Television receiver for microwave reception with provisions for reducing interference

Interference induced amplitude changes in the television signals received by a receiver which receives satellite signals are eliminated by delaying the signals for an integer multiple of the receiver line time and substituting the delayed signals for the undelayed signals for the duration of the amplitude changes.

BACKGROUND OF THE INVENTION 
In most television receivers no appreciable degradation of the displayed 
picture is visible under normal reception and viewing conditions. However, 
degradation of the displayed picture may arise due to impulsive 
interference, for instance from radar signals. 
Television receivers designed for reception of microwave signals, for 
instance from satellite transmissions, are particularly vulnerable to this 
type of interference, which may manifest itself as very small extreme 
white and dark areas superimposed on the picture information. 
It is an object of the invention to minimize the visibility of such 
interference by simple means within the receiver. 
SUMMARY OF THE INVENTION 
According to the invention a level detector is provided which produces a 
switching pulse simultaneously with the interference in the received 
signal, and a switch actuated by said pulse, switches the signal path 
during said interference to a source of a substitute signal, reducing the 
visibility of the interference within the picture. 
Said substitute signal may be a constant dc voltage of a mid-grey value of 
the luminance signal. It was found that a small interference area on the 
screen during interference is less visible if it is produced in grey and 
not in white or black. The substitute signal may also be the mean value of 
the luminance signal over a longer period of one or more lines. 
In a preferred embodiment of the invention the substitute signal is the 
signal of a previous line made available by a delay device with a delay of 
one line period. In this case one input of said switch is connected to the 
output of the delay device the input of which being connected to the 
signal path. Preferably the input of said delay device is connected to the 
output of said switch so that the signal re-circulates and can be repeated 
for more than one line if necessary. 
Preferably said level detector is fed with a baseband video signal and 
produces said pulse upon interference pulses extending beyond peak white 
and also upon interference pulses which extend beyond black level. In 
order to handle the composite video signal and also in order to maintain 
the color sub-carrier of the previous line preferably means are provided 
ensuring that an undelayed color carrier and a color carrier coming from 
said delay device fed to two inputs of said switch have the same phase. In 
one form of the invention adapted for signals the delay device has a 
delay of two line periods. According to an alternative solution a color 
carrier modifier is provided within the undelayed or within the delayed 
signal path connected to the input of said switch said modificator 
eliminating the phase alteration from line to line. Such a color 
carrier modificator is more fully described within DE-OS 1.814.879. 
For handling a SECAM signal preferably a delay device is provided having a 
delay of two line periods. This is necessary because the SECAM signal is a 
sequential signal carrying the two color difference signals on alternate 
lines. 
For a MAC signal a window signal generator is provided so that for an 
interference pulse during the luminance signal said switch is connected to 
the output of a delay device for one line whereas for an interference 
pulse during a color difference signal said switch is connected to the 
output of a delay device for two lines. For MAC and SECAM signals a switch 
is provided having three inputs the first input being supplied with a 
direct and substantially undelayed signal, the second input being supplied 
with a signal delayed by one line and the third input being supplied with 
a signal delayed by two lines. In another form of the invention a second 
delay device of a period just longer than the operation period of the 
level detector and switch is provided within the undelayed signal path of 
said switch, the input of said level detector being connected to the input 
of said second delay device. Thereby the interference pulse at the control 
input of the switch can be present a short time before interference within 
the signal fed to one input of said switch begins. In another form of the 
invention a memory is provided wherein the time of occurrence of the 
interference pulse is stored said stored pulse being used for signal 
processing of the signal stored within a frame memory.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1 a composite video signal CVBS (color video blanking sync) is fed 
from terminal 1 via delay device 2 having a delay of about 25-45 ns to 
first input of a switch S. Output of switch S is connected to the input of 
a delay device 3 for one line period the output of which is fed via delay 
device 4 to input b of switch S. Composite video signal from terminal 1 is 
further fed to amplitude level detector 5 producing an interference pulse 
P. Pulse P is fed to control input of switch S. 
The operation is as follows: During normal reception with on interference 
signal CVBS is fed from terminal 1 via delay device 2 and switch S to 
output terminal 6 without substantial modification except the small delay 
due to delay device 2. In this mode switch S remains in position a. If an 
interference pulse is detected by detector 5 modification pulse P is 
generated and actuates switch S to position b. Now instead of the degraded 
signal at the output of device 2 the corresponding signal of the previous 
line is inserted by using the output of delay device 3. It is unlikely 
that the signal of the previous line also has a spuriosity at the 
corresponding time of the line period. On the other hand there is usually 
a substantial correlation between the wanted information at corresponding 
points on successive lines. 
