GB2106357A - Video transmission system for transit vehicles - Google Patents

Abstract

Standard television transmission and reception is used at very low power for vehicle surveillance in a mass transit system which includes two short parallel conductors mounted beneath the vehicle and running along an open-wire transmission line disposed along the dedicated roadway of the vehicle.

Description

SPECIFICATION Video transmission system for transit vehicles The invention relates to a wayside communication system for moving cars or trains, in general, and more particularly to a television transmission and receiving system for public transportation.

For ensuring the safety and comfort of passengers in public transportation systems, it is desirable to have continuous television monitoring inside the car or cabin, especially in mass transit systems where free entry and exit of passengers exists with an absence of attendant personnel at the doors and in the car. Television monitoring systems are well known and are suggested to be installed on elevator systems. Despite the similarities in modern transportation between horizontal and vertical passenger transport, a wayside communication system on a moving car or train imposes different requirements mainly because of distance and track outlay.Length of transmission in the midst of an inhabited area requires an infrastructure and a high level of efficiency in the transmission and reception without undue interference with the broadcasting and television channels nor disturbances from power and telephone lines. Accordingly, to be valid, a wayside communication system should be simple, of low cost, easy to install, and provide a good television signal quality.

It is known to use television for surveillance of unmanned vehicles in prior art automated transit systems. However, the prior art uses a slow scan television system, in which data representing still frames is temporarily stored and then transmitted over a low frequency data link. By using data rates which are extremely low relative to the rate required for standard television transmission, slow scan television appeared to be the only viable solution in order to utilize existing narrow band links and avoid interference with the surrounding wide bandwidth communication channels. However, slow scan systems can only transmit still pictures every several seconds in order to accommodate the narrowband links. The information rate being very much lower, the quality is inferior to normal television transmission. In addition, a slow scan system is inherently expensive.

There is a need for a surveillance system of much higher quality, installed at much lower cost and having greater reliability. Wayside installation is also a problem where cost and reliability are important factors.

One approach has been to use a leaky coaxial cable, namely a cable having slots cut in a pattern to leak out signals causing localized radiation. This is shown in U.S. Patent No,3,909,751 where it is used for providing mobile radio bands in the UHF, VHF, ... regions. Also, in U.S. Patent No. 3,668,573 a high frequency cable is laid out on a highway, or a track, along which a vehicle moves with its receiver or transmitter equipment for television communication. Another leaky coaxial cable installation, according to U.S. Patent No. 3,729,740 is installed along the vehicle track while a coaxial type antenna is mounted aboard the vehicle.

A different approach is to use inductive coils. Inductive loop couplers can be used in slow scan surveillance systems because of the low data rate required. Inductive coils are also employed in U.S.

Patent No. 3,766,476. A short vertical radiator located on the side of the road, or track, is said to be inductively coupled with a sensitive whip antenna on the vehicle. Mention is also made in the patent of horizontally disposed conductors buried in the road at close range of the traffic lane which respond to vertically polarized signals. The idea is to discriminate against horizontally polarized signals in order to overcome power and telephone line interference. To this effect, receptivity to electric induction field is differentiated from receptivity to magnetic induction field.

Also in the inductive coupling category, U.S. Patent No. 3,585,505 discloses means for maintaining the characteristic impedance of an open wire transmission line including a pair of widely spaced insulated conductors.

These systems, however, would not do as a normal television bandwidth communication channel.

More generally, no simple, and inexpensive way of coupling television equipment on the vehicle in motion with ground equipment at a remote location has been disclosed by the prior art.

The present invention concerns a television communication system for a moving vehicle along a predetermined road track, which system is particularly applicable to surveillance within unmanned vehicles.

The invention resides in an apparatus including electric field coupling means for data transmission between a moving vehicle scheduled to move along a predetermined roadway, and a wayside station, comprising: two parallel conductors insulatively mounted on said vehicle connected to receive signals; an open-wire transmission line insulatively mounted along the roadway of said vehicle; said parallel conductors and transmission line being closely disposed in spatial relation to each other along a running portion of said transmission line for electrical field coupling therebetween while said vehicle in motion follows the roadway; means for producing a video data signal in waveform on board the vehicle for supply to said parallel conductors for transmission from said vehicle; said transmission line being responsive to the transmitted data signal from said parallel conductors for transmission to a remote ground location.

