Subliminal messages are messages that are sent in a manner such as to be undetectable consciously by an observer. Subliminal messages are hidden suggestions that can only be perceived by the subconscious. In video communication, a subliminal message can be flashed so quickly that the viewer will not be aware of having seen the message. The viewer can, nevertheless, be influenced by the message content. Consider, for instance, the case of a subliminal advertisement that is sent while the viewer is studying the latest televised news from the stock market. The advertisement may inform the viewer that ACME chocolate is good to eat, but is flashed so quickly that the viewer is unaware that he/she has been subjected to an advertisement. Some viewers, however, can be influenced subconsciously by the advertisement, and later feel an unexplainable longing for ACME chocolate.
The ground-based transmission of television channels are subject to ethical and legal constraints that are aimed towards preventing the above type of advertising. However, it is not possible to guarantee the prevention of the transmission of subliminal messages in many of the international satellite-based television channels that do not obey local laws and regulations. The protection of an observer from such messages is more difficult to achieve in modern types of communications, such as Internet and videotelephony, for instance. Subliminal messages can be hidden not only in a video sequence, but also in still images, or what the observer considers to be still images, and also in audio sequences.
Two mutually sequential images of an image sequence are seldom exactly the same. The fundamental concept of mediating movement with the aid of a plurality of mutually sequential images is that each image will differ slightly from the preceding image. When the images are shown at speed, this is perceived by the eye as a movement and not as a presentation of individual images, by virtue of the eye having a certain degree of inertia. In the majority of cases, only a small part of the image frame is involved in the actual movement; compare a walking person against a stationary background in this regard. This feature is used for different types of image sequence compression, such as MPEG2, for instance. MPEG2 saves space in the image sequence, by sending, among other things, approximative information that describes those pixels that change. However, this results in the introduction of errors in the image sequence, making it necessary to synchronise the image at regular intervals. This is achieved with a so-called I-image that contains all information necessary to compile a complete image.
Image sequences also include a row of different frames in order to enable a moving image to be transmitted in the most effective manner possible. A frame contains image information that is presented on a medium, possibly together with further frames, to form an image or picture. For instance, an interlaced image is comprised of two frames. The term frame will be used consistently throughout the following description. By frame is meant information that is used to compile an image. A frame can itself include a complete image, or solely parts of an image, or information from which an image can be calculated An I-frame is a complete frame that includes image information. Because an I-frame contains a great deal of information, it is expensive to transfer. A new P-frame can be formed from an I-frame or from a P-frame. A P-frame, (prediction frame) is formed by transferring to the receiver side movement vectors and DFD (Displaced Frame Difference) related to the preceding frame. The movement vectors describe how objects in the preceding frame shall be moved to form the P-frame. When the new P-frame is formed, errors will occur due to rounding-up, for instance. DFD describes how the calculated P-frame differs from the original image. The difference between the values of each pixel in the calculated frame and in the original frame can be calculated with regard to black-white frames. A colour frame that uses RGB (Red, Green, Blue) can be transformed to a form in which one portion consists of a luminance part. The luminance part can be used to calculate the DFD, in this case. A P-frame is more cost-effective than an I-frame, since movement vectors plus DFD contain much less information than a corresponding I-frame would contain. Also included are B-frames which are calculated from preceding and succeeding P-frames.
The expression subliminal message is also used to describe a code where a number of encrypted messages are encoded within the same set of symbols. This has no relationship at all with the present invention.
Described in U.S. Pat. No. 5,151,788 is a system for identifying and eliminating advertisements in and from a video signal, by detecting blank images. The concept of this solution cannot be applied to subliminal messages, because subliminal messages are not normally preceded by a blank image.
Described in FR 2,622,077 is a system for detecting discontinuities between images, by analyzing an analogue video signal line-by-line. The concept is not applicable to the present invention, since subliminal messages do not differ from other signals when considered line-by-line.