FR2608342A1 - Method and system for automatic control of the saturation level of at least one of the R, G and B colour video signals - Google Patents

Abstract

The present invention relates to a method and system for automatic control of the saturation level of an R, G, B colour video signal. The system of the invention is characterised in that it comprises three processing paths 1; 2; 3 for the R, G, B video signals, each comprising three transistors T4-T6; T10-T12; T16-T18 common-collector mounted and having their emitters connected in common forming an output terminal, the base of one T4; T10; T16 of the three transistors receiving the corresponding video signal; and a device 5, R15-R17 continuously shifting the amplitude level of the video signal received at the said base of one of the three transistors of each processing path by a value equal to 75% of the preset fixed maximum amplitude level of the video signals. The present invention finds an application in the video sector.

Description

 The present invention relates to the field of color video and particularly relates to a method and system for automatic control of the level of at least one of the color video signals R, G and B acting on their saturation.

We know that any color image is defined from three fundamental signals R, G and B, representative of the colors red, green and blue respectively. When a transmitted image is in black and white; the three signals R, G and B are identical to each other and equal to a third signal called Y or luminance signal, the expression of which is
Y = 0.59 V + 0.3 R + 0.11 B.

 From these signals, we can extract three new signals R-Y, V-Y and B-Y called chrominance signals.

When an image has very little color, the chrominance signals are of low amplitude and cancel each other out when the image is in black and white. The amplitude of the chrominance signals is, on the contrary, maximum when the image is very colorful, that is to say when the differences between R, G and B are maximum. These conditions are those defined by the different binary combinations of R, G and B evolving according to the following table
VRB Color
0 0 0 black
0 0 1 blue
0 1 0 red
0 1 1 magenta
1 0 0 green
1 0 1 cyan
1 1 0 yellow
1 1 1 white
The value (0) represents each color at its minimum level while the value (1) represents each color at its maximum level.

 Experience and statistical studies of colorimetry have shown that real images only very rarely show colors as saturated as those defined in the table above or, at least, maximum saturation is almost never associated with brightness maximum. In other words, the chrominance signals hardly ever reach their theoretical maximum levels.

 These findings have led to the allocation of these signals with a multiplier coefficient in order to optimize the behavior of transmissions when coding images in PAL, SECAM or NTSC systems (as well as in other derived systems or in new multiplexing systems. temporal).

 The standard has been established so that the R-Y and B-Y signals are considered nominal when their levels reach 75% of their theoretical maximum levels.

 In recent years, so-called synthetic machines have appeared, making it possible to manufacture synthetic, digital, analog or hybrid images.

 These machines work directly with the R, G and B signals, independently of each other and the maximum amplitude level of one of these signals may very well coincide with the minimum amplitude level of the other two, for example, which was never the case when the images were produced by a television camera.

 The R-Y and B-Y signals will therefore often reach their maximum amplitude levels, which is highly exceptional for natural images.

 However, the aforementioned coding systems are not suitable for the reception of video signals of amplitude levels clearly higher than their nominal amplitude levels. These systems will therefore react variously to these signals and cause various faults such as, for example, clipping, distortion, delay, which may appear at various points in the processing channel (video recorder, transmission device, etc.).

 To solve this problem, various solutions can be adopted.

 One of these solutions consists of direct action on the coders. By acting on appropriate potentiometers, it is possible to reduce the gain of the R-Y and B-Y signals. This solution has the merit of simplicity since it boils down to a simple adjustment of two amplifiers but it has the disadvantage of specializing the coder for synthetic images, since natural images will be too little saturated, unless resuming the adjustments initials.

 Another solution is to act directly on the generation of synthetic images. More specifically, graphic designers working on synthetic machines will need to know in particular that it will not be possible for them to make a color coexist at its maximum amplitude level with another color at its minimum amplitude level. However, such an approach constitutes a heavy constraint for graphic designers.

 A final solution is to technically limit the output level R, G, B to three quarters (75% of the nominal amplitude level). This solution, although working, leads to the impossibility of obtaining the strong luminosity in the whites.

 The present invention therefore aims to solve the problem of compatibility of natural images with synthetic images at the level of the R, G and B signals so that these images can be coded using a normal coder perfectly calibrated for natural images.

