TDA7400 datasheet(18/28 Pages) STMICROELECTRONICS

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PEAK+VTH are fed to a comparator which trig-

gers a re-triggerable monoflop. The monoflop’s

output activates the sample-and-hold circuits in

the signalpath for selected duration.

Automatic Noise Controlled Threshold Adjust-

ment (ATC)

There are mainly two independent possibilities for

programming the trigger threshold:

a the low threshold in 8 steps (bits D0 to D2 of

the noiseblanker byte)

b the noise adjusted threshold in 4 steps

(bits D3 and D4 of the noiseblanker byte,

see fig. 14).

The low threshold is active in combination with a

good MPX signal without any noise; the PEAK

voltage is less than 1V. The sensitivity in this op-

eration is high.

If the MPX signal is noisy the PEAK voltage in-

creases due to the higher noise, which is also

rectified. With increasing of the PEAK voltage the

trigger threshold increases, too.

This particular

gain is programmable in 4 steps (see fig. ...).

AUTOMATIC THRESHOLD CONTROL MECHA-

NISM

Automatic

Threshold

Control

the

Stereoblend Voltage

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. 11). 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 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. 23). the

RECTIFIER

LOWPASS

RECT

THRESHOLD

GENERATOR

VTH

PEAK

ADDITIONAL

THRESHOLD

CONTROL

MONOFLOP

HOLDN

MPX

D98AU856

MPX

CONTROL

Figure 17. Block Diagram of the Noiseblanker

TDA7400

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