FR2615061A1 - Device for transmitting multiplexed video signals over optical fibre - Google Patents

Abstract

The invention relates to a device for transmitting video signals, comprising means for multiplexing the said signals, electroluminescent means for transforming the multiplex signal at one end of an optical fibre and, at the other end of the optical fibre, photoreceiver means for receiving the said multiplex signal, and means for demultiplexing the multiplex signal and restoring the video signals. According to the invention, this device comprises means 10, 20, 30 for carrying out a binary analog/digital conversion of the said analog video signals before multiplexing, the multiplexing means being configured to produce a multiplex signal having more than two electrical levels; the demultiplexing means being configured to restore, from the multiplex signal, the video signals in binary digital form, means 310, 320, 330 being provided for carrying out a digital/analog conversion of the video signals. Application to remote surveillance.

Description

 The invention relates to a device for transmitting video signals, comprising means for multiplexing said signals available in analog electrical form into a single multiplex signal, electroluminescent means for transforming said multiplex signal in light form at one end of an optical fiber and , at the other end of the optical fiber, photoreceptor means for receiving said multiplex signal and rendering it in electrical form, and means for demultiplexing the multiplex signal and reproducing the video signals.

 The problem is to offer such a device at a moderate price, so without using very sophisticated electronic components.

 It is known to transmit video signals by analog multiplexing of these signals by frequency distribution, each signal being permanently present in its own frequency band. This solution requires a fairly large bandwidth. Furthermore, it requires the choice of linear electrical components, in particular for light-emitting and photoreceptor means, so that the frequency bands assigned to the different signals do not risk overlapping upon reception. However, the electroluminescent means and linear photoreceptors existing on the market are expensive.

 I1 is for example the diode-laser, which also does not have a high reliability.

 Another solution could have consisted in carrying out analog multiplexing of the video signals by wavelength distribution, each signal being permanently present in its own wavelength band. This solution requires the choice of expensive electroluminescent components, emitting selectively around a given wavelength.

 According to the invention, a time division multiplexing has been carried out, that is to say a multiplexing in digital form, authorizing the use of non-linear and inexpensive but reliable components, such as the light-emitting diode. In addition, with a view to reducing the bandwidth of the multiplex signal, the invention achieves a compromise between a conventional binary digital transmission at two electrical levels and an analog transmission having an infinity of electrical levels, by performing a transmission having a finite number of electrical levels, greater than two.

 The invention therefore relates to a transmission device of the type defined at the beginning of the description and characterized in that it comprises means for carrying out a binary analog / digital conversion of said analog video signals before multiplexing, the multiplexing means being arranged to producing a multiplex signal having a number of electrical levels greater than two, the demultiplexing means being arranged to restore, from the multiplex signal, the video signals in binary digital form, means being provided for carrying out digital / analog conversion of the signals video.

 Advantageously, the multiplexing means are arranged to simultaneously produce two binary multiplex signals, means being provided for producing, from these two binary multiplex signals, a single multiplex signal at four electrical levels, and means being provided for producing, from the four-level multiplex signal from the photoreceptor means, two binary multiplex signals received by said demultiplexing means.

 Advantageously, the device comprises a generator arranged to repeatedly produce a determined sequence of bits which is transmitted at the same time as the video signals, and a device arranged to recognize this sequence and which cooperates with said demultiplexing means.

 According to a preferred embodiment, said means for producing a multiplex signal at four electrical levels comprises a decoder receiving the two binary multiplex signals and three current sources connected in parallel which are selectively activated by the decoder according to each pair of bits from these binary multiplex signals.

 According to a preferred embodiment, said means for producing two binary multiplex signals from the four-level multiplex signal include three level comparators connected in parallel and producing three intermediate signals, and combinational logic arranged to reproduce the two binary multiplex signals from of these three signals.

 Advantageously, said multiplexing and demultiplexing means comprise two shift registers mounted in parallel.

 In the case where the device is arranged to receive three black and white video signals, each analog video signal is converted into six bit channels and an additional bit channel is provided for the transmission of said determined sequence and another additional bit channel for the transmission of a digital signal, these twenty binary channels being divided into two groups of ten channels from which the two above-mentioned binary multiplex signals are produced.

 Advantageously, the analog / digital conversion means comprise sampling means.

Other details and advantages of the invention will appear during the following description of a preferred but non-limiting embodiment, with reference to the appended drawings in which
Figure 1 shows schematically the
electronic circuit relating to the transmitter part of the
transmission device according to the invention.

