TDA7407 datasheet(18/30 Pages) STMICROELECTRONICS

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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

µs (programmable).

The block diagram of the noiseblanker is given in

fig.20.

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

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. 17).

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.

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. 14). In some cases

the behaviour of the noiseblanker can be im-

proved by increasing the threshold even in a re-

RECTIFIER

LOWPASS

RECT

THRESHOLD

GENERATOR

VTH

PEAK

ADDITIONAL

THRESHOLD

CONTROL

MONOFLOP

HOLDN

MPX

D98AU856

MPX

CONTROL

Figure 20. Block Diagram of the Noiseblanker

TDA7407

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