FR2563403A1 - Method and device for optical transmission of two video signals in particular by optical fibre - Google Patents

Abstract

The present invention relates to a method and system of optical transmission of two video signals each containing a vision signal and synchronising pulses. The system comprises an emitter device incorporating processing means able to invert one of the signals and shift the latter's level in such a way that the respective levels of the synchronising pulses determine the maximum and minimum levels respectively of the optical signal generated by an emitter of light. The signal also comprises a receiver system 100 including detection means 140, 142 measuring the maximum and minimum levels respectively of the optical signal received in order to control accordingly the gain of a variable-gain amplifier 130.

Description

 The present invention relates to the field of transmission of video signals.

 More specifically, the present invention relates to the optical transmission of video signals each containing, conventionally, a vision signal and synchronization pulses.

 The present invention particularly relates to the transmission of such video signals by optical fiber.

 The development of optical fiber transmissions is very promising, and many methods and devices for multiplexing optical fiber transmission channels have already been proposed.

 However, conventional optical transmission methods and devices prove to be relatively complex and expensive.

 The methods and devices hitherto proposed, in particular, have not been able to solve in a simple way, the control of the gain of the transmission chain in order to take into account the inevitable losses but difficult to predetermine, occasioned in the optical transmission medium. .

 The present invention now proposes a method of optical transmission of two video signals each containing a vision signal and synchronization pulses, which overcomes the aforementioned drawbacks.

The optical transmission method according to the present invention comprises the steps consisting in the transmission to be processed of the video signals,
- by reversing one of these signals and
- by shifting the level of these so that the respective levels of the synchronization pulses determine the respectively maximum and minimum levels of the signals after processing, to sample and to nested in time the video signals processed to form a signal of modulation, to modulate the emission of a light emitter disposed opposite an optical transmission medium, by means of the modulation signal, said optical transmission method according to the present invention further comprising the steps consisting in receiving
receiving the light signal transmitted by the optical transmission medium and converting it into an electrical reception signal,
detecting the respectively maximum and minimum levels of the electrical reception signal, comparing the difference between these respectively maximum and minimum levels with a reference value and controlling the gain of a gain control amplifier sensitive to the electrical signal reception, in the event of a difference between said level difference and the reference value, to establish equality between them, and
sampling the time components of the gain control amplifier output signal corresponding to each of the video signals, respectively.

 As will become clear on reading the detailed description which follows, the gain control process proposed according to the invention is particularly simple.

 It will be noted in particular that the present invention in no way requires the transmission of an additional signal to the video signals for the control of the amplifier stages with gain control, an additional signal commonly required in conventional optical transmission techniques, in particular by optical fiber.

 The optical transmission method according to the present invention advantageously further comprises the step of filtering, using a low-pass filter, the time components separated from the output signal of the gain control amplifier.

 The present invention also relates to a system for the optical transmission of two video signals each containing a vision signal and synchronization pulses.

The optical transmission system according to the present invention comprises
- a transmitter device comprising
two input channels receiving the video signals respectively, one of which includes an inverting amplifier,
at least one level-translating amplifier associated with one of the channels, such that the respective levels of the synchronization pulses determine the respectively maximum and minimum levels of the signals at the output of the input channels,
a basic clock which controls in phase opposition
two switches interposed respectively between the outputs of the input channels and
associated entries of a summator, and
. a light emitter attacked by the adder and capable of generating a light signal directed towards an optical transmission medium,
- a receiving device comprising
a light receiver sensitive to the optical signal transmitted by the optical transmission medium,
a variable gain amplifier connected to the light receiver output,
a first detection means which measures the maximum level of the signal at the output of the variable gain amplifier,
a second detection means which measures the minimum level of the signal at the output of the variable gain amplifier,
a differentiator which establishes the difference between the maximum level and the minimum level at the output of the variable gain amplifier,
an amplifier-comparator, comparing the output voltage of the differentiator with a reference voltage, and the output of which is connected to the gain control stage of the variable gain amplifier,
two output channels connected at the output of the variable gain amplifier and comprising respective switches controlled in phase opposition.

 The term “differentiator” is understood here to mean a member whose output delivers a direct voltage corresponding to the difference between the amplitudes of the signals applied to the inputs of this member.

