Input stages
The input circuits are the same as in preceeding
ST audioprocessors with exception of the CD in-
puts (see figure 1).
The typical input impedance is 100kΩ.
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.
To avoid audible clicks the audioprocessor is
muted before the loudness stage during this time.
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 TDA7403 could be switched in the "Auto
Zero Remain" mode (Bit 6 of the subaddress
byte). If this bit is set to high, the DATABYTE 0
could be loaded without invoking the AutoZero
and the old adjustment value remains.
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 2).
For timing purposes the Bit 3 of the I
2
C bus out-
put register is set to 1 from the start of muting un-
til the end of demuting.
D98AU951
100K
100K
100K
STEREODECODER
IN GAIN
CASSETTE
AM
MPX
Figure 1. Input stages
DESCRIPTION OF THE AUDIOPROCESSOR
1
EXT.
MUTE
+SIGNAL
REF
-SIGNAL
1
I
2
C BUS
OUT
Time
D97AU634
Figure 2. Softmute Timing
2dB
1dB
-1dB
-2dB
Time
D97AU635
VOUT
10ms
Figure 3. Soft Step Timing
Note: Please notice that a started Mute action is always terminated
and could not be interrupted by a change of the mute signal.
Note: For steps more than 1dB the softstep mode should be
deactivated because it could generate a 1dB error during the
blend-time
TDA7403
6/25
Obsolete Product(s) - Obsolete Product(s)
