TDA7409 Datasheet, PDF (8/25 Pages) STMicroelectronics – CARRADIO-SIGNAL-PROCESSOR

TDA7409
1.2 AutoZero
In order to reduce the number of pins there is no AC coupling between the In-Gain and the following stage, so
that any offset generated by or before 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 Mixing-block to eliminate all offsets generated by the Input-Stages and the In-Gain (Please notice
that externally generated offsets, e.g. generated through the leakage current of the coupling capacitors, are not
canceled).
The auto-zeroing is started every time the DATA-BYTE 0 (Input Selector/Gain) is selected and takes a time of
max. 0.3ms. To avoid audible clicks the Audioprocessor is muted before the loudness stage during this time.
1.2.1 AutoZero-Remain
In some cases, for example if the uP is executing a refresh cycle of the IIC-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 applications the TDA7409 could be switched in the AutoZero-Remain-Mode (I2 bit 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.
1.3 Mixing Stage
The 4 step Mixing stage offers the possibility to mix the rear selector signal or the phone signal to any other
source. Due to the fact that the mixing-stage is located behind the In-Gain-stage fine adjustments of the main
source level could be done in this way.
Figure 2. Signal-Flow of Mixing-Stage
SE1
SE2
SE3
SE4
BEEP
INPUT SELECTOR
INGAIN
+
-
25K
MIXING
MIXING LEVEL
INPUT / BEEP
0dB / -100dB
-3.5dB / -9.6dB
-6dB / -6dB
-12dB / -2.5dB
D00AU1181
LOUDNESS
1.4 Loudness
There are four parameters programmable in the loudness stage:
1.4.1 Attenuation
Figure 3 shows the attenuation as a function of frequency at f_C = 400Hz.
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