FR2511567A1 - TV signal transmission system for optical fibre cable - uses passband filters for sound and video channel multiplexing in transmitter and receiver - Google Patents

Abstract

The system includes a transmission module (10) which contains a frequency modulator (11) for the sound channel (S) to limit intermode effects in the video band. The modulator output and the video signal (V) are applied to a multiplexer (12) containing passband filters (14,13) for the respective channels. The multiplexer output is applied to a latching circuit (19) to ensure that the photodiode (17) is operated over the linear part of the transfer function characteristic. A voltage to current converter (18) drives the photodiode from the multiplexer output to transmit the signals along an optical fibre (30). At the receiver a photodiode output is applied to a demultiplexer containing passband filters to separate the sound and video signals. The sound channel signal output is demodulated.

Description

TELEVISION SIGNAL TRANSMISSION SYSTEM,
AND TRANSMITTER AND RECEIVER MODULES OF SUCH A SYSTEM
The present invention relates generally to television signal transmission systems and relates more particularly to transmitter and receiver modules used in the transmission of television signals.

 A system is already known for transmitting television signals. In general, it comprises a transmitter module and a receiver module connected together by a line or transmission channel constituted by a coaxial cable.

 To ensure the transmission of television signals constituted by video signals associated with a sound channel, on a coaxial cable, the transmitter module used essentially comprises a multiplexer of the video signals and the sound, and a current generator electrically connected to the coaxial cable and injecting into the latter, under the form of a current, the multiplexed signals. Similarly, the receiver module used mainly comprises a demultiplexer of the television signals transmitted over the coaxial cable, and reproducing the video signals and the sound.

 However, a system for transmitting television signals over coaxial cable of the type described above has drawbacks. Indeed, the use of a coaxial cable as a transmission line has electrical characteristics which are insufficiently efficient for the transmission of television signals; this is particularly characterized by the fact that a coaxial cable is particularly sensitive to electromagnetic interference of all origins, and has high losses. In addition, the respective structures of the transmitter and receiver modules of this transmission system use relatively complex and therefore expensive electronic circuits.

 The object of the present invention is to remedy these drawbacks by proposing a transmission of television signals carried out on a new type of transmission line, which is entirely satisfactory, and uses electronic equipment for transmission and reception of simple structures. and little crusty.

 To this end, the subject of the invention is a system for transmitting television signals intended to be transmitted over a transmission line, comprising a module transmitting television signals and a module receiving said signals, characterized in that the transmission line is an optical fiber arranged between the transmitter and receiver modules.

 Thus, the use of an optical fiber as a transmission line for television signals has all the technical advantages inherent in optical fibers, namely perfect insensitivity to electromagnetic interference, high transmission capacity, reduced bulk and weight. Consequently, the optical transmission system for television signals is very efficient and makes it possible to use long optical links.

 To transmit television signals over optical fiber, the transmitter module used mainly comprises a light source, such as a light emitting diode, optically coupled to one end of the optical fiber, and modulated by television signals.

 Similarly, the receiver module used mainly comprises a photodetector coupled to the other end of the optical fiber, and detecting the television signals transmitted on said fiber.

 Other characteristics and advantages of the invention will appear better in the detailed description which follows and refers to the appended drawings, given only by way of example and in which: - Figure I represents a general diagram of the transmission system according to the 'invention; - Figure 2 shows a diagram of the transmitter module for the system according to the invention, and - Figure 3 shows a diagram of the receiver module for the system according to the invention.

 According to an exemplary embodiment and with reference to FIG. 1, a system for transmitting television signals constituted by video signals associated with a sound channel, comprises a transmitter module 10, a receiver module 20 and a single mode optical fiber 30 or multimode, serving as a transmission line, and the ends A and B of which are optically coupled to the transmitter 10 and receiver 20 modules, respectively.

 FIG. 2 represents a diagram of the transmitter module 10 of the transmission system according to the invention.

 This transmitter first comprises a video signal channel V and a low frequency sound channel S connected to a frequency modulator 11, of conventional structure, intended to modulate the sound carrier at a frequency whose value is chosen so as to limit the influence of intermodes in the videotape. It should be noted that the frequency modulator 11 could be replaced by an amplitude modulator, without departing from the scope of the invention.

 Means 12 are shown in FIG. 2 for means for multiplexing the video signals and the frequency-modulated sound carrier. More specifically, the multiplexer 12 comprises a low-pass filter 13 whose input E1 is connected to the video channel V, and a band-pass filter 14 whose input E2 is connected to the sound channel S. The respective outputs of the two filters 13 and 14 are connected to each other at Q constituting the output of the multiplexer.

 The multiplexer 12 is connected to an emission base 16 comprising a light source 17, such as for example a light-emitting diode or a laser diode, the anode of which is connected to ground, and intended to emit, at a length of determined wave, light radiation corresponding to the television signals to be transmitted over the optical fiber 30.