 The present invention naturally also relates to the aforementioned receiver device and the transmitter device considered individually.

 According to an important characteristic of the present invention, each output channel of the receiving device comprises a low-pass filter, downstream of the associated switch.

 Furthermore, preferably, one of the output channels includes an inverter.

 The switches integrated into the output channels of the receiving device are advantageously controlled in phase opposition by an auxiliary clock of the same frequency as the base clock.

 This auxiliary clock is advantageously controlled by the output signal of the variable gain amplifier.

 Preferably, the receiver device according to the present invention further comprises a comparator which compares one of the minimum or maximum levels of the output signal of the variable gain amplifier with a second reference value, and a regulating member. level interposed between the light receiver and the variable gain amplifier, which is controlled by the comparator to enforce the equality of the amplitudes of the signals applied to it.

Other characteristics and advantages of the present invention will appear on reading the detailed description which follows and with reference to the appended drawings given by way of non-limiting example, and in which
FIG. 1 represents a schematic view of a transmitter device according to the present invention,
FIG. 2 represents a schematic view of a receiving device according to the present invention,
- Figure 3 illustrates the operation of the receiver device according to the present invention shown in Figure 1, and more specifically represents signals taken from different points of the aforementioned receiver device.

 The description which follows relates to a device for transmitting video signals by optical fiber in accordance with a preferred embodiment of the invention; this description should not however be considered as limiting, in particular in so far as the nature of the optical transmission medium is liable to be modified. For example, optical transmission may be carried out in air.

 We will first describe the structure of the transmitter device shown in Figure 1.

 This device comprises two input channels 10, 30 respectively receiving the video signals E1 and E2 illustrated on the first two lines of FIG. 3.

 As shown in these figures, each of the video signals comprises on the one hand a vision signal referenced SV in FIG. 3, of positive polarity with respect to a ground potential of the assembly referenced Ov in the figures, and on the other hand part of the synchronization pulses superimposed on a level of suppression of the vision signal, and formed of rectangular slots of negative polarity.

Input channels 10, 30 drive amplifiers
respective translating cators 12, 32. In addition, one of
these amplifiers, such as the associated amplifier 12
to the first input channel 10, is an inver amplifier
sister.

The amplitude of the translations of determined level
by each of the amplifiers 12, 32 is such that the
respective levels of the IS synchronization pulses
determine the maximum and minimum levels respectively
signals at the output of the input channels 10, 30 equipped
amplifiers. This provision is illustrated on the
third and fourth lines of figure 3, referenced
A and B respectively, and corresponding to the pre signals
sents on the outputs of amplifiers 12 and 32, res
pectively.

As can be seen from the examination of Figure 3,
the level of the IS synchronization pulses of the signal in
output of the translator amplifier 12 represents
effect the maximum level of signals after processing, tan
say that the level of the IS synchronization pulse
of the signal at the output of the 32-horn translator amplifier
responds to the minimum level of these signals.

Note also that the shift of
levels operated on each of the signals is identical, from
so that the maximum and minimum levels determined
by the IS synchronization pulses are symmetrical
of the ground potential Ov. More precisely, according to the illustration in FIG. 3, the level translation of the signals is carried out so that the signal A at the output of the amplifier 12 is of positive polarity while the signal B in output of amplifier 32 is of negative polarity.

 The outputs 14, 34 of the amplifiers 12, 32 are connected by the intermediary of respective switches 16, 36 to the inputs 42, 44 of an adder 40.

 Said switches 16, 36 are controlled in phase opposition by a base clock 20.

 For this1 the opposite phase outputs Q, Q of this clock, illustrated respectively on the fifth and sixth lines of FIG. 3, attack the single switch 16 associated with the first input channel, the other the switch 36 associated with the second input channel.

 As shown in the seventh and eighth lines of FIG. 3 which illustrates the signals present at the output of the switches 16 and 36, referenced respectively C and D, the summator 40 receives on its inputs a signal sampled at the rate of the oscillations of the basic clock 20.

 The summator 40 overlaps the processed video signals from the switches 16 and 36 over time.

 The signal referenced E obtained at the output of the summator 40 is represented on the ninth line of FIG. 3.

