DESCRIPTION OF THE AUDIOPROCESSOR
PART
Input Multiplexer
CD quasi differential
Cassette stereo
Phone differential
AM mono
Stereodecoder input.
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-
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 µP is executing
a refresh cycle of the I
2
C bus programming, it is
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
2
C bus pro-
grammable slope. The mute process can either
be activated by the softmute pin or by the I
2
C
bus. The slope is realized in a special S shaped
curve to mute slow in the critical regions (see fig-
ure 5).
15K 15K
100K
CD+
CD-
15K 15K
+
-
15K 15K
PHONE+
PHONE-
15K 15K
+
-
D98AU854A
100K
100K
100K
STEREODECODER
IN GAIN
CASSETTE
AM
MPX
1
1
100K
CD OUT
Figure 4. Input stages
TDA7400D
12/28
Obsolete Product(s) - Obsolete Product(s)