A preferred embodiment described herein teaches a television transmitter on a moving vehicle; stationary open transmission lines running along a dedicated roadway for said vehicle; a television receiver location: and primary open lines coupled to said transmitter and mounted on said vehicle in close relationship for electrical coupling with and for movement relative to said stationary open transmission lines.

More specifically, the television signals from said transmitters are transmitted at low power, said transmitter being operatively connected to said primary open lines through a coaxial cable and balun coupling combination, said receiver being operatively connected to said stationary open transmission lines through another coaxial cable and balun coupling combination.

Preferably, the primary open lines have a length of a quarter wavelength of the television signal carrier frequency and are separated at a distance which is a substantially small fraction of said above length. The stationary secondary open lines have a spacing which is equal to the spacing of the primary open transmission lines. They are preferably in two planes parallel to the dedicated roadway, the primary open lines consisting of two parallel conductors insulatively mounted beneath the vehicle and in proximity and alignment to said stationary transmission lines.

A more detailed understanding of the invention can be had from the following description of preferred embodiment given by way of example and to be studied in conjunction with the accompanying drawing wherein: Figure 1 shows a block diagram of the television system according to an embodiment of the present invention.

Fig. 2 shows, with more details, the coupling line and the transmission line of the television system of Fig. 1.

Figs. 3A, 3B, 3C illustrate with curves the frequency responses of coupling lines of three different lengths.

Figs. 4A, 4B indicate the coupling loss vs. frequency for various vertical separations and various horizontal displacements of the coupling line relative to the transmission line.

Fig. 5 is specific circuitry for the modulator of Fig. 1 built around a commerical solid state device.

Automated transit systems that employ unmanned vehicles running on specal predetermined roadways are now operating at a number of major airports and amusement parks, while similar systems are coming into vogue for the downtown areas in cities. For downtown transit systems, the transmission of television surveillance information from inside the moving vehicles to central control sites is highly desirable. Slow scan television, however, does not meet the requirements of quality, low cost and reliability. Rather, the use of standard television is much preferred and this involves the consideration of possible means of transmission which can, at a very low-cost technique, transmit standard television signals.

Referring to Fig. 1 , the system according to a preferred embodiment of the present invention is shown in block diagram form. A signal from a standard television camera 1, including a sound signal if desired, modulates, at 2, a carrier in the VHF frequency range, typically a carrier at about 80 MHz. For standard AM modulation, the modulator 2 is a single low cost integrated circuit such as sold on the open market under the trade name "National LM 1 889". This is a chip conventionally used to couple TV game signals into television receivers. The modulated signal is conveyed via a coaxial cable 3 to a balun 4, which consists in a transformer, coupling the unbalanced signal from cable 3 to the balanced coupling line 5. The coupling line 5 will be discussed in more detail hereinafter by reference to Fig. 2.

The coupling line 5 is used for coupling the signal derived on line 6 from balun 4 on the moving vehicle to a stationary transmission line 7 running along the roadway beneath the vehicle. Transmission line 7 is itself connected at the receiving end by another balun 8 coupling the balanced signal derived on line 9 from the transmission line to an unbalanced coaxial cable 1 0 leading to a standard commercial television receiver 11.

Fig. 2 illustrates with more details the coupling line 5 and the transmission line 7. The transmission line 7 consists of a pair of open wires W1, W2, mounted on a support so as to maintain a spacing relative to one another and to the coupling line 5. The coupling line consists of two short wires L1, L2, mounted on the vehicle in alignment with the transmission line. The spacing between the transmission line wires typically is approximately 100 mm. (4 inches). The two short wires L1, L2, of the coupling line are aligned with the transmission line and positioned above it at a distance that is very small relative to the wavelength of the transmitted frequency. Typically such distance is about 1-1/2 inches.

The wires that make up the coupling line are approximately one-quarter wavelength long, or about 33 inches for an 80 MHz carrier. For this length, the unterminated coupling line 5 when by itself, e.g. not coupled to the transmission line, appears as a short circuit to the balun 4 that drives it. When it is coupled to the transmission line 7, a resistive component is produced corresponding to power induced in the transmission line by the electric field from the coupling line.

Since television receivers are very sensitive, very little transmitter power is required into the coupling lines. Transmission of the least power possible is desirable in order to avoid excessive radiations. The power output from the chip used as modulator 2 is only 10 microwatts. This power level has proven to be adequate for good quality pictures and sound from any point on a 245 meter (750foot) track. Since the stations in a down-town People Mover system are about 327 meters (1000 feet) apart, this power level should also be adequate for surveillance of vehicles approaching or departing from the stations.

