TDA7420 Datasheet, PDF (10/29 Pages) STMicroelectronics – MULTIFUNCTIONAUDIO PROCESSOR

TDA7420
DESCRIPTION
DESCRIPTION OF THE NOISEBLANKER
In the normal automotive environment the MPX
signal is disturbed by ignition spikes, motors and
high frequency switches etc.
The aim of the noiseblanker 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 40µs (average spike du-
ration).
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 triggerstage a pulse former generates
the 40µs ”blanking” pulse.
In the following section all of these circuits are de-
scribed in their function and their programming,
too (see fig.1).
1.1 Normal Trigger Path (RECT-PEAK, ACT,
PEAK-COMP, BLANK-COMP, BIAS-MONO)
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 gain of the rectifier can be controlled by the
bit D2 of the noiseblanker byte2.
If programming bit D2 to zero the gain is only half
of the nominal value.
All trigger thresholds must be roughly doubled in
this case. The rectified signal, RECT, is used to
generate by peak-rectification a signal called
PEAK, which is available at the PEAK pin.
Also noise with a frequency >100KHz increases
the PEAK voltage. The value of the PEAK voltage
influences the trigger threshold voltage Vth (block
ATC).
Both signals, RECT and PEAK+Vth are fed to a
comparator (block PEAK-COMP) which outputs a
sawtooth-sharped waveform at the TBLANK pin,
it is triggered.
A second comparator (block BLANK-COMP)
forms the internal blanking duration of 40µs.
The noiseblanker is supplied by his own biasing
circuit (block BIAS-MONO).
1.2 Automatic Threshold Control (ATC)
There are two independent possibilities for pro-
gramming the trigger threshold:
a)the minimum threshold in 8 steps (bits D0-D2,
10/29
NB-byte 1)
b)the maximum threshold in 4 steps (bits D3-
D4, NB-byte 1) (see fig.2)
The low threshold is used in combination with a
good MPX signal without any noise.
The sensitivity in this operation is high, depending
only on the programmed ”Low Trigger Threshold”,
bits D0-D2 of the noiseblanker byte 1.
It is independent of the PEAK voltage.
The MPX signal is noisy (low fieldstrength) the
PEAK signal increases due to the higher noise,
which is also rectified (see part 1.1).
With increasing of the PEAK voltage the trigger
threshold voltage increases, too. This particular
gain is programmable in 4 steps (see fig.2).
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 reason for implementing such a second con-
trol 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.3).
But in some cases the behaviour of the noise-
blanker can be improved by increasing the
threshold even in a region of higher fieldstrength,
for the MPX signal often shows distortion in this
range.
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 in 3
steps or switched off (see fig.3).
1.4 Over Deviation Detector (MPX-RECT)
Sometimes when listening to stations with a
higher deviation than 75KHz the noiseblanker
triggers on the high frequency modulation.
To avoid this blanking, which causes noise in the
output signal, the noiseblanker offers a deviation-
dependent threshold adjustment.
By rectifying the MPX signal a further signal rep-
resenting the actual deviation is obtained.
It is used to increase the PEAK voltage.
Offset and gain of this circuit are programmable in
3 steps (the first step turns off the detector, see
fig.4).