Figure 2 shows the electro circuit
nique relating to the corresponding receiving part.

Figure 3 specifies the composition of the train
digital four-level broadcast on the single channel.

 Figure 4 is a time diagram of the four-level digital train.

Figure 5 shows the tempo diagrams
of the three binary signals resulting from the processing
of the signal of figure 4 by the three comparators
241, 242, 243.

The transmitter of the transmission device represented
shown in Figure 1 includes three analog converters
that / digital 10, 20, 30 respectively receiving their
input 17, 27, 37 a video signal relating to an image of
black and white television. These signals come from
for example three remote surveillance video cameras
arranged on a site, near the trans device
mission to which they are connected by three electrical cables
coaxial triques.

Each analog / digital converter
encodes the input signal: the video signal ana
logic is sampled at a frequency fe each sample
tillon being transformed into a determined binary number tra
doubling its amplitude. Given the application envisaged
and the fact that the image is in black and white, it has been
considered that a six-bit binary number was sufficient
to restore an image.

Furthermore, the maximum frequency of the spectrum of analog black and white video signals being gold
dre of 5 MHz, the sampling frequency was
e
chosen equal to 9.6 MHz.

 The analog / digital converters are those sold by the company ANALOG DEVICE under the reference AD 9000.

 The transmitter of FIG. 1 also comprises a digital generator 40 arranged to repeatedly produce a determined sequence of sixteen bits bearing the reference 47 in FIG. 3, called the pseudo-random sequence. The bits "0" or "1" constituting this sequence are transmitted successively at a determined frequency. As indicated below, this pseudo-random sequence will allow resynchronization of the video signals on reception.

 The transmitter also has an input 57 for a digital signal, for example a video camera control signal, which may have a bit rate less than or equal to I Mbits / s.

 According to the invention, the aforementioned twenty digital channels are divided into two groups of ten channels each which respectively attack two shift registers 71, 72. Each shift register comprises ten bistable flip-flops connected in series and therefore has ten parallel inputs and one output series. Thus, the six digital channels of the converter 10 and four digital channels of the converter 20 attack the register 71, while the other two digital channels of the converter 20, the six channels of the converter 30 and the two digital channels relating to the pseudo-random sequence and at digital signal attack register 72.

 In a manner known per se, under the effect of a loading control signal C, all the flip-flops simultaneously assume a state corresponding to the bit present at their input, and under the effect of an offset signal D, the container of each flip-flop is shifted towards the flip-flop connected to its output until the ten loaded bits are output sequentially from the shift register.

 A clock signal H of frequency 96 MHz imposes the rate of shift of the registers 71, 72, so that at the output of these are produced two digital binary trains of bit rate 96Mbits / s according to two voltage levels e and " 1 ".

 In practice, each shift register with ten flip-flops is obtained by combining two shift registers with five flip-flops, themselves composed of a register with four flip-flops to which a flip-flop is added.

 Two scramblers 81, 82 are respectively connected to the outputs of the shift registers 71, 72 and themselves include two shift registers with four rockers, looped back. In a manner known per se, the function of a scrambler is to produce at its output bits "O" and "1" in a pseudo-random manner, while the information present at its input does not necessarily have this randomness . The goal is to create a large number of transitions favoring the recovery of the line of the clock signal by the receiver.

A logic 90 receives at its inputs e1, e2 the signals coming from the two scramblers 81, 82, and at an input e3 the clock signal H. According to the invention, this logic is arranged to produce at its output a signal whose l amplitude is proportional to
s = el + 2e2 which can still be defined by the table (I below, where e1, e2, s denote the signals present at the inputs and the corresponding output and "1" denotes the maximum amplitude of the signal considered
TABLE I
Nature of e e2
signal 1 2
OO levels
electric 1 0
possible 0 1
for this 1 1
signal
It will be noted that one transforms two binary multiplex signals e1, e2, that is to say at two electrical levels, arriving on two channels into a single multiplex signal s at four electrical levels which will be transmitted on a single channel.

 In practice, three current sources 94 to 96 are advantageously used, arranged in parallel and producing currents of prospective amplitudes 1/3, 2/3 and 1. Each current source is connected in series with a gate 91, 92 , 93 controlled by a decoder 97, which receives the two signals e1, e2 as input. In use, the decoder 97 receiving for example at its input the values (0, 1), it commands the closing of the gate 92 to transmit a level 2/3 current produced by the current source 95, until the arrival of two new values (el, e2).

