LEVEL INPUT

LP 2.2KHZ

1.th ORDER

LEVEL

FM_L

FM_R

MPX

100K

D05AU1614

GAIN 0..10dB

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