frequency is reduced down to 1kHz to improve

the signal to noise ratio. Furthermore the condi-

tions of the stereo separation (see fig.2) can be

controlled through the signal applied to Pin 16

(VSB). Both signal levels (VSB and VHCC) are

referred to Pin 17 (VR), with the characteristic

By modifying the feedback resistor value of the

output stages (Pin 9 - 10, Pin 11 - 12) the total

gain of the stereodecoder can be modified.

Pin 7 and Pin 8 have an additional function. By

pulling them to ground the VCO-OFF (Pin 7) and

the MUTE (Pin 8) function are activated. The

MUTE signal disconnects the MPX-signal from

the circuit, while in combination with VCO-OFF

also the output buffers are disconnected from the

circuit. In this mode the output buffers can be

used for AM-stereo, cassette play back and other

purposes.

AM Mono Mode

By selecting VCO-OFF (Pin 7 to GND) the VCO

is switched off and the SB and HCC are disabled.

The deemphasis time constant is changed to

40

DESCRIPTION OF THE NOISE BLANKER

In the normal automotive environment the MPX

signal is disturbed by ignition spikes, motors and

high frequency switches etc.

The aim of the noise blanker part is to cancel the

influence of the spikes produced by these compo-

nents. Therefore the output of the stereodecoder

is switched off for a time of 40

µs (average spike

duration).

In a first stage the spikes must be detected but to

avoid a wrong triggering on high frequency noise

a complex trigger control is implemented.

Behind the trigger stage a pulse former generates

the 40

µs ”blanking” pulse.

This duration of 40

µs can be varied by changing

the capacitor at pin 7.

1.1 Trigger Path

The incoming MPX signal is highpass-filtered,

amplified and rectified (block RECT-PEAK).

The second order highpass-filter has a corner-fre-

quency of 140KHz.

The rectifier signal, RECT, is used to generate by

peak-rectification a signal called PEAK, which is

available at the PEAK pin 8.

Also noise with a frequency >100KHz increases

the PEAK voltage. The value of the PEAK voltage

influences the trigger threshold voltage Vth (block

ATC). The higher the noise level the higher the

threshold.

Both signals, RECT and PEAK+Vth are fed to a

comparator (block PEAK-COMP) which outputs a

sawtooth-shaped waveform at the TBLANK pin 7.

A second comparator (block BLANK-COMP)

forms the internal blanking duration of 40

µs.

The noise blanker is supplied by his own biasing

circuit (block BIAS-MONO) to avoid any cross talk

to the signal path (block BIAS-MONO).

1.2 Noise Controlled Threshold Adjustment

(ATC)

The behaviour of the noise controlled threshold

adjustment is shown in fig. 4. It can be influenced

slightly by adding a resistor in parallel to the

resistor to GND will decrease the threshold

recommended to choose one of the internal

thresholds by use of the PROG pin (see table 1)

1.3 Automatic Threshold Control by the

Stereoblend voltage (ATC-SB)

Besides the noise controlled threshold adjustment

there is an additional possibility for influencing the

trigger. It is controlled by the difference between

Vsb and Vr, similar to the Stereoblend. The rea-

son for implementing 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 trig-

ger adjustment is fixed (fig.5). But in some cases

the behaviour of the noiseblanker can be im-

proved by increasing the threshold even in a re-

gion of higher fieldstrength, for the MPX signal

often shows distortion 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 con-

trolled by Vsb and Vr. This threshold increase is

programmable(see fig. 5).

1.4 Blend Mode

Another possibility to avoid a disturbing triggering

on modulation is to use the spikes on the field-

strength signal (LEVEL pin). But in the range of

higher fieldstrength the signal saturates and no

more spike detection is possible. For this reason

the TDA7338 offers the ”BLEND MODE”. When

”BLEND MODE” is activated 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 con-

nected to GND if not used).

TDA7338

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