 For this, the present invention relates to an automatic method for controlling the level of at least one of the video signals of color R, G, B acting on their saturation with a view to their subsequent coding in order to record them for example in a video recorder or broadcast them by means of a transmission device and characterized in that it consists, after one of the video signals R, G, B has exceeded an amplitude level equal to 75% of the level d fixed maximum amplitude of the video signals, to increase the amplitude level of the other two video signals in proportion to the increase in the amplitude level of said video signal between the amplitude level equal to 75% of the maximum amplitude level and the maximum amplitude level so that the amplitude level of the other two video signals is not less than a value equal to the difference between the amplitude level of said video signal and the value corresponding to 75% of the level of maximum amplitude.

 The present invention also relates to the automatic control system for the level of at least one of the video signals of color R, G, B acting on their saturation for the implementation of the method defined above and characterized in that it comprises three channels for processing video signals R respectively; V; B, each comprising three transistors mounted in a common collector having their transmitters connected in common forming an output terminal, the base of one of the three transistors receiving the corresponding video signal; a device continuously shifting the amplitude level of the video signal received by said base from one of the three transistors of each processing channel by a value equal to 75% of the pre-established fixed maximum amplitude level of the video signals, the level amplitude offset from the video signal of each of the processing channels being applied to the bases of a pair of transistors formed by two transistors respectively of the two other processing channels; and in that the transistors of each pair become conductive when the amplitude level of the video signal received on one of the processing channels reaches and exceeds the value equal to 75% of the maximum amplitude level so that the level d amplitude of the other two video signals at the two output terminals respectively of the other two processing channels increases in proportion to the increase in the amplitude level of the video signal between the amplitude level equal to 75% of the maximum amplitude level and said maximum amplitude level, the amplitude level of the other two video signals being not less than a value equal to the difference between the amplitude level of said video signal and the amplitude level corresponding to 75% of the level maximum amplitude.

 According to another characteristic of the system of the invention, the shift device comprises three resistors of equal ohmic values connected respectively to the bases of the transistors receiving the video signals R; V; B and three equal constant current generators connected respectively to the three resistors, the common connection point of each generator to a resistor delivering the aforementioned corresponding offset video signal.

 According to yet another characteristic of the system of the invention, each aforementioned current generator is constituted by a transistorized assembly.

 According to still another characteristic of the system of the invention, the value of the constant currents produced by the aforementioned generators is adjustable so as to be adjusted for example to a value such that the difference between the amplitude level of the video signal received at one processing channels and the amplitude level of the corresponding shifted video signal is 1.5 volts.

The invention will be better understood and other objects, characteristics, details and advantages thereof will appear more clearly during the explanatory description which follows, made with reference to the appended schematic drawings given solely by way of example illustrating a mode of the invention and in which
FIG. 1 represents the electronic diagram of the automatic control system according to the invention and
- Figure 2 shows the shape of the three signals R, G and B at the output of the system of the invention.

The detailed description of the electronic circuit of FIG. 1 will be made with reference to video signals R, G and B, whose maximum voltages
ER, EV and EB for the three colors have been fixed for practical reasons at + 2 volts, it being understood that other maximum voltage values are possible without altering the operating principle of the system of the invention. In addition, in the video field, the standard requires that the R, G and B signals as well as Y (monochrome) are conveyed in the transmission cables at a level of 0.7 volts on an impedance of 75 ohms. In the circuit of FIG. 2, the input amplifiers making it possible to bring this level to 2 volts, and the output stages making it possible to bring the signals back to their normalized level of 0.7 volts / 75 ohms have not been shown for clarity.

 Referring to FIG. 1, the references 1, 2 and 3 respectively designate the three processing channels of the video signals R, G and B.

 The video signals of voltage ER, EV, EB at the level of 2 volts and under low impedance are applied respectively to the inputs El, E2 and E3 of the three processing channels.

A device 4 comprising clamp circuits 4a-4c known per se and each receiving the clamp pulse i, supplies the video voltage signals ER, EV and
EB respectively on the bases of transistors T1, T7,
T13 with a zero level (black) perfectly aligned with the O volt potential of the system. This clamping operation being already well known in the video field need not be detailed. Transistors T1, T7 and T13 are respectively connected to transistors T2, T8 and
T14, the respective bases of which are brought to the continuous potential of +2.1 volts via resistors R1 and R2 respectively connected to the potential + V (+ 6 volts for example) and -V (-6 volts). The collectors of the pairs of transistors T1, T2; T7, T8; T13, T14 are connected in common and coupled to the potential of -6 volts via resistors R3, R4 and R5. In addition, the transmitters connected n common to each pair of transistors T1, T2; T7, T8 and T13, T14 are connected to the potential of +6 volts via the resistors
R6, R7 and R8.

