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. 15). 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.
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
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).
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.
=
+V
CC
9V
100nF
CASS R
CASS L
100nF
100nF
CASS R
CASS L
OUTRR
OUTRR
OUTLR
OUTLR
OUTRF
OUTRF
OUTLF
OUTLF
CREF
10µF
D98AUxx5
LEVEL
SMUTE
SCL
SDA
LEVEL
SMUTE
SCL
SDA
GND
V
S
MPX
220nF
MPX
AM
220nF
AM
Figure 19. Application Example.
TDA7403
16/25
Obsolete Product(s) - Obsolete Product(s)