Therefore, the signal of the previous line can be used as a substitute 
signal and filling up the time where the original signal is degraded. So 
at output 6 a signal is available wherein the interference is 
substantially eliminated. 
Delay device 3 is a CCD (Charge Coupled Device) delay line and controlled 
by clock pulses C with double color sub-carrier frequency 2*fsc. Clock 
pulses C are locked to the burst and delay device 3 has half a cycle less 
delay than the exact line period. Device 3 is controlled in such a way 
that the color sub-carriers at input a and b of switch S have the same 
phase in spite of offset and phase alternation. It is also 
possible to eliminate phase alternation by a so-called color carrier 
modifier. Such a circuit is also controlled by a clock signal C with a 
frequency of 2*fsc and "mirrors" the color carrier in every second line to 
the fixed U-axis. This means that the color phase alternation is 
eliminated so that the color sub-carrier has no phase alternation and is 
similar to that in the NTSC signal. 
In FIG. 2 the composite video signal 7 has two interference pulses N1 and 
N2, N1 being a transient to beyond peak white level W and interference 
pulse N2 going to beyond black level B. Both spuriosities N1 and N2 
produce video interference pulses P by amplitude detection. 
In FIG. 3 the insertion pulse produced by detector 5 begins at t1 a short 
time before the beginning t2 of interference pulse N. The time interval 
between t1 and t2 is usually a few nano-seconds. This is effected by delay 
device 2 causing the signal to reach input a after pulse P is produced. 
Pulse P ends at t4 after the end of interference N at t3. So insertion 
pulse P is lengthened or stretched with respect to interference N in order 
to extend over the full time of interference N even with tolerances. 
FIG. 4 shows a circuit adapted to process MAC signals and SECAM signals by 
the introduction of small modifications. Additionally to delay devices 3 
and 4 according to FIG. 1 connected to input b of a further path is 
provided including a further delay device 8 of one line period and an 
incremental delay device 9 connected to a third input c of switch S. First 
the operation for MAC mode is described. With MAC signals two features 
have to be taken into account. First during each horizontal line there are 
transmitted sequentially a color difference signal and a luminance signal 
both compressed in time. Therefore, it is necessary to take into account 
whether the interference and the pulse P occur during said color 
difference signal or during said luminance signal. Secondly the two MAC 
color difference signals are transmitted on successive lies. Therefore, 
during an interference within color difference signal the color difference 
signal of the previous line cannot be used as a substitute signal because 
it carries the wrong color difference information. Therefore, it is 
necessary to use within line n the color difference signal from line 
(n-2). 
In FIG. 4 two window signals W1, W2 are generated within window generator 
10. The first window signal W1 indicates the time of the color difference 
signal whereas the second window signal W2 defines the time of the 
luminance signal. The window signals W1, W2 are combined with pulse P in 
stage 11 the output of which is controlling switch S in the following way: 
If the pulse P occurs during luminance signal Y the window signal W2 acts 
upon the control of switch S in such a way that the switch S is connected 
to input b so that the signal of the previous line is inserted as in FIG. 
1. If pulse P arises during color difference signal R-Y window signal W1 
acts upon control of switch S in such a way that switch S is connected to 
input c. Now during line n the signal of line n-2 delayed by two lines by 
delay devices 3, 8 is inserted as the substitute signal. If for example 
the interference is within signal R-Y in line n during pulse P the 
corresponding color difference signal R-Y of line n-2 is inserted. The 
bandpass filter 12 and the notch filter 13 shown are not used and not 
provided for MAC mode. 
If circuit according to FIG. 4 is adapted for SECAM window generator 10 and 
stage 11 are not present whereas bandpass filter 12 and notch filter 13 
are introduced. The operation is similar to that of MAC. If the substitute 
signal is to be inserted only within luminance signal Y upon occurrence of 
pulse P switch S is connected to input b as described with FIG. 1. If, 
however, a substitute signal is to be introduced within the color 
difference signal than switch S is connected to input c which is a signal 
delayed by two lines. Bandpass filter 12 is designed to transmit only FM 
SECAM color carrier and its sidebands and suppressing the luminance 
components outside said frequency band of the color carrier. Notch filter 
13 is adapted to suppress color carrier frequency and corresponding 
sidebands.