GB2165129A - Digital data transmission with a video signal - Google Patents

Abstract

Digital data is transferred from a video screen, such as from the CRT screen of a television receiver, into a micro-computer, by modulating a small area of the screen on successive television field scans. On each field scan, a plurality of data bits are modulated respectively onto a plurality of adjacent lines within the area. Thus, e.g. one or two (Fig 3) complete bytes can be transferred on each field scan. The data may be detected on the CRT screen by a light-pen device. In a further embodiment (Fig 2) many data channels may be transmitted by modulating several areas simultaneously. Each area may comprise either complete or partial horizontal line scans. <IMAGE>

Description

SPECIFICATION Digital data transmission with a video signal This invention relates to the transmission of digital data with a video signal.

There is now a demand for the transmission of computer programs or other digital data to, for example, the receiver of a television broadcast. It has been proposed to transmit this data over the television sound channel, but this necessitates an interruption in the main content of the broadcast and the sound itself is strident.

It has been proposed in our British Patent Specification 2,044,446 to transmit data using the video display itself. In the example shown in the drawings of that specification, a small area of the screen is reserved at the appropriate time, for this data transmission and is illuminated on successive television field scans in accordance with respective data bits to be transmitted. A light pen (i.e. a manipulable electronic light sensor) can be held against this area of the screen to sense the illumination and provide an electrical output which, after appro- priate waveform shaping, gives a digital representation of the data. Thus data can be transmitted at a rate of 50 bits per second and this data can for example by used to control receiver functions. Although well suited to this limited purpose, this system can handle only relatively low data rates.

Other known proposals exist by which a user can select an option displayed on a video screen with the aid of a light pen. Examples are United States Patents 3,181,154; 3,492,657 and 4,017,680. In these systems several screen areas are identified and the user selects one of the areas by holding a light pen or the like against that area. The selected area is detected by circuitry which relates the instant at which the light pen produces an output to the scanning raster so as to identify the position at which the light pen is being held on the screen.

The circuitry which provides this timelposition or co-ordinate detection system is complex, and again the data transfer rate is low.

United States Patent 3,292,489 describes a video terminal in which various items displayed on the screen each have displayed next to them a unique code, which is in effect somewhat like a bar code.

A light pen can be swept across the screen by a user to read the code. This system essentially constitutes a mechanical analogue of the now wellknown printed bar code system. The data rate is again small as the light pen has to be moved across the screen to read each data word.

The present invention provides an improvement over these known systems and is defined in the appended claims.

An embodiment of the invention will be described in more detail in which the digital data is transmitted over the television video channel so as to provide a pulsating image on the screen of the cathode ray tube of a television receiver. This image can be confined to a small area of the display, the remaining area of the screen being used for normal picture transmission and the audio channel being used for normal sound transmission. The data-carrying area of the screen is monitored by an appropriate sensor, for example a photo-sensitive device such as a light pen or a device which responds to secondary emissions due to the electron beam impinging on the screen. The sensor provides output pulses which may be shaped to provide recovered data.

Each data bit is transmitted as part of one or more horizontal lines of the scanning raster. The sensor is large enough to cover several scanning lines, and thus one or more multi-bit characters can be transmitted in every field period. The data may be recovered using conventional synchronous or asynchronous techniques; the bits arising serially as the successive lines in the scanning raster are scanned.

The main content of the broadcast is uninterrupted, either in its sound or vision aspects. However, many data channels can be established, if desired, on different areas of the screen or using different colours for the different channels.

The invention will be described in more detail, by way of example, with reference to the drawings, in which: Figure 1 is a diagram representing a small area of screen used for transmitting a single eight-bit data byte with start, parity and stop bits; Figure 2 is a diagram representing a video screen being used both for normal video picture information and for nine silmultaneous data channels; and Figure 3 is a diagram similar to Figure 1 showing how two eight-bit bytes can be transmitted in the same frame period.

The apparatus required to implement the present invention can be based on that of British Patent Specification 2,044,446, to which reference should be made, and will not therefore be described in detail. An indication of the changes necessary is given below.

Figure 1 shows a small area of the screen, which can be defined by a line round it forming a square, and illustrates a small part of each of eleven successive lines of the horizontal line scan. Each line in this area is used to indicate a different bit. However it will be appreciated that a binary data bit can alternatively be transmitted as adjacent lines.

The next data bit follows on the next line though it would alternatively be possible to separate data bits by one or more dark lines.

In Figure 1 is illustrated the asynchronous transmission of an 8-bit data byte complete with start, parity and stop bits. As shown the width of the area used to transmit the byte is only part of the horizontal line scan of the screen, though for simplicity the full width of the screen could be used.

