GB2214388A - Video Data Indicating Apparatus - Google Patents

Abstract

To convey hidden messages (words) on a TV display (12), the word(s) in digital format cause field to field background brightness variation in selected areas (13) without affecting the detail within the areas. The variation in brightness is not detectable by a human user, but only by a photo detector probe coupled to a decoder. By making the number of "0" and "1" bits in a word always different, flickering is reduced. <IMAGE>

Description

VIDEO DATA INDICATING APPARATUS The present invention relates to video data indicating apparatus in which coded signals can be displayed on a video display screen, eg. a television screen, and detected and decoded using a sensing probe. With such a system coded signals can be applied to the picture of a television screen which can be arranged to be undecipherable when viewed by a person watching the television, but which can be detected by a sensing probe handled by the person and then deciphered. In one application of this invention, the apparatus may be used, eg. as a teaching aid to test the responses of the person viewing the television picture to questions displayed on the screen.For example, if a question is displayed in a multiple answer form, with each of a number of possible answers at different locations on the television screen the sensing probe can be applied to a part of the screen associated with a chosen one of the possible answers and arranged to react to coding on the screen to indicate whether or not the answer is the correct one.

U.S. Patent No. 4608601 describes apparatus of this kind in which patches or selected areas on the screen are encoded with binary data by alternatively raising and lowering the luminance of the patch. In this prior art disclosure, the luminance level represented by the luminance signal in the selected patch or area is alternatively raised and lowered without destroying the original luminance signal spacial variations in the area, whereby the original picture content of the area is not destroyed.

It may be noted additionally that the above referred prior art specification recommends that the encoding format used to encode the patches or selected areas on the screen is such that each successive binary word has substantially the same number of binary "0" and binary "1". It is said that, as a result, the overall luminance of a coded patch or area during the course of one binary word is uneffected since the number of frames in which the luminance is raised is equal to the number of frames in which the luminance is lowered.

Whilst the above described prior art arrangement works quite satisfactorily, there is still produced some flickering of the encoded patch or area which is discernable to the eye of the viewer, though not decipherable. This flickering effect is distracting and a purpose of the present invention is to reduce it.

According to the present invention, video data indicating apparatus comprises a video display screen; video signal generating means for generating display control signals to display a video picture on the screen and incorporating modulation means for varying the display control signals to produce detectable variations of the picture in at least a selected area on the display screen without destroying the original video picture content in said area and encoding means controlling the modulation means to vary the display control signals in accordance with a desired binary code, the rate of variation and the selection of code format being such that the code is visually undecipherable when viewed on the display screen; and sensing means including a photo sensitive element to respond to said detectable picture variations and including decoding circuitry arranged to decode the binary code represented by said variations, wherein the encoding means is arranged to vary the display control signals in accordance with an asymmetrical code format having a majority of bits of one type.

Quite contrary to the teaching of the above mentioned prior art specification, the present inventions calls for the code format to be asymmetrical so that in a continuing binary encoded sequence displayed on the screen there is a preponderance of bits of one type eg. binary "1".

With an arrangement producing such a code format the effect of flicker can be substantially reduced. In the prior art arrangement, where the number of bits of one type in any one word is maintained the same as the number of bits of the other type, the effect is to accentuate the visible flickering, particularly when the bit rate produced on the screen is synchronized with the field repetition rate of the screen (particularly 50Hz or 60Hz). The result is to produce a flicker rate below 25 (or 30) Hz which is substantially more visible at such frequencies.

Preferably, the code format used in the present invention has not more that two bits of the other type in series. Conveniently then, the code format comprises successive words of predetermined bit length with each word incorporating a header section having two bits in series of the other type and a data section in which a bit of said other type is always immediately followed by one or more bits of said one type.

For a substantial reduction in visible flicker, a bit of said other type in the data section of the word is always followed by a successive plurality of bits of said one type and preferably at least four successive bits of said one type.

Alternatively, or as well, a bit of said other type may be preceded always by at least four bits of said one type.

By providing this coding format in which successive bits are primarily of this one type, eg.

binary "1", with only occasional changes usually for only one single bit, to the other type (binary "O"), the appearance of flicker on the screen is much reduced.

