Television – Special applications – Hazardous or inaccessible
Patent
1992-05-26
1994-08-02
Kostak, Victor R.
Television
Special applications
Hazardous or inaccessible
348726, 348192, H04N 718
Patent
active
053350065
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention concerns the transmission of video signals in confined spaces such as tunnels and mines.
Video transmission is used in systems for remote surveillance of the environment or of a mobile plant moving in an ill-defined environment subject to interference, with the aim of viewing and guiding or controlling mobile plants; one particular example is the high data rate, analog transmission of information and telecontrol signals to a machine in a mine tunnel, a drainage tunnel or, more generally, any tunnel with a diameter in the order of 1 to 10 meters.
Until now video signals have been transmitted in tunnels for broadband communications between the mobile plant and a fixed (or at least temporarily fixed) equipment in the tunnel using radiating structures; these might be coaxial cables operating in the 200-900 MHz frequency band, for example, or microwave waveguides radiating frequencies above 1 GHz. In this way it is possible to secure continuous transmission between a mobile transmitter moving near the waveguide and a receiver at the end of the waveguide. A communication system of this kind is bidirectional as the waveguide can also be used for transmission from the fixed equipment carrying the receiver to the mobile plant carrying the transmitter.
These known systems have the disadvantage that the mobile plant is required to remain in the immediate vicinity of the waveguide (within about 1 meter) if the benefits of the homogeneity of the field are to be obtained. More importantly, these systems have the major disadvantage of requiring the installation of radiating strutures representing a costly infrastructure. The transmission system is, therefore, a fixed system devoid of any flexibility for future adaptation.
The invention is directed to alleviating these disadvantages by providing a microwave video transmission system establishing continuous communication with mobile plants at any position in a tunnel, the communication quality being good at all times in spite of fluctuations in the received signal resulting from the numerous reflections from the tunnel walls and from the usual presence of metal obstacles between the transmitter and the receiver and attenuation due to bends or branches in the tunnel.
SUMMARY OF THE INVENTION
To this end the invention proposes a method for transmission in a confined space between mobile or fixed transmit and receive areas of a video signal formed by a succession of video picture line signals beginning with a known format synchronization signal of known period representing a known line synchronization frequency. In the transmit area the video signal modulates a carrier signal at a frequency in the microwave band, the modulated carrier is transmitted on a microwave link to the receive area, in the receive area two signals are received on two angularly offset privileged axes, the received signals are demodulated in respective identical processing channels, the demodulated received signal from a predetermined initial channel is intially applied to picture forming equipment for as long as there is detected in it the presence of a periodic signal with a frequency equal to the video line synchronization frequency, and the channel is changed when this periodic signal is no longer present on this channel but is detected on the other channel, and so on.
The essential characteristic of the invention is, therefore, fast picture discrimination by testing for the presence of the synchronization signal line or over a predetermined number of lines or frame by frame. This produces a clear enhancement of the picture reconstituted from the signals received in the receiving area. Even if there are significant fluctuations, given the very short response time (response time of one line to switch from one channel to the other), under no circumstances can more than a few lines be lost, which in practice may be imperceptible. This is much better than discrimination based on comparing the amplitudes of signals on the two channels.
This low sensitivity to
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Baudet Jacques
Charpentier Dominique
Degardin Daniel
DeGaugue Pierre
Charbonnages de France
Kostak Victor R.
VanOphem Remy J.
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