 Furthermore, the clock signal e3 is transmitted directly to the output s.

 This signal s is amplified by an amplifier 100 which attacks a fast light-emitting diode 110 emitting laterally infrared radiation of wavelength equal to 850 or 1300 nm.

The light emitting diode 110 emits at the end of an optical fiber of the index gradient type, for example a fiber 50/125 having a core diameter equal to 50 m and a cladding diameter equal to 125 / ut
As a variant and in replacement of the decoder 97, of the doors 91 to 93, and of the current sources 94 to 96, one could use a summator with two resistances.

 It should be noted however that, if this second solution can be used, that of FIG. 1 is preferable because it allows faster production of the signal at four levels s.

 The composition of the signal s at four levels is specified in FIG. 3. Each elementary information of this signal translates two binary information originating simultaneously from the shift registers 71, 72, for example the bits (11, 25) present respectively on the first channel digital of converter 10 and on the fifth channel of converter 20, then the bits (12, 26), then the bits (13, 31) present respectively on the third digital channel of converter 10 and on the first digital channel of converter 31, etc ... down to bits (24, 51).

 The entire content of registers 71, 72 is thus transferred repeatedly over the optical fiber.

With regard to the information 41-successively coming from the pseudo-random sequence generator 40, it will be noted that it corresponds to the different bits of this sequence, read in order. In FIG. 3, the two pieces of information 41 correspond to the first two bits "0", "O" of the sequence.

 The receiver of the transmission device is shown in FIG. 2. The photoreceptor means comprise a PIN diode 200 disposed at the other end of the above-mentioned optical fiber, which drives an amplifier 210. An automatic gain control device 211 is mounted in feedback on this amplifier and allows in a manner known per se to restore the electrical signal s at four levels whose maximum amplitude has a constant value, equal here to 1.5 V.

 The DC component of the four-level signal, canceled before its emission on the optical fiber, is recovered in a manner known per se by locking in a device 220.

The means for recovering, from the digital signal or train at four levels, two binary digital signals or trains, comprise three comparators 241 to 243 mounted in parallel at the output of the device 220 for recovering the DC component. These are ultra-fast comparators marketed by the Company
ANALOG DEVICE under the reference AM 685.

 These three comparators 241 to 243 produce at their output respectively three binary signals c1, c2, C3 at two electrical levels "O" or "1" depending on whether the level of the signal at their input is less than or greater than a determined and equal threshold. 1/6, 3/6 and 5/6 respectively of the maximum amplitude of the four-level signal.

In FIG. 5, the amplitude of the signals c1, c2, c3 is represented as a function of time. In FIG. 4, the amplitude of the signal s at four levels is represented as a function of time, and the thresholds of the three comparators 241 to 243 are represented in dotted lines.

 From the above table (I) and FIGS. 4 and 5, the following table G (II) can be drawn, which specifies the relationship between on the one hand the levels of the signals c1, c2, c3 and on the other hand that of the signal s, that is to say that of the signals e1 and e2.

TABLE II
s c1 c2 c3 e1 e2
1st level
none O 0 0 0 0
2nd level 1 0 0 1 0
3rd level 1 1 0 0 1
4th level 1 1 1 1 1
The following two equations can be deduced from table (II) and show that we can find the signals e1 and e2 produced by the transmitter, from the three signals c1, c2, c3
e = c3 + c1 'c2 e2 = C2
The production of the two signals e1, e2 from the signals c1, c2, c3 is ensured by a combinational logic 250. Then, the two signals e1, e2 are processed by two descramblers 261, 262 which are dual circuits of the scramblers 81, 82 of the transmitter and therefore restore the signals as they appear at the input of these scramblers 81, 82.

These signals are received respectively by two shift registers 271, 272 having a serial input and ten parallel outputs. Control signals
(D, A) registers make it possible to ensure the offset of the information arriving at their input to load the registers, then the stopping of this offset to transmit this information in parallel. At the outputs of the two registers 271, 272 are connected three digital / analog converters 310, 320, 330 reproducing at their outputs 317, 327, 337 the three analog video signals present at the input of the transmitter, while the digital signal is supplied directly by the register 272 to an output 357.