In another aspect, this invention also provides apparatus comprising a video display screen; video signal generating means for generating display control signals to display a video picture on the screen and incorporating modulation means for varying the display control signals to produce detectable variations of the picture in at least a selected area on the display screen without destroying the original video picture content in said area and encoding means controlling the modulation means to vary the display control signal in accordance with a desired binary code; and sensing means including a photo sensitive element to respond to said detectable picture variations and including decoding circuitry arranged to decode the binary code represented by said variations, wherein the encoding means is arranged to produce detectable variations of the colour balance of a colour picture on the display screen and the sensing means is sensitive to said colour balance variations.

An example of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 is a simplified block schematic diagram showing an example of a video signal generator and display for producing encoded areas on the screen; Figure 2 is a schematic drawing of the sensing probe itself; and Figure 3 is a graphical representation of an example of the proposed new coding format compared with the prior art format.

Referring firstly to Figure 1, video control signals are generated on a line 10 by a generator 11 for supply to drive a video display 12 which may typically take the form of a television display with the usual raster scanning having predetermined line scan and field rates. In the generator 11, patches or areas 13 on the display are defined in T.V. scan time by picture area defining circuitry 14 which operates in accordance with principles well know to those practised in this art. The selection of the patches or areas to be encoded is typically by manual control eg. through a keyboard input device (not shown) to the generator 11. An encoder 15 provides a serially encoded binary signal on line 16 synchronized with the field repetition rate of a video signal on line 17 from an external video source.The binary signals on line 16 are timed, in response to control from the picture area defining means 14 to coincide with those parts of successive video line scans corresponding to the selected area on the display at which the coded patch is to appear. Video mixer 18 modulates the video signal from the external source in accordance with the binary signals on line 16 to produce a modulated video control signal on line 10 which incorporates the binary encoded patches or selected areas.

In accordance with the arrangement disclosed in prior art U.S. Specification 4608601, the encoding in the selected areas may be such as to alternatively raise and lower the luminance signal in the selected patches on the screen, without destroying the original spacial variations of the luminance signal, so that any picture content in the selected areas is retained.

Instead of modulating the luminance signal, the present invention may operate by modulating other components of a colour video signal, eg. so as to vary the colour balance of the resultant display in accordance with the binary code. Such variations in colour balance may be selected to reduce further the visual flickering effect on the screen, compared with varying the brightness. Variations in the colour balance may still be readily detectable by photo detector means using an appropriate coloured filter.

It is known that the physiological response of the eye to colour changes is slower than the response to changes in brightness, so that a rapid variation in accordance with the coding of the red content of a patch (for example) without changing the overall brightness level of the patch, maybe far less discernible to the eye than a corresponding variation in brightness itself.

Any encoding, whether of the brightness or colour, of a patch 13 on the screen of display 12 can be detected and decoded by a probe such as illustrated in Figure 2. The probe is arranged to be hand held and has a photo detector 20 at one end feeding signals to a decoding circuit 21 powered from a battery 22. In order to detect the coding of a patch on the screen, the detector end of the probe is touched or brought close to the patch on the screen and the probe immediately responds in accordance with the decoded data from the patch. The response of the probe is signaled to the viewer by means of a sound transducer 22 and light emitting diodes 23.

Referring now to Figure 3, a novel coding format for the encoded area on the screen is compared with the prior art format. In each case the encoding comprised a succession of sixteen bit words of which one is illustrated in the Figure. In the prior art code format, it was considered desirable for the coding to be essentially symmetrical, with no preponderance of binary "1" compared to binary "0".

Further in the example shown in the upper part of Figure 3, the sixteen bit word is divided into a five bit header section (the first five bits in figure 1) followed by an eleven bit data section. Only in the header section are there more than two successive bits of the same type. In the sixteen bit word as a whole there are an equal number of bits of each type. When encoding a patch on a television display at a bit rate corresponding to the field rate of 50Hz, this coding format produces a substantial flicker component at about half the field repetition frequency, ie. 25Hz and the eye is relatively sensitive to flicker at this frequency. Thus, even with relatively small variations in luminance, flicker could be discerned.

By comparison, the lower part of Figure 1 illustrates the novel coding format exemplifying the present invention in which it can be seen that the format is asymmetrical, having in each word twelve bits corresponding to binary "1" and four bits corresponding to binary "0". Further only in the header, which is now reduced to just two bits of the word, are there two successive bit at binary "0".