 The output 46 of the adder 40 is connected to a voltage / current converter 50 which drives a light emitter 60, such as a laser diode or a light-emitting diode or any other functionally equivalent means whose light emission is directed towards a fiber optics schematically referenced 70.

 The power P of the optical signal emitted by the light emitter 60 is represented on the tenth line of FIG. 3.

 Of course, the evolution of this optical signal, the average power of which is referenced PO in FIG. 3 is similar to the evolution of the electrical signal obtained at the output 46 of the adder 40.

 We will now describe the structure of the receiving device 100 shown in FIG. 2.

 This receiving device comprises, opposite the output of the above-mentioned optical fiber 70, a light receiver 110, such as a PIN photodiode.

 This light receiver 110 transforms the light signal transmitted by the optical fiber and illustrated on the last line of FIG. 3 into a corresponding electrical signal applied to the input 114 of a preamplifier 116. The latter is provided, as shown in the figure 2, of a feedback loop, for example resistive, referenced 118.

 The output 120 of the preamplifier 116 drives the first input 124 of a summing member 122 used for offset adjustment, and connected by its second input 126 to a feedback loop whose structure will be described in more detail below. .

 The summing member 122 is adapted to establish an algebraic summation of the signals applied to its inputs 124, 126.

 The output of the summing member 122 is connected to the input 132 of an amplifier 130 with variable gain.

 The control input of the gain control stage of the variable gain amplifier 130 is referenced 134 in FIG. 2.

 The output 136 of the variable gain amplifier 130 drives two output channels referenced 180, 190.

 The first output channel 180 comprises in series a switch 182 and a low-pass filter 184, the output of channel 180 being referenced 186 in FIG. 2.

 The second output channel 190 comprises in series a switch 192, an inverter 198 and a low-pass filter 194, the output of the second channel 190 being referenced 196 in FIG. 2.

 The above-mentioned switches 182 and 192 are controlled in phase opposition, in a similar manner to the switches 16 and 36 integrated in the transmitting device.

 More precisely, the switches 182 and 192 are controlled in phase opposition by an auxiliary clock 170 whose outputs 172, 174 of opposite phase, drive the switches 182 and 192 respectively.

 The rising and falling edges of the control pulses of the switches 182 and 192, generated by the clock 170 must be strictly in phase with the rising and falling edges of the signal at the output of the variable gain amplifier 130.

 The auxiliary clock 170 therefore has a beat frequency identical to that of the base clock 20 integrated into the transmitter device and synchronized with the beats of the signal at the output of the variable gain amplifier 130.

 For this, advantageously, the auxiliary clock 170 can be controlled by the output signal of the variable gain amplifier 130 for example using a phase locked loop.

 Furthermore, as appears in FIG. 2, according to an important characteristic of the present invention, the receiver device 100 comprises a first detection means 140 which measures the maximum level of the signal at the output of the variable gain amplifier (corresponding to the level of the synchronization pulse IS of the first video signal E1) as well as a second detection means 142 which measures the minimum level at the output of the variable gain amplifier (corresponding to the level of the synchronization pulse IS of the second video signal E2).

 The detection of the extreme levels of the signal at the output of the variable gain amplifier 130 defined by the detection means 140, 142, can be carried out by means of the charging of a capacitance, through a diode, with discharge of the capacitance to through a resistive circuit according to a time constant greater than the period of superimposition of the synchronization pulses 15 on the video signals.

 The outputs 144 and 146 of the detection means 140, 142 are respectively connected to the non-inverting inputs 152 and inverting inputs 154 of a differentiating member 150. Here, the term "differentiator" means a member whose output delivers a direct voltage corresponding to the difference between the amplitudes of the signals applied to the inputs of this organ. The latter consequently establishes the difference between the maximum level and the minimum level of the signal at the output of the variable gain amplifier 130. The output 156 of the differentiator 150 drives a first input 160 of a comparator 158 which receives on its second input 162 a first reference voltage Ref. 1.

 The output 164 of the comparator 158 is connected to the input 134 of the gain control stage of the amplifier 130.

 The comparator 158 therefore defines on its output 164 a correction signal whose amplitude and direction represent the value of the difference between the first reference value applied to the input 162 and the difference between the maximum level and the level signal at the output of the variable gain amplifier, applied to input 160.