For a given level of transmitted power, the factor limiting transmission range is attenuation of the transmitted signal as it propagates along the transmission line. The attenuation has been experimentally determined to be åpproximateiy 1.5 decibels per 32.7 meters (100 feet) of transmission line. As the distance of the vehicle from the reception point increases, the received signal level decreases. On a sufficiently long line it would decrease until the picture on a television receiver would eventually become too noisy for use in surveillance. However, the transmission range associated with a given transmitter power level can be greatly extended, if necessary, by inserting repeater amplifiers at periodic intervals along the transmission line to compensate for line attenuation.Single module wideband amplifiers are currently available that can be used for this purpose.

Also, as illustrated in the preferred embodiment, electric coupling of coupling lines like L1, L2, on Fig. 2 with an open wire transmission line like WI , W2, on Fig. 2, is effected in the VHF frequency band, typically at a frequency of at least 70 MHz. The quarter wavelength at such carrier frequency permits the use of relatively small coupling lines, with wires L1, L2 at relatively close distance to each other.

Experiments have shown that when using the LM 1889 modulator chip for modulator 2, e.g. with a power of only 10 microwatts, no amplification is required to operate on television channels 5 or 6, which have center frequencies of 79 MHz and 85 MHz respectively. As a result, a standard camera picture and sound transmitter 1 and a standard television receiver 11 tuned on channel 5 or 6 can be used for the system of Fig. 1.

The coupling line 5 typically is a short section of open transmission line made of two parallel conductors L1, L2 mounted on an insulation plate disposed beneath the vehicle undercarriage.

Conductors L1, L2, are aligned with the transmission line that runs in a plane along the roadway and is itself suspended above the roadway, as close to the plane of conductors L1, L2 as possible. The vertical separation between the coupling line and the transmission line typically is about 2 inches to provide clearance at all points along the track. Signal power is coupled from the coupling line to the transmission line via the electric field produced by the coupling line.

The length of the coupling line is approximately one quarter wavelength at the selected television channel frequency. Since the coupling response is broadband, this length is not too critical. Figs. 3A, 3B, and 3C illustrate the coupling responses of three substantially different lengths of coupling line as measured by a spectrum analyzer in a lab test setup. The coupling and transmission lines were each 4 inches wide, and the coupling line to transmission line spacing was 1-3/8 inches. The frequency band shown in each curve is 50 to 100 MHz. The frequency of peak response and the response at 80 MHz (channel 5) for each line is as follows: Curve Line Length Peak Frequency 80 MHz Coupling A (14 inches) 355 mm.Above 100 MHz -23dB B (28 inches) 710 mm. 87 MHz -14 dB C (42 inches) 1065 mm. 70 MHz -16dB The length selected for the coupling line used corresponds to peak coupling at 82 MHz, the center frequency of the band occupied by TV channels 5 and 6.

The optimum width of the coupling line and the associated transmission line is a function of the vertical separation of the lines. For a typical vertical separation of 38 mm. (1-1/2 inches), experiments have shown that a width of 100 mm. (4 inches) is approximately optimum: Decreasing the width to (3 inches) 75 mm. increases the coupling loss of 2 dB and makes lateral alignment more critical; increasing the width to 5 or 6 inches provides little or no coupling improvement while increasing the likelihood of excessive radiated interference.

Figs. 4A and 4B illustrate the effects of various vertical separations and horizontal displacements of a (4-inch) 100 mm. coupling line relative to a (4-inch) 100 mm. transmission line. From Fig. 4A it appears that the coupling loss at 80 MHz varies with vertical separation as follows: Curve Vertical Separation Coupling Loss (80 MHz) A (7/8 inches) 22 mm. 11 dB B (1-3/8 inches) 35 mm. 14dB C (2-5/16 inches) 58 mm. 18 dB D (3-3/4 inches) 95 mm. 24 dB From Fig. 4 it appears that the coupling loss at 80 MHz varies with horizontal displacement of the coupling line as follows:: Curve Horizontal Displacement Coupling Loss (80 MHz) A (0 inches) O mm. 14dB B (1 inch) 25 mm. 1 7 dB C (2 inches) 50 mm. 24 dB The stationary secondary line is mounted insofar as possible away from material objects that could absorb power. Insulated supports at regular intervals are required for the line to maintain alignment with the roadway and to minimize the vertical separation from the coupling line mounted on the vehicle. It is desirable to minimize both vertical separation and horizontal displacement of the coupline line. Vertical separation can be varied at different positions along the roadway depending on clearance.

