TDA7412 datasheet(30/56 Pages) STMICROELECTRONICS

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TDA7412

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3.0 FUNCTIONAL DESCRIPTION OF STEREODECODER

Figure 23. Block diagram of Stereodecoder

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.

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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