TDA7400D datasheet(15/28 Pages) STMICROELECTRONICS

15 / 28 page

FUNCTIONAL DESCRIPTION OF STEREODE-

CODER

The stereodecoder part of the TDA7400D (see

Fig. 12) contains all functions necessary to de-

modulate the MPX signal like pilot tone depend-

ent

MONO/STEREO

switching

well

"stereoblend" and "highcut" functions.

Stereodecoder Mute

The TDA7400D has a fast and easy to control

RDS mute function which is a combination of the

audioprocessor’s softmute and the high-ohmic

mute of the stereodecoder. If the stereodecoder

is selected and a softmute command is sent (or

activated through the SM pin) the stereodecoder

will be set automatically to the high-ohmic mute

condition after the audio signal has been soft-

muted.

Hence a checking of alternate frequencies could

be performed. To release the system from the

mute condition simply the unmute command must

be sent: the stereodecoder is unmuted immedi-

ately and the audioprocessor is softly unmuted.

Fig. 13 shows the output signal VO as well as the

internal stereodecoder mute signal. This influ-

ence of Softmute on the stereodecoder mute can

be switched off by setting bit 3 of the Softmute

byte to "0". A stereodecoder mute command (bit

0, stereodecoder byte set to "1") will set the

stereodecoder in any case independently to the

high-ohmic mute state.

If any other source than the stereodecoder is se-

lected the decoder remains muted and the MPX

pin is connected to Vref to avoid any discharge of

the coupling capacitor through leakage currents.

Ingain + Infilter

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 nose and acts as an anti al-

lasing filter for the following switch capacitor fil-

ters.

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 cancelled.

For reaching a high channel separation the

TDA7400D offers an I2C bus programmable roll-

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

MPINFL

LEVEL INTERN

CONTROL

VHCCH

VHCCL

MULTIPATH

DETECTOR

MP_OUT

LEVEL INPUT

LP 2.2KHZ

1.th ORDER

MP_IN

LEVEL

FM_L

FM_R

MPX

100K

D98AU855

GAIN 0..10dB

MPLEVELOUT

QUALITY DETECTOR

QUAL

NOISE

Figure 12. Block Diagram of the Stereodecoder

TDA7400D

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