The present invention relates to image processing and more particularly to a system for decomposing NTSC color video signals into their Y-I-Q components to facilitate subsequent data compression.
Because travel costs are rising and because a traveler's time in transit can seldom be used productively, there is an increasing interest in the use of teleconferencing as an alternative to face to face business meetings between people from different locations. In a typical teleconferencing system, people in different cities or even different countries meet in special teleconferencing rooms at their respective home locations. Each room normally includes a room camera for capturing a wide angle view of the people, a document camera which can be focused on letters, drawings or other documents, a room monitor for permitting people in one room to see those in the other, and a document monitor for viewing documents being presented in the other room. Communications between the two rooms are established over conventional teleprocessing links, such as leased or switched telephone lines or satellite communication channels.
There has been a good deal of interest in the use of color video techniques in teleconferencing systems because information presented in the form of a color image is generally considered easier to comprehend than the same information presented in the form of a monochrome or gray scale image.
It is, of course, possible to use conventional video equipment and transmission techniques to provide what is referred to as full-motion teleconferencing; that is, teleconferencing in which the people in one room can watch those in the other room move about during the teleconference. The communications costs for conventional full-motion teleconferencing, particularly using color video, are high. A considerable amount of data must be transmitted at high rates, making it necessary to use a transmission medium having a high bandwidth. Communications costs are generally proportional to bandwidth. Therefore, any requirement for a high bandwidth runs counter to one of the primary reasons for using teleconferencing to begin with, namely, to reduce costs associated with the conduct of meetings.
To reduce communications costs, freeze-frame teleconferencing techniques may be employed. The video image captured by a room camera is updated only periodically, either at fixed intervals or on command of an operator. People at the receiver see the same "frozen" room image between updates. Audio signals are transmitted on a real time basis so that there is no perceptible delay in voice communications. Document images are updated only when the person presenting a document pushes a "send" button in the teleconferencing room.
There are two basic ways to reduce bandwidth requirements in a freeze-frame teleconferencing system. One of those ways is to reduce the amount of data that must be sent in order to recreate an acceptable image at the receiving location. The other of those ways is to use a lower bandwidth and simply take longer to transmit the data required to recreate an acceptable image at the receiving location.
The time required for transmission of necessary image data is important in any freeze-frame teleconferencing system since it determines the frequency with which images can be updated during the course of a teleconference. If meeting participants must sit and wait what they consider to be an excess amount of time for an expected video image, they are likely to become irritated, reducing the effectiveness of the teleconference.
In monochrome freeze-frame teleconferencing systems, the amount of data that must be sent can be reduced using known gray-scale data compression and run length coding techniques. Because monochrome image data can be manipulated and reduced using such techniques, it is possible to transmit necessary data at low bandwidths without requiring an excessive amount of time for the transmission.
A greater amount of data is required to define a single picture element (pel) in a color image than is required to define the same pel in a monochrome image. Because of the complexity of the data required to define a color pel, it has been generally assumed that color images could not be processed using the same kinds of data compression techniques that have been used on monochrome images.
Known freeze-frame color videoconferencing systems have avoided the potential problems and technical difficulties of color image compression by the simple expediency of transmitting uncompressed color data between teleconferencing locations. Where such systems use a high bandwidth transmission medium, the frequency with which images can be updated remains high, but so do the communications costs. Where such systems use low bandwidth transmission medium, an undesirable delay may be required between image updates.
Before it is possible to consider compression of color data in a video system, it is necessary to decompose each color signal to be processed into its components. Any color can be defined either in terms of R-G-B components or, alternatively, in terms of Y-I-Q components. These terms are described in greater detail below.
It is possible to analyze a given color using known vector analysis techniques and equipment. Such equipment is not suitable for use in a videoconferencing system, however, because it operates strictly in an analog domain and thus is not readily compatible with the digital data processing equipment normally used to control the operation of a videoconferencing system. Moreover, vector analysis equipment represents an added cost in a videoconferencing system. Finally, vector analysis equipment does not separate luminance and chrominance information with the accuracy required for a videoconferencing application.
It is also known to decompose a color signal using comb filtering techniques. A comb filtering technique is a spatial averaging technique in which samples on successive active video lines form a weighted average which can be used to determine the luminance and chrominance of a given point in an image. Comb filtering is not considered desirable in a video conferencing application since the necessary line-to-line averaging results in a loss of image resolution, usually in a vertical direction.