TDA7513T Datasheet, PDF (26/59 Pages) STMicroelectronics – SINGLE-CHIP FM/AM TUNER WITH STEREO DECODER AND AUDIO PROCESSOR

TDA7513T
5.7 AUTOMATIC THRESHOLD CONTROL MECHANISM
5.7.1 Automatic Threshold Control by the Stereoblend Voltage
Besides the noise-controlled threshold adjustment there is an additional possibility to influence the trigger
threshold which depends 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 trigger adjustment is fixed. In some cases the behavior of the noise-
blanker can be improved by increasing the threshold even in a region 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 tran-
sition it can be controlled by stereoblend.
This threshold increase is programmable in 3 steps or switched off.
5.7.2 Over Deviation Detector
If the system is tuned to stations with a high de-viation the noiseblanker might be erroneously triggered
on the higher frequencies of the modulation. To avoid this unnecessary muting of the signal, the noise-
blanker offers a deviation-dependent threshold adjustment.
By rectifying the MPX signal a further signal representing the actual deviation is obtained. This is used to
increase the PEAK voltage. The circuit offset, gain (and enabling) are programmable in 3 steps.
5.8 MULTIPATH DETECTOR
Using the stereo decoder 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 pin #MULTIPATH-
TC is externally connected to a capacitor of about 47nF and the MPIN signal is internally connected to the
unfiltered field strength. To avoid losing the information stored in the external capacitor during AF checks
but at the same time to allow some fast multipath detection capability during the same AF check period,
the external capacitor is disconnected by the MP-Hold switch. This switch is controlled directly by the pin
#RDSMUTE.
Moreover, selecting the "internal influence" in the configuration byte, the channel separation is automati-
cally reduced during a multipath condition according to the voltage appearing at the pin #MULTIPATHTC.
5.8.1 Programming
To obtain a good multipath performance an adaptation 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 properly choosing the value of the external capacitor.
5.9 QUALITY DETECTOR
The device offers a quality detector output voltage representing the quality of the FM reception conditions.
This voltage is derived from MPX noise information and multipath information according to the following
formula:
Quality = 1.6 (V_noise -0.8V)+ a (REF5V- V_MPOUT)
The noise signal is the PEAK signal of the noise blanker without additional influences. The multipath in-
formation weight "a" can be programmed between 0.7 and 1.15. The circuit output pin #QUALITY is a low
impedance output able to drive external circuitry as well as suitable to be simply fed to an A/D converter
for RDS applications.
5.9.1 AF Search Control
The device is supplied with several functionality to support AF-checks using the stereodecoder. As already
mentioned before the high ohmic mute feature at the stereo decoder input avoids any clicks during the
jump condition.
It is possible at the same time to evaluate the noise- and multipath-content of the alternate frequency by
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