TDA7460ND Stereo decoder part
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5.5 Description of stereo decoder
The stereo decoder part of the TDA7460ND (see Figure 20.) contains all functions
necessary to demodulate the MPX signal like pilot tone dependent mono/stereo switching
as well as "stereo blend" and "high cut" functions.
Adaptations like programmable input gain, roll-off compensation, selectable de-emphasis
time constant and a programmable fieldstrength input allow to use different IF devices.
Figure 21. Block diagram of the stereo decoder
5.5.1 Stereo decoder mute
The TDA7460ND has a fast and easy to control RDS mute function which is a combination
of the audioprocessor soft mute and the high-ohmic mute of the stereo decoder. If the stereo
decoder is selected and a soft mute 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. 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. 18 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 set the stereo
decoder in any case 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.
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 75μs
REF 5V
VSBL
MPINFL
LEVEL INTERN
HC
CONTROL
VHCCH
VHCCL
-
MULTIPATH
DETECTOR
D
A
MPOUT
LEVEL INPUT
LP 2.2KHZ
1.th ORDER
MPIN
LEVEL
FM_L
FM_R
MPX
100K
D97AU762
GAIN 0..10dB
Obsolete Product(s) - Obsolete Product(s)
