REF5V

The gain can be programmed through 4 bits in

the ”Stereodecoder-Adjustment”byte.

The MONO voltage VMO (0dB channel separa-

tion) can be choosen selecting VSBL

All necessary internal reference voltages like

REF5V are derived from a bandgap circuit.

Therefore they have a temperature coefficient

near zero. This is useful if the fieldstrength signal

is also temperature compensated.

But most IF devices apply a LEVEL voltage with a

TC of 3300ppm. The TDA7400D offers this TC

for the reference voltages, too. The TC is select-

byte.

Highcut Control

The highcut control setup is similar to the

stereoblend control setup : the starting point

VHCH can be set with 2 bits to be 42, 50, 58 or

66% of REF5V whereas the range can be set to

be 17, 22, 28 or 33% of VHCH (see fig. 21).

FUNCTIONAL DESCRIPTION OF THE NOISE-

BLANKER

In the automotive environment the MPX signal is

disturbed by spikes produced by the ignition and

for example the wiper motor. The aim of the

noiseblanker part is to cancel the audible influ-

ence of the spikes.

Therefore the output of the stereodecoder is held

at the actual voltage for a time between 22 and

38

µs (programmable).

The block diagram of the noiseblanker is given in

fig.17.

In a first stage the spikes must be detected but to

avoid a wrong triggering on high frequency

(white) noise a complex trigger control is imple-

mented. Behind the triggerstage a pulse former

generates the ”blanking” pulse. To avoid any

crosstalk to the signalpath the noiseblanker is

supplied by his own biasing circuit.

Trigger Path

The incoming MPX signal is highpass filtered,

amplified and rectified. This second order high-

pass-filter has a corner frequency of 140kHz.

The rectified signal, RECT, is lowpass filtered to

generate a signal called PEAK. Also noise with a

frequency 140kHz increases the PEAK voltage.

The resulting voltage can be adjusted by use of

the noise rectifier discharge current.

The PEAK voltage is fed to a threshold generator,

which adds to the PEAK voltage a DC depend-

ent threshold VTH. Both signals, RECT and

INTERNAL

VOLTAGES

t

D97AU639

VSBL

REF 5V

SETUP OF VST

INTERNAL

VOLTAGES

t

33%

REF 5V

SETUP OF VMO

LEVEL

LEVEL INTERN

FIELDSTRENGHT VOLTAGE

VST

VMO

LEVEL INTERN

42%

50%

58%

VSBL

VST

VMO

FIELDSTRENGHT VOLTAGE

Figure 15. Relation Between Internal and External LEVEL Voltage and Setup of Stereoblend

LOWPASS

TIME CONSTANT

D97AU640

FIELDSTRENGHT

VHCH

VHCL

3

•

Figure 16. Highcut Characteristics

TDA7400

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