TDA7461N Datasheet, PDF (20/31 Pages) STMicroelectronics – CAR RADIO SIGNAL PROCESSOR

TDA7461N
Automatic Threshold Control
Besides the noise controlled threshold adjust-
ment there is an additional possibility for influenc-
ing the trigger threshold. It is depending on the
stereoblend control.
The point where the MPX signal starts to become
noisy is fixed by the RF part. Therefore also the
starting point of the normal noise-controlled trig-
ger adjustment is fixed (fig. 16). In some cases
the behaviour of the noiseblanker can be im-
proved by increasing the threshold even in a re-
gion of higher fieldstrength. Sometimes a wrong
triggering occures for the MPX signal often shows
distortion in this range which can be avoided
even if using a low threshold.
Because of the overlap of this range and the
range of the stereo/mono transition it can be con-
trolled by stereoblend. This threshold increase is
programmable in 3 steps or switched off with bits
D0 and D1 of the fieldstrength control byte.
used to increase the PEAK voltage. Offset and
gain of this circuit are programmable in 3 steps
with the bits D6 and D7 of the stereodecoder byte
(the first step turns off the detector, see fig. 15).
FUNCTIONAL DESCRIPTION OF THE MULTI-
PATH DETECTOR
Using the internal detector the audible effects of a
multipath condition can be minimized. A multipath
condition is detected by rectifying the 19kHz
spectrum in the fieldstrength signal.
Selecting the ”internal influence” in the configura-
tion byte, the channel separation is automatically
reduced during a multipath condition according to
the voltage appearing at the MPOUT pin.
To obtain a optimal performance an adaptation is
necessary. Therefore the gain of the 19kHz band-
pass is programmable in four steps as well as the
rectifier gain. The attack and decay times can be
set by the external capacitor value.
Over Deviation Detector
If the system is tuned to stations with a high de-
viation the noiseblanker can trigger on the higher
frequencies of the modulation. To avoid this
wrong behaviour, which causes noise in the out-
put signal, the noiseblanker offers a deviation de-
pendent threshold adjustment.
By rectifying the MPX signal a further signal rep-
resenting the actual deviation is obtained. It is
TEST MODE
During the test mode which can be activated by
setting bit D0 of the testing byte and bit D5 of the
subaddress byte to ”1” several internal signals
are available at the CASSR pin. During this
mode the input resistance of 100kOhm is discon-
nected from the pin. The internal signals available
are shown in the software specification.
Figure 23. Block diagram of the Multipath Detector
LEVEL
MPIN
VDD
DC=1µA
BANDPASS
19KHz
RECTIFIER
GAIN
2 BITS
GAIN
2 BITS
to SB
-
int. INFLUENCE
MPOUT
220nF
D97AU880
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