TDA7402 datasheet(40/71 Pages) STMICROELECTRONICS

40 / 71 page

Functional description of stereo decoder

TDA7402

40/71

Functional description of stereo decoder

Figure 28.

Block diagram of stereo decoder

The stereo decoder-part of the A619 (see

Figure 28) contains all functions necessary to

demodulate the MPX-signal like pilot tone dependent Mono/Stereo switching as well as

"stereo blend" and "highcut". Adaptations like programmable input gain, roll off

compensation, selectable de-emphasis time constant and a programmable fieldstrength

input allow to use different IF devices.

6.1

Stereo decoder mute

The A619 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 stereo decoder. If the stereo

decoder is selected and a SoftMute command is sent (or activated through the SM-pin) the

stereo decoder will be set automatically to the high-ohmic mute condition after the audio-

signal has been softmuted. Hence a checking of alternate frequencies could be performed.

Additionally the PLL can be set to "Hold" mode, which disables the PLL input during the

mute time. To release the system from the mute condition simply the unmute command

must be sent: the stereo decoder is unmuted immediately and the audioprocessor is softly

unmuted. Fig. 26 shows the output-signal VO as well as the internal stereo decoder mute

signal. This influence of SoftMute on the stereo decoder mute can be switched off by setting

bit 3 of the SoftMute byte to "0". A stereo decoder mute command (bit 0, stereo decoder

byte set to "1") will also set the stereo decoder independently to the high-ohmic mute state.

If any other source than the stereo decoder is selected the decoder remains muted and the

MPX pin is connected to Vref to avoid any discharge of the coupling capacitor through

leakage currents. No further mute command should be applied.

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

NOISE

SB CONTROL

MULTIPATH DET.

DEEMPHASIS

+ HIGHCUT

t=50.62..5.75,100

μs

(37.5,47,56,75

μs)

REF 5V

VSBL

MPHCINFL

MPHCOUT

MPSBOUT

CONTROL

VHCCH

VHCCL

5 bits

LEVEL INPUT

LEVELSB-LP

LEVEL

MPSBINFL

FM_L

FM_R

MPX

100K

AM-IF

100K

D00AU1135

LEVELHC-LP

GAIN 0..6dB

QUALITY

DETECTOR

QUAL

MP-OUT

MP-IN

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