The pairs of transistors T1, T2; T7, T8 and
T13, T14 therefore behave like clipping stages avoiding accidental overshoot of voltage levels and which, in principle, should never be used.

The outputs of the clipping stages are respectively applied to the bases of three transistors
T3, T9 and T15, the collector of each of which is connected to the potential of + 6volts. The transistors T3, T9 and T15 supply the low-impedance voltage video signals ER, EV and EB following the mounting by their transmitters, after having compensated by their Vbe for the offset caused by the Vbe of the transistors T1, T7 and T13.

So far, the circuit of Figure 1 is completely "transparent" to video voltage signals.

 The voltage video signals from the transmitters of T3, T9 and T15 are applied directly to the bases of transistors T4, T10 and T16 respectively, which are mounted as a common collector.

The processing channels 1, 2 and 3 additionally comprise two pairs of transistors T5 respectively,
T6; T11, T12 and T17, T18. The transistors of each pair are also mounted in a common collector with the emitters of the pair T5, T6 connected in common to the emitter of transistor T4, the emitters of transistors T11, T12, connected in common to the emitter of transistor T10 and the emitters of the transistors T17, T18 connected in common to the emitter of the transistor T16. The outputs of the circuit formed by the three transistors
T4-T6; T10-T12; T16-T18 of each channel are applied respectively to the bases of transistors T22, T23 and T24 and, via resistors R9, R10 and R11 to the collectors thereof, the common junction of resistors R9, R10 and R11 to the collectors transistors T22, T23 and T24 being connected to the potential of -6 volts. The emitters of the transistors T22, T23 and T24 constitute the output terminals SR, SV and
SB of the system of the invention, these transmitters being connected via resistors R12, R13 and R14 at the potential of +6 volts.

Three resistors R15, R16, R17 are connected respectively, by one of their terminals, to the bases of the transistors T4, T10 and T16 and, by their other terminals, to a device 5 generating constant currents. This device 5 comprises a transistor assembly comprising the transistors T19, T20 and T21 having their bases connected in common to the common junction of two resistors R18 and19, the latter also being connected to the potential of -6 volts while the resistor R18 is connected to ground (O volt). Transistor emitters
T19-T21 are connected to the potential of -6 volts via resistors R20-R22. The collectors of these three transistors forming the output of device 5 are respectively connected to resistors R15-R17 having equal ohmic values.

 The resistors R15-R17 are each traversed by a constant current of the device 5 such that there appears at the terminals of each of them a fixed potential difference equal to 1.5 volts and the negative pole of which is that connected to the collector of the transistors T19-T21. I1 therefore appears at the collector of transistors T19-T21 the video signals of voltage E'R, E'V, E'B with a continuous offset of -1.5 volts.

 The video signal of offset voltage E'R appearing at the collector of transistor T21 is applied to the bases of transistors T11 and T17 of processing channels 2 and 3 respectively. The video signal of offset voltage E'V appearing at the collector of transistor T19 is applied to the bases of the transistors T6 and T18 respectively of the processing channels 1 and 3. Finally, the shifted voltage video signal E'B appearing at the collector of the transistor T20 is applied to the bases of the transistors T5 and T12 respectively of the processing channels i and 2 .

 It is understood that the output of each of the processing channels 1, 2 and 3 will be controlled by one of the three transistors mounted as a common collector, the base of which will be at the highest potential. Thus, if we consider the processing channel 2, the output SV will be driven by the transistor T10, T11 or T12 at the highest potential.

In the case of weak signals, the output SV will be controlled by the transistor T10 since the base of the transistor T11 is supplied by the video signal with offset voltage E'R and the base of the transistor T12 is supplied by the video signal with offset voltage E 'B.

 The operation of the system of the invention already follows in part from the above description and will be described below in more detail.

 As an example, we will start from the assumption that a video signal of voltage EV applied to the terminal E2 varies from 0 volts to its maximum level, therefore from O to +2 volts and no signal on the red processing channels. 1 and blue 3 is not applied.