 This correction signal applied to the input 134 of the variable gain amplifier 130 modifies the gain of the latter, by increasing or decreasing according to the direction of the correction signal at the output of comparator 158, in order to cancel this correction signal .

 Of course, the amplitude of this reference signal applied to the input 162 of the comparator 158 is determined as a function of the amplitude of the translations of levels determined by the translating amplifiers 12, 32, of the transmitting device.

 Furthermore, the detection means 142 which measures the minimum level of the signal at the output of the variable gain amplifier 130 drives a first input 166 of a comparator 165 whose second input 167 receives a second reference signal Ref. 2 and the output 168 is connected to the input 126 of the summing member 122.

 The detection means 142 and the comparator 165 constitute the feedback loop for the offset adjustment previously mentioned.

 The output signal of the comparator 165 acts on the signaling member 122 to modify the level translation of the signal applied to the input 132 of the variable gain amplifier 130, in the event of an inequality between the minimum signal level. at the output of this amplifier detected by the means 142 and the second reference signal Bef. 2 applied to the input 167 of the comparator 165. The level shift then introduced by the summing member 122 is of course adapted to tend to cancel the difference between the signals applied to the comparator 165.

 The operation of the receiving device 100 shown in Figure 2 can be summarized as follows.

 The optical signal transmitted by the optical fiber 70 is transformed into an electrical signal by the light receiver 110. This electrical signal is amplified by the preamplifier 116.

 The loop comprising the detection means 142, the comparator 165 and the summing member 122 provides offset adjustment.

 The gain of the variable gain amplifier is controlled by the loop comprising the detection means 140, 142, the differentiator 150 and the comparator 158.

 The signal at the output of the variable gain amplifier 130 is sampled by the switches 182, 192.

Thus, the time components of the output signal of the gain control amplifier corresponding to each of the original video signals are separated.

 These sampled signals are then filtered by the elements 184, 194, after inversion thanks to the element 198, for one of the output channels corresponding to the first video signal E1.

 It will be noted that the gain control of the receiving device 100 shown in FIG. 2, in accordance with the invention, is particularly simple insofar as this gain control is based on a detection of the level of the synchronization pulses IS transmitted, without any 'it is necessary to add to the transmitted signal other additional signals.

 As a result, the transmission system proposed by the present invention is both simple and economical.

 Of course, the illustration of the transmitting device and the receiving device, respectively in FIGS. 1 and 2, is only schematic.

 The practical realization of each of the previously described and represented functional blocks is within the domain and skills of the person skilled in the art.

 In particular, in practice, the amplifiers, comparators or differentiators shown in the figures and previously mentioned, will for the most part be produced in the form of operational amplifiers comprising feedback loops. These have not been shown in the figures to simplify the drawing.

Claims (12)