The modulator 2 preferably consists of a National LM 1 889 integrated solid state circuit 100 in association with the external tuning circuits recommended by National. As shown in the schematic given in Fig. 5, to the standard chip a 2N4126 transistor amplifier has been added between pins 12 and 5 of chip 100 to provide the required video input. Thus, pin 5 is connected to proceed, pin 12 is connected to the collector of the transistor; pin 14 is connected to a + 1 5v source potential and pin 13 is connected to the sliding arm of a potentiometer mounted across a ground and the + 15v potential, for modulation depth adjustment. Also a MV21 03 varactor has been added to frequency modulate the sound carrier from line 102.A one volt audio input signal applied on line 101 at the base of the transistor produces a standard 50 kHZ frequency deviation. A one volt audio input signal on line 102 is applied through the varactor device which modulates the audio carrier frequency at pin 15. At the output, the tuned circuit for channel 6 is connected between pins 8 and 9, while the tuned circuit for channel 5 is connected between pins 6 and 7. The audio signal is outputted on pin 11 for channel 6. and on pin 10 for channel 5. A channel selector switch allows the selection of channel 5 or channel 6 while selecting the audio to be outputted on the radio frequency cable.

The baluns of Fig. 2 used could be ordinary television matching transformers obtained from Radio Shack (Archer Catalog No. 1 5-1140).

To summarize, at extremely low transmitted power levels which would satisfy the FCC imposed limits, good quality reception has been demonstrated for distances up to 750 feet. Since this is roughly the distance between gates on downtowm People Mover systems, surveillance within cars approaching and departing from each gate is effective. The transmission range can be greatly extended without increasing stray radiation by merely inserting single integrated circuit repeaters periodically along the transmission line.

Although the invention has been described in the context of television transmission and reception, it is understood that the electrically coupled parallel wires or the vehicle and transmission line on the wayside could be used to more generally transmit a data signal carrying information for instance in binary form, as well.

Claims (12)

1. Apparatus including electric field coupling means for data transmission between a moving vehicle scheduled to move along a predetermined roadway, and a wayside station, comprising: two parallel conductors insulatively mounted on said vehicle connected to receive signals; an open-wire transmission line insulatively mounted along the roadway of said vehicle; said parallel conductors and transmission line being closely disposed in spatial relation to each other along a running portion of said transmission line for electrical field coupling therebetween while said vehicle in motion follows the roadway; means for producing a video data signal in waveform on board the vehicle for supply to said parallel conductors for transmission from said vehicle;; said transmission line being responsive to the transmitted data signal from said parallel conductors for transmission to a remote ground location.

2. The apparatus of claim 1 with said data signal being applied to said parallel conductors from said vehicle through a first combination of a coaxial cable and a balun circuit, said balun circuit having a transformer with a primary coupled to said coaxial cable and a secondary coupled to said parallel conductors.

3. The apparatus of claim 2 with said open-wire transmission line being comprised of two parallel wires.

4. The apparatus of claim 3 with said two parallel wires and said parallel conductors having respective spacings which are substantially of the same order.

5. Apparatus as in claim 4 with said two parallel conductors having a length of the order of a quarter wavelength of said data signal.

6. Apparatus as in claim 5 with said conductors being aligned with portions of said wires of same length, said conductors and wire portions being in substantially parallel planes in close relationship.

7. Apparatus as in claim 6 with the transversal spacing of said conductors and wire portions being a substantially small fraction of a quarter wavelength of said data signal.

8. Apparatus as in claim 7 with said open-wire transmission line being connected to a remote receiver through another balun circuijt and coaxial cable combination at said remote location.

9. Apparatus as in claim 8 with said data signal being a television signal generated by a standard transmitter in said vehicle, and said receiver being a standard television receiver.

10. Apparatus as in claim 9 with said television signal being generated from a television camera and a modulator responsive to said camera and connected to the coaxial cable of said first combination.

11. Apparatus as in claim 10 with said modulator being an integrated circuit device.

12. Apparatus as in claim 11 with said balun being a double-tuned, two transformer circuit.