Under these conditions, there will be 0 volts on the bases of the transistors T4 and T16 and -1.5 volts on the bases of the transistors T11, T17, T5 and T12. The potential on the base of transistor T10 will progressively pass from O to +2 volts and the video signal of shifted voltage E'V appearing at the collector of transistor T19 will be applied to the bases of transistors T6 and T18, the potential at the bases of the latter two transistors therefore varying from -1.5 volts to +0.5 volts. On the processing channel 2 of the video signal V, the output signal at the terminal SV will faithfully follow the input signal while on the processing channels 1 and 3 respectively of the video signals R and B, at the input of which none signal is not applied, nothing will happen as long as the video signal of voltage EV applied on processing channel 2 does not reach the value of 1.5 volts. However, when the video signal of voltage EV reaches and exceeds the value of 1.5 volts1 the video signal voltage shifted
E'V reaches and exceeds O volt. Under these conditions, the bases of the transistors T6 and T18 become more positive than the bases of the transistors T4 and T16, respectively.

A corresponding output video signal will therefore appear at the output terminals SR and SB, the voltage of which increases in proportion to the increase in voltage of the video signal ER between the values of 1.5 and 2 volts.

When the video signal of voltage EV reaches its maximum value (+2 volts), the outputs SR and SB of the processing channels 1 and 3 deliver a corresponding video signal of 0.5 volt.

FIG. 2 perfectly illustrates the variations in voltage at the outputs SR and SB of the corresponding video signals of red and blue as a function of the variation of the video signal of green at output SV of the processing channel 2. The signal represented in dotted lines indicates l 'shape of the video signal of offset voltage E'V produced at the collector of transistor T19 for in the case where no signal is applied to the input terminals El and
E3 and that a video signal of voltage EV varying from 0 to +2 volts is applied to the terminal E2. It is therefore noted from FIG. 2 that when the video signal of voltage EV reaches and exceeds 1.5 volts, the video video signals ER and EB respectively at the outputs SR and SB evolve in an identical manner to the video signal of offset voltage E 'V.

 Furthermore, if under the conditions represented in FIG. 2, the video signal of voltage EB for example reached and exceeded the level of 0.5 volts at the input E3, the potential on the base of the transistor T16 would become higher than that present on the base of the transistor T18 and the video signal appearing at the output SB would exactly reflect the video signal of voltage EB appearing at the input E3.

 The reasoning which has just been explained above also applies when a video signal of ER or EB voltage varies greatly from O to +2 volts while the two other inputs of the processing channels are applied a corresponding video signal. of zero voltage, the diagram in Figure 1 being permutative.

 In summary, for the signals R, G and B of an amplitude level lower than 1.5 volts (that is to say 75% of the maximum amplitude level), the system of figure 1 is completely transparent. On the other hand, as soon as one of the video signals R, G, B reaches and exceeds this level of amplitude, the other two video signals increase in amplitude in proportion to the increase in the video signal having exceeded the level of amplitude equal to 75% of the maximum amplitude level of video signals, unless they already have a higher level. By the system of the invention, it cannot therefore appear at the outputs of the system of FIG. 1 a difference of more than 1.5 volts between the signals R, G, B.

 When using real images, the system of the invention is therefore completely transparent since the different amplitudes between the video signals R, G and B are in this case less than 75% of their maximum level (it will be recalled that for a black and white image the video signals R, G and B are equal, therefore their difference is zero and that the difference between the video signals R, G, B will be higher the more the image is more colorful). The system of the invention therefore behaves like a "difference limiter" and only acts when the differences between the video signals R, G and B are too great.

 Thus, when a video image from a rigging device has normally saturated parts as is the case for example with a natural image, and excessively colored parts (graphic tablet, text synthesizer), only the too colored parts since the system of the invention is completely transparent to normally saturated parties. When a graphic designer wants, for example, to obtain a more or less red shape, he will act on his machine and obtain pure red up to 75% of his maximum level. If it exceeds this level, the video signals of green and blue will not remain at zero but will increase slightly in voltage as described previously. On the display screen, it will always appear red and the graphic designer will hardly be aware that it becomes a little less saturated because a color resulting from the mixture of red at 100% of the maximum amplitude level and green and blue at 25% of the maximum amplitude level is always a red color. This reasoning is of course the same for the other colors.

