TDA7412 View Datasheet(PDF) - STMicroelectronics

Part Name

Description

MFG CO.

TDA7412

CARRADIO SIGNAL PROCESSOR

STMicroelectronics 

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TDA7412

3.0 FUNCTIONAL DESCRIPTION OF STEREODECODER

Figure 23. Block diagram of Stereodecoder

MPX

AM

100K

100K

AM-IF

INGAIN

3.5 ... 11dB

STEP 2.5dB

D00AU1135

INFILTER

LP 80KHz

4.th ORDER

PLL +

PILOT-DET.

F19

F38

STEREO

NOISE BLANKER

HOLDN

NOISE

DEMODULATOR

- PLOT CANC

- ROLL-OFF COMP.

- LP 25KHz

DEEMPHASIS

+ HIGHCUT

t=50.62..5.75,100µs

(37.5,47,56,75µs)

SB CONTROL

5 bits

REF 5V

VSBL

HC

CONTROL

D

A

FM_L

FM_R

VHCCH

VHCCL

MPSBINFL

-

MPHCINFL

MULTIPATH DET.

MPHCOUT

-

MPSBOUT

MP-IN

MP-OUT

LEVELSB-LP

LEVEL INPUT

GAIN 0..6dB

LEVELHC-LP

QUALITY

DETECTOR

+

LEVEL

QUAL

The stereodecoder-part of the A679 (see Fig. 23) contains all functions necessary to demodulate the MPX-sig-

nal like pilottone-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

fieldstrength input allow to use different IF-devices.

3.1 Stereodecoder-Mute

The A679 has a fast and easy to control RDS-Mute function which is a combination of the audioprocessor's Soft-

Mute 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 con-

dition 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 stereodecoder is unmuted

immediately and the audioprocessor is softly unmuted. Fig. 26 shows the output-signal VO as well as the internal

stereodecoder mute signal. This influence 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 selected the decoder remains muted and the MPX-pin is connect-

ed to Vref to avoid any discharge of the coupling capacitor through leakage currents. No further mute command

should be applied.

30/56

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