TDA7400D Datasheet, PDF (15/28 Pages) STMicroelectronics – ADVANCED CAR SIGNAL PROCESSOR

TDA7400D
FUNCTIONAL DESCRIPTION OF STEREODE-
CODER
The stereodecoder part of the TDA7400D (see
Fig. 12) contains all functions necessary to de-
modulate the MPX signal like pilot tone depend-
ent MONO/STEREO switching as well as
"stereoblend" and "highcut" functions.
Stereodecoder Mute
The TDA7400D 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 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
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 stereodecoder is unmuted immedi-
ately and the audioprocessor is softly unmuted.
Fig. 13 shows the output signal VO as well as the
internal stereodecoder mute signal. This influ-
ence of Softmute on the stereodecoder mute can
Figure 12. Block Diagram of the Stereodecoder
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 se-
lected the decoder remains muted and the MPX
pin is connected to Vref to avoid any discharge of
the coupling capacitor through leakage currents.
Ingain + Infilter
The Ingain 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 80KHz and is used to
attenuate spikes and nose and acts as an anti al-
lasing filter for the following switch capacitor fil-
ters.
Demodulator
In the demodulator block the left and the right
channel are separated from the MPX signal. In
this stage also the 19 kHz pilot tone is cancelled.
For reaching a high channel separation the
TDA7400D offers an I2C bus programmable roll-
MPX
100K
INGAIN
3.5 ... 11dB
STEP 2.5dB
INFILTER
LP 80KHz
4.th ORDER
PLL +
PILOT-DET.
F19
F38
STEREO
NOISE BLANKER
HOLDN
NOISE
D98AU855
DEMODULATOR
- PLOT CANC
- ROLL-OFF COMP.
- LP 25KHz
DEEMPHASIS
+ HIGHCUT
t=50 or 75µs
FM_L
FM_R
SB CONTROL
REF 5V
VSBL
HC
CONTROL
D
A
VHCCH
VHCCL
MPINFL
LEVEL INTERN
-
MULTIPATH
DETECTOR
MP_OUT
LEVEL INPUT
LP 2.2KHZ
1.th ORDER
GAIN 0..10dB
QUALITY DETECTOR
MPLEVELOUT
MP_IN
+
LEVEL
QUAL
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