 If the preferred scheme of the system of the invention is that shown in FIG. 1 because the exchange of signals between transistors working in coupling of transmitters does not present secondary phenomena such as dragging, non-linearity, low bandwidth, which could hinder the functioning of the video system, the fact remains that other modifications can be made to it without departing from the scope of the invention.

 Thus, instead of implementing the system of the invention using discrete components, it is possible to envisage production from integrated networks.

 In addition, as already explained previously, the choice of voltages varying from 0 to +2 volts maximum of the video signals R, G and B is not limiting, the system of the invention also being able to operate with signals of levels of much weaker or much stronger amplitude.

 Also, the diagram in FIG. 1 can be produced symmetrically using PNP transistors in place of the NPN transistors shown in FIG. 1 and NPN transistors in place of the PNP transistors, by reversing the positive and negative power supplies and by deciding that the zero level represents the black level and the -2 volts level represents the maximum amplitude level of each color.

I1 should also be noted that the potential difference se occurring at the terminals of the resistors R15,
R16 and R17 can be adjusted and fixed at will by action on the potential of the bases of the transistors T19,
T20 and T21 thus modifying the value of the constant currents delivered by these transistors. In addition, if the potential difference across the resistors R15 -R17 is adjusted to a value greater than 2 volts, the system of the invention will be completely transparent and if this difference is zero, only a black and white image can go through the system.

Claims (5)

 1. Method for automatic control of the level of at least one of the color video signals R, G and B acting on their saturation with a view to their subsequent coding in order to record them for example in a video recorder or to broadcast them via a transmission device; characterized in that it consists, after one of the video signals R, G, B of exceeding an amplitude level equal to 75% of the predetermined fixed maximum amplitude level of the video signals, in increasing the level d amplitude of the other two video signals in proportion to the increase in the amplitude level of said video signal between the amplitude level equal to 75% of the maximum amplitude level and the maximum amplitude level so that the amplitude level of the other two video signals is not less than a value equal to the difference between the amplitude level of said video signal and the value corresponding to 75% of the maximum level.  2. Automatic control system for the level of at least one of the color video signals R, G, B acting on their saturation for the implementation of the method defined in claim 1, characterized in that it comprises three processing channels (1; 2; 3) respectively of the R video signals; V; B, each comprising three transistors (T4-T6; TlQ-T12; T16-T18) mounted in common collector having their emitters connected in common forming output terminal, the base of one (T4 T10; T16) of the three transistors receiving the corresponding video signal; a device (5, R15-R17) continuously shifting the amplitude level of the video signal received at said base of one of the three transistors of each processing channel by a value equal to 75% of the maximum fixed amplitude level preset video signals, the shifted amplitude level (E'R; E'V; E'B) of the video signal from each of the processing channels being applied to the bases of a pair of transistors (T11, T17 T6, T18; T5, T12) formed by two transistors respectively of the other two processing channels; and in that the transistors of each pair become conductive when the amplitude levels of the video signal received on one of the processing channels reaches and exceeds the value equal to 75% of the maximum amplitude level so that the level d amplitude of the other two video signals at the two output terminals respectively of the other two processing channels increases in proportion to the increase in the amplitude level of the video signal between the amplitude level equal to 75% of the maximum amplitude level and said maximum level, the amplitude level of the other two video signals being not less than a value equal to the difference between the amplitude levels of said video signal and the amplitude level corresponding to 75% of the amplitude level maximum.  3. System according to claim 1, characterized in that the above-mentioned shifting device comprises three resistors (R15, R16, R17) of equal ohmic values connected respectively to the bases of the transistors (T4; T10; T16) receiving the video signals (R , V, B) and three generators (T21; T20; T19) of equal constant currents connected respectively to the three resistors, the common connection point of each generator to a resistor delivering the above-mentioned corresponding offset video signal.  4. System according to claim 2 or 3, characterized in that each aforementioned current generator is constituted by a transistorized assembly.  5. System according to one of claims 2 to 4, characterized in that the value of the constant currents produced by the aforementioned generators is adjustable to be adjusted for example to a value such as the difference between the amplitude level of the video signal received at one of the aforementioned processing channels and the amplitude level of the corresponding shifted video signal either 1.5 volts or 75% of the nominal level chosen of 2 volts.