TDA7400 Datasheet, PDF (15/28 Pages) STMicroelectronics – ADVANCED CAR SIGNAL PROCESSOR

TDA7400
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 V_O as well as the
internal stereodecoder mute signal. This influ-
ence 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 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
Figure 12. Block Diagram of the Stereodecoder
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-
off adjustment which is able to compensate the
lowpass behaviour of the tuner section. If the
tuner attenuation at 38kHz is in a range from
13.8% to 24.6% the TDA7400D needs no exter-
nal network in front of the MPX pin. Within this
range an adjustment to obtain at least 40dB
channel separation is possible.
The bits for this adjustment are located together
with the fieldstrength adjustment in one byte. This
gives the possibility to perform an optimization
step during the production of the carradio where
the channel separation and the fieldstrength con-
trol are trimmed.
The setup of the Stereoblend characteristics
which is programmable in a wide range is de-
scribed in 2.8.
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
MPLEVELOUT
MP_OUT
MP_IN
LEVEL INPUT
LP 2.2KHZ
1.th ORDER
GAIN 0..10dB
QUALITY DETECTOR
+
LEVEL
QUAL
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