 So that the multiplexed signals can be correctly routed to the corresponding outputs of the shift registers 271, 272 and in a manner known per se, a device 340 for recognizing the pseudo-random sequence 47 (FIG. 3) receives the relative demultiplexed signal as an input. to this sequence and coming from the shift register 272, and outputs a control signal to the two shift registers 271, 272. This recognition device 340 comprises in a manner known per se a shift register and a logic controller connected in series .

 A device 230 for recovering the clock signal H is mounted at the output of the amplifier 210.

It comprises, in a manner known per se, a very narrow band phase-locked loop formed from a
VCO (voltage controlled oscillator). The clock signal
H recovered is applied to combinatorial logic 250 and shift registers 271, 272.

 All electronic functions of the transmitter and receiver are carried out using ECL 10,000 technology circuits.

 It will be noted that the transmission device described above offers great flexibility of use since it has nineteen digital channels usable for transmitting information, for example nineteen digital signals with a 1Mbit / s speed, by eliminating the analog converters. / digital and digital / analog.

Any other assignment is also possible, for example that of keeping six digital channels for the transmission of a black and white video signal and thirteen channels for the transmission of thirteen digital signals.

 The transmission device according to the invention can find different applications in which it is necessary to transmit information, and in particular images, remotely. The useful transmission distance for this device is 9 km on a 50/125 fiber. It is thus possible to connect an underwater observation vehicle to a boat, a remote surveillance camera placed on a site to a surveillance room, etc.

 In the present example, the number of levels of the multiplex signal s on the single channel has been chosen equal to four, which leads to a degradation of 4dB of the signal / noise ratio in comparison with the two binary signals e1, e2. The choice of a larger number of levels is possible, but may lead to an unacceptable deterioration of the signal / noise ratio and force the replacement of the digital comparators of the receiver by complex analog devices.

 Doubling the number of levels advantageously makes it possible to divide the transmission frequency in a ratio 2.

Claims (8)

 (200) for receiving said multiplex signal and restoring it in electrical form, and means (271, 272, 310, 320, 330) for demultiplexing the multiplex signal and restoring the Video signals, characterized in that it comprises means ( 10, 20, 30) for carrying out a binary analog / digital conversion of said analog video signals before multiplexing, the multiplexing means being arranged to produce a multiPlex signal having a number of electrical levels greater than two, the demultiplexing means being arranged to restore , from the multiplex signal, the video signals in binary digital form, means (310, 320, 330) being provided for performing digital / analog conversion of the video signals.  (110) for transforming said multiplex signal into light form at one end of an optical fiber and, at the other end of the optical fiber, photoreceptor means l.Video signal transmission device, comprising means (10, 20, 30, 71, 72) for multiplexing said signals available in analog electrical form into a single multiplex signal, electroluminescent means 2. Device according to claim 1, in which the multiplexing means are arranged to produce two binary multiplex signals simultaneously, means (90) being provided for producing, from these two binary multiplex signals, a single four-level multiplex signal electrical, and means (241 to 243, 250) being provided for producing, from the four-level multiplex signal from the photoreceptor means, two binary multiplex signals received by said demultiplexing means. 3. Device according to claim 1 or claim 2, which comprises a generator (40) arranged to repetitively produce a determined sequence of bits which is transmitted at the same time as the video signals, and a device (340) arranged to recognize this sequence and which cooperates with said demultiplexing means. 4. Device according to claim 2, wherein said means for producing a multiplex signal at four electrical levels comprises a decoder (97) receiving the two binary multiplex signals and three current sources (94, 95, 96) connected in parallel which are selectively activated by the decoder according to each pair of bits from these binary multiplex signals. 5. Device according to claim 2, in which said means for producing two binary multiplex signals from the four-level multiplex signal comprise three level comparators (241, 242, 243) connected in parallel and producing three intermediate signals, and logic combinatorics (250) arranged to reproduce the two binary multiplex signals from these three signals. 6. Device according to revenidcation 2, wherein said multiplexing and demultiplexing means comprise two shift registers (71, 72; 271, 272) mounted in parallel. 7. Device according to claim 3 arranged to receive three black and white video signals, in which each analog video signal is converted into six bit channels and an additional bit channel is provided for the transmission of said determined sequence and another bit channel. additional for the transmission of a digital signal, these twenty binary channels being divided into two groups of ten channels from which the two aforementioned binary multiplex signals are produced. 8. Device according to any one of the preceding claims, in which analog / digital conversion means comprise sampling means.