TDA7400 ST Microelectronics, TDA7400 Datasheet - Page 12

• Manufacturer Part Number: TDA7400
• Description: ADVANCED CAR SIGNAL PROCESSOR
• Manufacturer: ST Microelectronics

TDA7400
Input Multiplexer
Input stages
Most of the input stages have remained the same
as in preceeding ST audioprocessors with excep-
tion of the CD inputs (see figure 4).
In the meantime there are some CD players in
the market having a significant high source im-
pedance which affects strongly the common-
mode rejection of the normal differential input
stage. The additional buffer of the CD input
avoids this drawback and offers the full common-
mode rejection even with those CD players.
The output of the Cd stage is permanently avail-
able of the Cd out-pins
AutoZero
In order to reduce the number of pins there is no
AC coupling between the In-Gain and the follow-
ing stage, so that any offset generated by or be-
fore the In-Gain stage would be transferred or
even amplified to the output.
To avoid that effect a special offset cancellation
stage called AutoZero is implemented.
This stage is located before the volume-block to
eliminate all offsets generated by the Stereode-
coder, the Input Stage and the In-Gain (Please
notice that externally generated offsets, e.g. gen-
Figure 4. Input stages
12/28
CD quasi differential
Cassette stereo
Phone differential
AM mono
Stereodecoder input.
CASSETTE
PHONE+
PHONE-
MPX
AM
CD+
CD-
100K
100K
100K
100K
100K
1
1
STEREODECODER
15K
15K
15K
15K
+
+
-
15K
-
15K
15K
15K
erated through the leakage current of the cou-
pling capacitors, are not cancelled).
The auto-zeroing is started every time the DATA-
BYTE 0 is selected and takes a time of max.
0.3ms. To avoid audible clicks the audioproces-
sor is muted before the volume stage during this
time.
AutoZero Remain
In some cases, for example if the P is executing
a refresh cycle of the I
not useful to start a new AutoZero action because
no new source is selected and an undesired mute
would appear at the outputs. For such applica-
tions the TDA7400D could be switched in the
”Auto Zero Remain mode” (Bit 6 of the subad-
dress byte). If this bit is set to high, the DAT-
ABYTE 0 could be loaded without invoking the
AutoZero and the old adjustment value remains.
Multiplexer Output
The output signal of the Input Multiplexer is avail-
able at separate pins (please see the Blockdia-
gram). This signal represents the input signal am-
plifier by the In Gain stage and is also going into
the Mixer stage.
Softmute
The digitally controlled softmute stage allows
muting/demuting the signal with a I
grammable slope. The mute process can either
be activated by the softmute pin or by the I
bus. The slope is realized in a special S shaped
curve to mute slow in the critical regions (see fig-
ure 5).
For timing purposes the Bit 3 of the I
put register is set to 1 from the start of muting un-
D98AU854A
CD OUT
IN GAIN
2
C bus programming, it is
2
2
C bus pro-
C bus out-
2
C