TDA7402_07 Datasheet, PDF (40/71 Pages) STMicroelectronics – Car radio signal processor

Functional description of stereo decoder
6
Functional description of stereo decoder
TDA7402
Figure 28. Block diagram of stereo decoder
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 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 V_O 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 V_ref to avoid any discharge of the coupling capacitor through
leakage currents. No further mute command should be applied.
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