When part only of the line scan is used several data channels can be accommodated on the same horizontal strip of screen simultaneously by transmitting them adjacent to each other, as shown in Figure 2 with channels 4 to 8. More data channels still can be accommodated by utilising areas on other horizontal strips of the screen as shown for channels 1 to 3 and 9. Thus the example of Figure 2 utilises 9 channels for data, but as the data can be placed anywhere on the screen and need only consist of a very small portion of the horizontal and vertical scan, many more could be a accommodated. It will be appreciated that Figure 2 is very diagrammatic in this respect and the data-carrying areas will normally be much smaller than shown. The portions of the screen not used as above can still be used for normal picture transmission, as indicated.

Any desired area of screen, illuminated as above, can be monitored by either a photo-sensitive device such as a light pen or some other sensor which detects the presence of, or secondary effects from, the electron beam. The pulses obtained from the sensor are appropriately shaped, and appear as recovered data at an outgoing serial data port. One data byte can thus be recovered on each field scan.

The equipment can be based on that of Figure 3 of our specification 2044446, modified to take account of the higher data rate now obtained in accordance with this invention.

The system can thus be used to down-load broadcast computer programs into a local microcomputer via its serial input data port. A baud rate converter or other buffering device may be found to be desirable. However, the invention is also applicable to non-broadcasting systems. For example, it provides a very simple system by which the user of a micro-computer can select one of a number of options displayed on the VDU screen by placing a light pen at a spot associated with that option. In either event it will usually be convenient to apply the output to some form of electronic storage device. The bit rate can thus be increased to 400 data bits per second if all the interlaced scans of the raster scan are used. Alternatively alternate fields could be blank over the spot on one set of the alternate scans, in which case the bit rate is halved.

The sensor can be coupled to the data-carrying area of the screen by an optical fibre which has its input end directed at that area of the screen by the user. This end of the optical fibre may conveniently be held by a rubber suction element which adheres by its suction to the television screen.

The optimum acceptance angle or effective area for the sensor or the input end of the fibre which is placed against the screen needs to be found empirically. Clearly it must be large enough to cover all the lines being used for data transmission. It should also be a little larger than that to allow for some tolerance in placement on the screen. However it should not be so much bigger that the information in the data-carrying areas is diluted by light from other adjacent areas on the screen.

It is possible to transmit more than a single byte of data in any single frame period if the acceptance angle or area of the sensor is made sufficiently wide. Figure 3 shows two 8-bit bytes, with no parity, being transmitted in the same frame period.

This is done by utilising a multiple of the number of lines used to transmit a single byte.

The transmitted data can be introduced at the transmitter using apparatus based on Figure 2 of our specification 2,044,446. Again, account has to be taken of the increased data rate so that the different lines in each area defined by the windows carry information relating to different data bits.

Further implementational details may be found in Electronics Letters Vol 21, No. 10, pages 444 to 445.

It will be seen that the system illustrated provides a method of transmitting digital data via a video screen and also apparatus for monitoring the image on the screen and recovering the digital data from it, in which the data transfer rate can be several times higher than with the system of our specification 2,044,446. Nevertheless the data can be continously transferred by keeping the sensor fixed on the appropriate spot on the screen.

Claims (10)

1. An electronic data transfer system comprising a two-dimensional raster scanned display, means for modulating by the data to be transferred one or more areas of the display during successive field scans, sensor means adapted selectively to be fixedly placed in front of a small part only of the display for data transfer purposes, and detecting means connected to the sensor means to detect the modulation in the sensor output to form a pulse train of coded information, and in which a plurality of data bits are transmitted on each field scan be modulation of different regions of the or each said area.

2. A system according to claim 1, in which the said regions are defined by different lines of the scanning raster.

3. A system according to claim 1 or 2, in which the display comprises a cathode ray tube.

4. A system according to claim 1, 2 or 3, in which the display is part of a television receiver.

5. A system according to any preceding claim, in which the sensor means comprises a manipulable photo- sensitive device.

6. An electronic data transfer system substantially as herein described with reference to the accompanying drawings.

7. A video signal formed by raster scanning, in which one or more small areas of the image represented by the signal carry data modulated thereon during successive field scan, and in which a plurality of data bits are transmitted on each field scan by modulation of different scan lines in the or each said area.

8. A video signal transmitter comprising signal generating means for generating a video signal by raster scanning, and data modulating means for including in the video signal one or more signal portions corresponding to small areas of the image represented by the video signal which carry data modulated thereon during successive field scans, and in which the data modulating means modulates a plurality of data bits into the video signal on each field scan by modulation of different scan lines in the or each said area.

9. A method of data transfer, comprising the steps of providing a two-dimensional raster scanned display, modulating by the data to be transferred one or more areas of the display during successive field scans, selectively piercing a sensor in front of a small part only of the display including one of the said areas for data transfer purposes, and detecting the modulation in the sensor output to form a pulse train of coded information, in which a plurality of data bits are transmitted on each field scan by modulation of different regions of the or each said area.

10. A method according to claim 9, in which the said regions are defined by different lines of the scanning raster.