E-TDA7461ND View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'E-TDA7461ND' PDF : 48 Pages View PDF
Stereo decoder part
TDA7461
4.5
Description of stereo decoder
The stereo decoder part of the TDA7461 (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 20. Block diagram of the stereo decoder
MPX
100K
INGAIN
3.5 ... 11dB
STEP 2.5dB
INFILTER
LP 80KHz
4.th ORDER
DEMODULATOR
- PLOT CANC
- ROLL-OFF COMP.
- LP 25KHz
DEEMPHASIS
+ HIGHCUT
t=50 or 75μs
FM_L
FM_R
PLL +
PILOT-DET.
F19
F38
STEREO
NOISE BLANKER
HOLDN
D97AU762
SB CONTROL
REF 5V
VSBL
HC
CONTROL
D
A
VHCCH
VHCCL
MPINFL
LEVEL INPUT
LEVEL INTERN
LP 2.2KHZ
-
1.th ORDER
LEVEL
GAIN 0..10dB
MULTIPATH
DETECTOR
MPIN
MPOUT
4.5.1
4.5.2
Stereo decoder mute
The TDA7461 has a fast and easy to control RDS mute function which is a combination of
the audioprocessor 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 soft muted.
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. Figure 21 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 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.
Input stages
The In gain stage allows to adjust the MPX signal to a magnitude of about 1Vrms internally
which is the recommended value. The 4th order input filter has a corner frequency of 80 kHz
and is used to attenuate spikes and noise and acts as an anti aliasing filter for the following
switch capacitor filters.
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