TDA7407 Datasheet, PDF (19/30 Pages) STMicroelectronics – ADVANCED CAR SIGNAL PROCESSOR

Figure 21. Block Diagram of the Multipath Detector
TDA7407
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. 18).
FUNCTIONAL DESCRIPTION OF THE MULTI-
PATH DETECTOR
Using the internal multipath 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.
An external capacitor is used to define the attack
and decay times (see block diagram fig. 21). the
MPOUT pin is used as detector output connected
to a capacitor of about 47nF and additionally the
MPIN pin is selected to be the fieldstrength input.
Using the configuration an external adaptation to
the user’s requirement is given in fig.21.
To keep the old value of the Multipath Detector
during an AF-jump, the external capacitor can be
disconnected by the MP-Hold switch. This switch
can be controlled directly by the AFS-Pin.
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 MP_OUT pin. A
possible application is shown in fig. 21.
Programming
To obtain a good multipath performance an adap-
tation is necessary. Therefore tha gain of the
19kHz bandpass is programmable in four steps
as well as the rectifier gain. The attack and decay
times can be set by the external capacitor value.
QUALITY DETECTOR
The TDA7407 offers a quality detector output
which gives a voltage representing the FM recep-
tion conditions. To calculate this voltage the MPX
noise and the multipath detector output are
summed according to the following formula:
Quality = 1.6 (Vnoise -0.8V)+ a (REF5V- VMPOUT)
The noise signal is the PEAK signal without addi-
tional influences. The factor "a" can be pro-
grammed from 0.7 to 1.15. the output is a low im-
pedance output able to drive external circuitry as
well as simply fed to an A/D converter for RDS
applications.
19/30