 The transmission base 16 also includes means 18 for converting the voltage delivered by the multiplexer 12 into a current called modulation of the light emitting diode 17. This converter 18, of conventional structure, comprises for example two amplifiers (one of voltage and one of current) connected in series with a feedback loop.

 FIG. 2 shows in FIG. 2 means for blocking or locking the bias voltage of the light emitting diode 17 at a determined value corresponding for example to the average value of the optical power emitted by said diode 17.

These blocking means 19 thus ensure a prepolarization of the diode 17 on the linear part of its known transfer characteristic (output power as a function of the current).

 FIG. 3 represents a diagram of the receiver module 20 of the transmission system according to the invention.

 This receiver first comprises a photodetector 21, such as for example an avalanche photodiode, the anode of which is connected to ground via a load resistor R, and the cathode of which is connected to a supply + U.

 The photodetector 21 converts the light radiation received after propagation in the optical fiber 30 into an electrical signal generated at the terminal T of said photodetector.

 This electrical signal is then presented to a demultiplexer 22 intended to restore the video signals and the sound composing the television signals emitted by the transmitter module as described in FIG. 2. This demultiplexer 22 comprises a low-pass filter 23 and a filter bandpass 24, the respective inputs of which are connected together in N constituting the input of the demultiplexer 22.

The low-pass filter 23 generates at its output S1 the video signals, and the band-pass filter 24 generates at its output S2 the sound channel S,
near passage in a demodulator 26, for example of frequency, and of conventional structure.

By way of illustration, the Applicant has produced a system for transmitting television signals over a one kilometer optical fiber, with video signals having a bandwidth of the order of 6 MHz, and with a sound channel having a bandwidth of the order of 50 KHz. The light emitting diode used is a light emitting diode with a wavelength equal to 0.86> μm, and the photodetector used is a semiconductor diode of the type
PINE.

 Of course, the invention is in no way limited to the embodiment described and shown and includes all the technical equivalents of the means described as well as their combinations if these are carried out according to the spirit of the invention and implemented in the context of the following claims.

Claims (14)

 1. A system for transmitting television signals intended to be transmitted over a transmission line, comprising a transmitter module (10) for television signals and a receiver module (20) for said signals, characterized in that the transmission line is a optical fiber (30) arranged between the transmitter and receiver modules.  2. System according to claim 1, characterized in that the television signals comprise video signals (V).  3. System according to claim 2, characterized in that the video signals are associated with a sound channel (S).  4. Transmitter module of a television signal transmission system intended to be transmitted over a transmission line, comprising a video signal channel (V), characterized in that the transmission line is an optical fiber (30), and in that the module (10) comprises a light source (17) disposed opposite the optical fiber (30), the video signals modulating the light source.  5. transmitter module according to claim 4, characterized in that it also comprises means (19) for blocking the bias voltage of the light source (17) at a determined value, the output of the blocking means being connected to the light source.  6. Transmitter module according to one of claims 4 and 5, characterized in that it further comprises a converter (18) voltages current of video signals, connected between the blocking means (19) and the light source (17).  7. Transmitter module according to one of claims 4 to 6, characterized in that it comprises a sound channel (S) and means (12) for multiplexing video signals and sound, the signals generated by the multiplexing means modulating the light source.  8. Transmitter module according to claim 7, characterized in that the multiplexing means (12) comprise a bandpass filter (14) whose input (E2) is connected to the sound channel, and a low pass filter (13) whose input (El) is connected to the video channel, the respective outputs of the two filters being connected to each other.  9. Transmitter module according to one of claims 7 and 8, characterized in that the sound channel (S) comprises a frequency modulator (11) connected to the input of the multiplexing means (12).  10. Transmitter module according to one of claims 4 to 9, characterized in that the light source (17) is a light emitting diode.  He. Receiver module for a system for transmitting television signals transmitted over a transmission line, characterized in that the transmission line is an optical fiber (30), and in that the module (20) comprises a photodetector (21) arranged opposite the optical fiber (30) and detecting the transmitted video signals.  12. receiver module according to claim 11, characterized in that the video signals are associated with a sound signal, and in that the module also comprises means (22) for demultiplexing the video signals and the sound, having an input (N ) connected to the photodetector (21) and two outputs (S1; S2) connected to the video channel (V) and the sound channel (S), respectively.  13. Receiver module according to claim 12, characterized in that the demultiplexing means (22) comprise a bandpass filter (24) whose output (S2) is connected to the sound channel, and a lowpass filter (23 ) whose output (S1) is connected to the video channel, the respective inputs of the two filters being connected together, thus constituting the input (N) of the demultiplexing means.  14. Receiver module according to one of claims 12 and 13, characterized in that the output (S2) of the demultiplexing means (22) connected to the sound channel is connected to a frequency demodulator (26).  15. Receiver module according to one of claims 11 to 14, characterized in that the photodetector (21) is an avalanche photodiode.