With this arrangement, the variation of the picture from the level corresponding to an encoded binary "1" to that corresponding to binary "0" is only momentary (corresponding to one frame) except once per sixteen bit word for the header section. As a result the largest flicker component in the resulting display is at a lower frequency than in the prior art and importantly has a reduced amplitude for the same luminance variation.

With the described new code format, the position of the occasional binary "0" in the word may be used to represent the required data. The pattern of bits employed in the new format may also be chosen so as not to produce a regular flicker frequency component of significant amplitude. It is also desirable to form the code with an odd number of bits.

In order to retain the original time averaged perceived brightness level (or colour balance) in the coded patch, the variation from the original video signal levels may also be asymmetrical. Thus, in the described example, a binary "1" (which is preponderant) may cause a relatively small increase in luminance (or colour shift in one direction) whilst a binary "0" causes a relatively larger reduction in luminance (or colour shift in the opposite direction).

It should be appreciated also that the output from the generator 11 need not be supplied directly to a T.V. display 12, as illustrated for convenience in Figure 1. Instead the link may be via a T.V. broadcast or cable link. Further, the encoded video signal from the generator 11 may first be recorded for later replay, transmission and display.

With the reduced flicker, it is feasible to apply the coding to a relatively large area on the screen, or even the entire displayed picture. This may be useful when the code is to be-detected at some distance from the screen, for example, by T.V.

interactive toys or games. A sensor may then be provided fitted with a lens for focussing light from the T.V. screen on to the photo detector.

Claims (11)

CLAIMS:

1. Video data indicating apparatus comprising a video display screen; video signal generating means for generating display control signals to display a video picture on the screen and incorporating modulation means for varying the display control signals to produce detectable variations of the picture in at least a selected area on the display screen without destroying the original video picture content in said area and encoding means controlling the modulation means to vary the display control signal in accordance with a desired binary code, the rate of variation and the selection of code format being such that the code is visually undecipherable when viewed on the display screen; and sensing means including a photo sensitive element to respond to said detectable picture variations in said area and including decoding circuitry arranged to decode the binary code represented by said variations, wherein the encoding means is arranged to vary the display control signals in accordance with an asymmetrical code format having a majority of bits of one type.

2. Apparatus as claimed in-Claim 1, wherein the code format has not more than two bits of the other type in series.

3. Apparatus as claimed in Claim 2, wherein the code format compares successive words of predetermined bit length and each word incorporates a header section having two bits in series of the other type and a data section in which a bit of said other type is always immediately followed by one or more bits of said one type.

4. Apparatus as claimed in Claim 3, wherein a bit of said other type in the data section is always followed by a successive plurality, of bits of said one type.

5. Apparatus as claimed in Claim 4, wherein a bit of said other type in the data section is always followed by at least four successive bits of said one type.

6. Apparatus as claimed in Claim 4 or Claim 5, wherein a bit of said other type is always preceded by at least four bits of said one type.

7. Apparatus as claimed in any preceding claim wherein the code format comprises successive words having a predetermined odd number of bits length.

8. Video data including apparatus comprising a video display screen; video signal generating means for generating display control signals to display a video picture on the screen and incorporating modulation means for varying the display control signals to produce detectable variations of the picture in at least a selected area on the display screen without destroying the original video picture content in said area and encoding means controlling the modulation means to vary the display control signal in accordance with a desired binary code; and sensing means including a photo sensitive element to respond to said detectable picture variations and including decoding circuitry arranged to decode the binary code represented by said variations, wherein the encoding means is arranged to produce detectable variations of the colour balance of a colour picture on the display screen and the sensing means is sensitive to said colour balance variations.

9. Apparatus as claimed in Claim 8 wherein the modulation means is arranged to vary the colour balance in accordance with the binary code in such a way that the time averaged colour balance over successive bits of the binary code remains unchanged.

10. Apparatus as claimed in Claim 8 or Claim 9 wherein the sensing means includes a colour filter.

11. Apparatus as claimed in any preceding claim wherein the video signal generating means further incorporates picture area defining means for defining at least one selected area of the video picture and said modulation means produces said detectable variations only in said at least one selected area.