 1. A method of optical transmission of two video signals (E1, E2) each containing a vision signal (SV) and synchronization pulses (IS), characterized in that it comprises the steps consisting in the transmission:  to process video signals  - by reversing (12) one of these signals and  - by shifting (12, 32) the level of these so that the respective levels of the synchronization pulses IIS) determine the respectively maximum and minimum levels of the signals after processing,  to sample (16, 36) and to nested (30) in time the video signals processed to form a modulation signal, to modulate the emission of a light emitter (60) arranged opposite a transmission medium optical (70) by means of the modulation signal, the transmission method being further characterized by the fact that it comprises the steps consisting, upon reception  receiving (110) the light signal transmitted by the optical transmission medium (70) and converting it into an electrical reception signal,  detecting (140, 142) respectively the maximum and minimum levels of the electrical reception signal,  comparing (158) the difference between the maximum and minimum levels respectively with a reference value (162) and controlling the gain of a gain control amplifier (130) sensitive to the electrical reception signal, in the event of difference between said level difference and the reference value (162) to establish equality between them, and to separate by sampling (182, 192) the time components of the output signal of the amplifier  gain control encoder (130) corresponding respectively to each of the video signals (E1, E2).  2. Transmission method according to claim 1, characterized in that the optical transmission medium is formed of an optical fiber (70).  3. Transmission method according to one of claims let2, characterized in that it further comprises the step of filtering (184, 194) using a low-pass filter the time components separated from the signal gain control amplifier output (130).  4. Optical transmission system of two video signals (E1, E2) each containing a vision signal (SV) and synchronization pulses (IS), characterized in that it comprises  - a transmitter device (1) comprising  two input channels (10, 30) receiving the video signals, one of which includes an inverting amplifier,  at least one level translating amplifier (12, 32) associated with one of the input channels, such that the respective levels of the synchronization pulses (IS) determine the respectively maximum and minimum levels of the signals at the output of the channels d 'Entrance,  a basic clock (20) which controls in phase opposition  two switches (16, 36) interposed respectively between the outputs of the input channels and  associated inputs (42, 44) of a summator (40) and  a light emitter (60) driven by the adder (40) and capable of generating a light signal directed towards an optical transmission medium (70),  - a receiving device (100) comprising  a light receiver (110) sensitive to the signal transmitted by the optical processing medium (70),  a variable gain amplifier (130) connected to the output of the light receiver,  a first detection means (140) which measures the maximum level of the signal at the output of the variable gain amplifier (130)  a second detection means (142) which measures the minimum level of the signal at the output of the variable gain amplifier,  a differentiator (150) which establishes the difference between the maximum level and the minimum level of the  signal at the output of the variable gain amplifier (130)  an amplifier-comparator (158) comparing the output voltage of the differentiator (150) with a reference voltage (162), and whose output (164) is connected to the gain control stage of the variable gain amplifier (130),  two output channels (180, 190) connected at the output of the variable gain amplifier (130) and comprising respective switches (182, 192) controlled in phase position.  mé of an optical fiber (70).  by the fact that the optical transmission medium is for  5. System according to claim 4, characterized  6.Receiver device for an optical transmission system of two video signals each containing a vision signal and synchronization pulses, according to one of claims 4 and 5, characterized in that it comprises  - a light receiver (110) sensitive to the optical signal transmitted by an optical transmission medium (70),  - a variable gain amplifier (130) connected to the output of the light receiver,  a first detection means (140) which measures the maximum level of the signal at the output of the variable gain amplifier (130),  a second detection means (142) which measures the minimum level of the signal at the output of the variable gain amplifier (130),  - a differentiator (150) which establishes the difference between the maximum level and the minimum level at the output of the variable gain amplifier,  an amplifier-comparator (158) comparing the output voltage of the differentiator (150) with a reference voltage (162) and the output of which is connected to the gain control stage of the variable gain amplifier,  - two output channels (180, 190) connected at the output of the variable gain amplifier and comprising respective switches (182, 192) controlled in phase opposition.  7. Receiving device according to one of claims 4 to 6, characterized in that each output channel (180, 190) comprises a low-pass filter (184, 194) downstream of the associated switch (182, 192).  8. Receiving device according to one of Claims 4 to 7, characterized in that one of the output channels (180, 190) comprises an inverter (198).  9. Receiver according to one of claims4 to 8, characterized in that the switches (182, 192) integrated into the output channels (180,190) are controlled in phase opposition by an auxiliary clock (170) of the same frequency as the 'base clock (20).  10. Receiving device according to claim9 characterized in that the auxiliary clock (170) is controlled by the output signal of the variable gain amplifier (130).  11. Receiving device according to one of claims 4 to 10, characterized in that it further comprises a comparator (165) which compares one of the minimum or maximum levels of the signal at the output of the variable gain amplifier (130) with a second reference value (167) and a level adjustment member (122), interposed between the light receiver (110) and the variable gain amplifier (130), which is controlled by the comparator ( 165) to enforce the equality of the amplitudes of the signals applied to it.  12. Transmitter device for an optical transmission system of two video signals (E1, E2) each containing a vision signal (VS) and synchronization pulses (IS), according to one of claims 4 and 5, characterized by the fact that he understands  - two input channels (10, 30) receiving the video signals, one of which includes an inverting amplifier,  - at least one level translating amplifier (12, 32) associated with one of the channels such that the respective levels of the synchronization pulses (IS) determine the respectively maximum and minimum levels of the signals at the output of the input channels,  - a basic clock (20) which controls in phase opposition  two switches (16, 36) interposed respectively between the outputs of the input channels (10, 30) and of the inputs (42, 44) associated with an adder (40) and  - a light emitter (60) driven by the adder (40) and able to generate a light signal directed towards an optical transmission medium (70).