TDA7461N datasheet(20/32 Pages) STMICROELECTRONICS

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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.

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).

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.

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.

RECTIFIER

BANDPASS

19KHz

to SB

D97AU880

MPIN

GAIN

2 BITS

GAIN

2 BITS

DC=1

µA

VDD

MPOUT

LEVEL

int. INFLUENCE

220nF

Figure 23. Block diagram of the Multipath Detector

TDA7461N

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