7/12

TDA7338

4.3.2 Noise Controlled Threshold Adjustment (ATC)

The behaviour of the noise controlled threshold adjustment is shown in fig. 8. It can be influenced lightly

by adding a resistor in parallel to the PEAK capacitor at Pin 8 either to GND or VDD. A resistor to GND

will decrease the threshold whereas a resistor to VDD will increase it. But it is recommended to choose

one of the internal thresholds by use of the PROG pin (see table 5)

4.3.3 Automatic Threshold Control by the Stereoblend voltage (ATC-SB)

Besides the noise controlled threshold adjustment there is an additional possibility for influencing the trig-

ger. It is controlled by the difference between Vsb and Vr, similar to the Stereoblend. The reason for im-

plementing such a second control will be explained in the following: 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 (fig.9). But in some cases the behaviour of the noiseblanker can be improved

by increasing the threshold even in a region of higher fieldstrength, for the MPX signal often shows distor-

tion in this range, which leads to an undesired triggering.

Because of the overlap of this range and the range of the stereo/mono transition it can be controlled by

Vsb and Vr. This threshold increase is programmable (see fig. 9).

4.3.4 Blend Mode

Another possibility to avoid a disturbing triggering on modulation is to use the spikes on the fieldstrength

signal (LEVEL pin). But in the range of higher fieldstrength the signal saturates and no more spike detec-

tion is possible. For this reason the TDA7338 offers the "BLEND MODE". When "BLEND MODE" is acti-

vated a smooth transition between the LEVEL- and the MPX-signal is used to detect the spikes either on

LEVEL or on MPX.

In the lower fieldstrength range mainly the LEVEL-signal is used whereas in the higher range mainly the

MPX is used.

This switching is controlled also by the normal Stereoblend signal to avoid additional pins. "BLEND MODE

OFF" is activated by connecting the LEVEL pin to GND (LEVEL must be also connected to GND if not

used).

Figure 7. Block Diagram of the Noise Blanker

80KHz LP

SIGNAL PATH

ATC

AUTOMATIC

THRESHOLD

CONTROL

PEAK+VTH

RECT-PEAK

BUF

PEAK

AMP

RECT

PEAK COMP

REF.

BLANK COMP

ADDITIONAL THRESHOLD

CONTROL

(ATC-SB)

40

µs

LEFT

RIGHT

to OUTPUTS

MPX IN

VR

VSB

CPEAK

47nF

CBLANK

330pF

D95AU368

140KHz HP

120KHz HP

LEVEL

2V

7V

THRESHOLD L/H

ADDITIONAL THRESHOLD

CONTROL on/off

PROG

-

+

BLEND

ON/OFF

0.1V

BLEND

CONTROL