FR2559633A1 - Method and device for inverting a television video signal. - Google Patents

Abstract

The invention relates to a method and device for inverting a television video signal. The method of inverting the signal consists in preserving the signal, during the line-blanking period, with the original modulation and in inverting the useful line signal with respect to an axis passing mid-way between the white level and black level of the video signal. The device consists in setting a first electrode of a capacitor C1 at the white level potential, amplified + g by a differential amplifier 1, by way of a black-alignment circuit 2 and of a sample-and-hold circuit 3; the other electrode is brought to the black level potential amplified - g by the differential amplifier 1. The output is taken, by a multiplexer P2, P3, 4 at the input A of the sample-and-hold circuit or on the first electrode of the capacitor C1. The invention applies in particular to the scrambling of television broadcasts.

Description

 The present invention relates to a method and a device for inverting a television video signal.

 Such a device is particularly advantageous in the transmission of scrambled television signal where, for example, the picture can be provided with randomly reversed lines.

 In some countries, video signals are transmitted in negative modulation, but in this case the synchronization pulses are also transmitted in negative modulation.

 The present invention relates to a device for inverting a video signal in which the synchronization pulses are kept in original modulation and in which only the useful line signals are inverted.

 The object of the invention is therefore to make the inverted signal easily reversible, that is to say that what is "coded" must be able to be "decoded".

 According to a characteristic of the invention, the video signal is inverted so that, during the period of suppression of vines, the video signal is multiplied by a positive constant and that, during the duration of the useful line signal, one multiply said video signal by the same negative constant, then add the output of the signal value at the white level and the signal value on the black level, which is also multiplied by the positive constant.

Other characteristics and advantages of the present invention will appear during the following description, said description n made in relation to the attached drawings in which the figure represents a device for implementing the method according to
the invention; FIG. 2 represents a line signal and the notations used ;;
and - Figure 3 shows the video signal of Figure 2 inverted along 1. ' inve tion
In FIG. 19 the video signal to be inverted is injected on the input of a differential amplifier 1 whose two outputs deliver, on the one hand, the video signal multiplied by a positive gain + g, on the other hand, said video signal multiplied by a negative gain -g. The positive output of the differential amplifier sends a signal on a black dialing circuit 2 intended to fix a constant value of the suppression level of each line signal; the output of this circuit is then transmitted to a sampler-blocker 3, controlled by a logic signal (white control) which is at the high level during an instant of the frame where the video signal is at the reference white, for example the level of white usually sent to line 17 or line 330, and to the low level elsewhere. Said blocker sampler will therefore charge (or discharge) the capacitance CS at the signal voltage at the white level, at each frame of the video signal. This white level voltage will be applied, via a controlled Pi gate by a logic signal (black command), on one of the two electrodes B with a capacity Ci. This charging or discharging of said capacity Cl takes place only when the door Pi is closed, that is to say when the signal logic (black control) is high, either during the suppression levels, before or after the line synchronization pulse. The other electrode of capacitance Cl is connected to the negative output of differential amplifier 1, and therefore is charged or discharged during the same closing time of the door Pi, at the black level voltage; the capacity therefore has a potential difference equal to the sum of the black level and the white level multiplied by a multiplying constant g.

 It should be noted that the capacities C1 and CS must keep their charge for the duration of at least one frame; the output of the assembly must therefore flow on a system having a sufficiently high input impedance.

 The output of the system is done via two doors P2 and P3 controlled by a logic signal (Tl / T2 wave). Thus, the door P2 is closed when the wave T1 / T2 is at the high level, that is to say during the duration of line suppression; the output video signal, taken from the input of the sampler-blocker 3, is then equal to the input video signal multiplied by the positive gain + g. The door P3 is closed when the Tl / T2 wave is at the level low, thanks to the logic inverter 4, therefore for the duration of the useful signal; the output video signal, taken from the electrode B of the capacitor Ci, is equal to the input video signal multiplied by the negative gain -g to which the potential difference across the capacitors CI is added.

The method for inverting a television video signal described here therefore retains the synchronization pulses of the video signal in the original modulation and reverses the useful line signal with respect to an axis passing halfway between the white level and the black level, corme indicates in Figures 2 and 3 where the constant g is equal
a ; this process allows the inverted signal to be easily reversible with the same device.

 Thus, thanks to the controls of the doors P2 and P3, the output is equal either to the inverted video signal as we have just seen, or to the direct video signal if the door F2 is closed for the entire duration of the line signal, and l 'We can control this inversion or not, pseudo-randomly.

 Although the present invention has been described with the aid of an example circuit, it is clear that it is not limited to said example and that it is capable of modification without going beyond its ambit.

Claims (4)

 1. Method for inverting a television video signal, characterized in that one adds to the useful line signal, amplified by a negative gain (-g), an additive constant, amplified by the same gain (+ g ) positive; and in that the signal, for the duration of line suppression, amplified by the same positive gain (+ g), remains in original modulation.  2. Device for inverting a television video signal for implementing the method according to claim 1, characterized in that the video signal is injected at the input of a differential amplifier (1) with two outputs a opposite gains, whose positive gain output is sent to a black alignment circuit (2), which supplies a sample-and-hold circuit (3) controlled by a first logic signal (white control), and the negative gain output on an electrode of a capacity (C1), the other electrode of which is connected, via a gate (fl) controlled by a second logic signal (black control), to the output of the sampler-blocker, and in that the inverted signal is obtained by a multiplexing (4, P2, P3), controlled by a third logic signal (wave TI / T2), between the output of the black alignment circuit (2) and the electrode the capacity (C1) connected to the logic gate (P1).  3. Device for inverting a television video signal according to claim 2, characterized in that the additive constant is equal to the sum of the amplified voltage (+ g) of the signal at the white level supplied by said sampler-blocker (3) and the amplified voltage (-g) of the signal at the black level supplied by the differential amplifier (1), this constant being obtained by charging or discharging said capacitance (C1).  4. Device for inverting a television video signal according to one of claims 2 or 3, characterized in that the first logic signal (white control) is at the high level at an instant in the frame where the signal is at white level, at low level elsewhere, the second logic signal (black command) is at high level during an instant of each line where the signal is at black level, at low level elsewhere, and the third logic signal (TI / T2 wave) is high at the line drop signal, low elsewhere.