TDA7461ND datasheet(19/31 Pages) STMICROELECTRONICS

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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 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

µs.

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 gen-

erate a signal called PEAK. Also noise with a fre-

quency 140kHz increases the PEAK voltage. 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

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 40

µs.

The block diagram of the noiseblanker is given in

fig.22.

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. 14).

LOWPASS

TIME CONSTANT

D97AU640

Deemp

FIELDSTRENGHT

VHCH

VHCL

•

Deemp

Figure 21. Highcut characteristics

RECTIFIER

LOWPASS

RECT

THRESHOLD

GENERATOR

VTH

PEAK

ADDITIONAL

THRESHOLD

CONTROL

MONOFLOP

HOLDN

MPX

D98AU861

HIGH PASS

Figure 22. Block diagram of the noiseblanker

TDA7461N

19/31

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