TDA7402_07 Datasheet, PDF (47/71 Pages) STMicroelectronics – Car radio signal processor

TDA7402
Functional description of the multipath detector
8
Functional description of the multipath detector
Using the multipath detector the audible effects of a multipath condition can be minimized. A
multipath condition is detected by rectifying the spectrum around 19kHz in the fieldstrength
signal. An external capacitor is used to define the attack and decay times for the stereo
blend (see block diagram, Figure 34). Due to the very small charge currents this capacitor
should be a low leakage current type (e.g ceramic). Using this configuration an adaptation to
the user's requirement is possible without effecting the "normal" fieldstrength input (level) for
the stereo decoder. This application is given in Figure 34. Another (internal) time constant is
used to control the Highcut through the multipath detector
Selecting the "internal influence" in the configuration byte the stereo-blend and/or the
highcut is automatically invoked during a multipath condition according to the voltage
appearing at the MP_OUT pin.
Figure 35. Block diagram of the multipath detector
High-Cut-Infl.
Attach<500μs
Decay=2/10ms
VDD
=
LOWPASS
100Hz
to HCC
INT.
INFLUENCE
LEVEL
LOWPASS
53KHz
LOWPASS
2.2KHz
SHIFT
0 ... 1.5V
VDD VDD
=
CHARGE
0.25μA/0.5μA
=
300μA
FAST
CHARGE
to SB
INT.
INFLUENCE
Stereoblend-Infl.
Attach=1ms
Decay=4/8s
MP_IN
BANDPASS
19KHz
GAIN 2 bits
MP HOLD
RECTIFIER
GAIN 2 bits
D00AU1166
MP_OUT
470nF
8.1
Quality detector
The A619 offers a quality detector output which gives a voltage representing the FM
reception conditions. To calculate this voltage the MPX-noise and the multipath-detector
output are summed according to the following formula :
V_Qual = 1.6 (V_Noise-0.8 V)+ a (REF5V-V_Mpout).
The noise-signal is the PEAK signal without additional influences (see the noiseblanker
description). The factor 'a' can by programmed to 0.7 .... 1.15. The output is a low
impedance output able to drive external circuitry as well as simply fed to an AD converter for
RDS applications.
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