1. Field of the Invention
This invention relates generally to a broadband data compression/expansion system and more particularly, to an improved system for rapid transmission of black/white and color video images via telephone lines for use in teleconferencing applications.
2. Description of the Prior Art
The rapid transmission, reception, and display by commercial television stations and receivers of quality television images composed of 10.sup.6 pixels or more with a satisfactory number of gray intensity levels per pixel is commonplace. However, a substantial bandwidth of several megahertz is required for transmission of complete data with respect to the gray-level intensity of each pixel. The cost to obtain this bandwidth is prohibitive for certain business applications like teleconferencing.
It is common in the art to use narrow bandwidth mediums, such as telephone lines, to transmit these television images. However, the television images are necessarily transmitted more slowly over the narrow bandwidth media. Certain techniques were developed to accommodate high data rate sources to narrow band communication channels. These techniques have been integrated into systems which have particular application to the transmission of video information over voice grade communications channels at a data rate that can be accommodated in the limited bandwidth available. Such systems are sometimes called "slow scan" television systems.
It is well known in the art to use slow scan systems for certain applications like security or surveillance where high resolution is not required. In such systems, a frame of 128 pixels by 128 pixels with each pixel being represented by sixteen gray levels can be transmitted in 128.times.128.times.4 bits=2.sup.7 .times.2.sup.7 .times.2.sup.2 =2.sup.16 bits. Such a system is embodied in the Robot Model 530 produced by Robot Research Inc. of San Diego, Calif. However, these systems are capable of only low resolution and cannot process high quality video images of a 480.times.640 pixel array with 64 gray levels or 225.times.2.sup.13 bits. Moreover, such security systems do not posses the ability to automatically improve resolution.
The time necessary for present slow scan systems using current compression schemes to transmit sufficient information for a video presentation of acceptable quality requires in the order of 90 to 120 seconds per display frame at transmission rates of 9600 bits/sec. For example, a normal video picture requires about 2 million bits of information. At the normal rate of transfer of 9,600 bits per second over telephone lines, approximately 200 seconds are required to reassemble a complete video picture of satisfactory quality. This process has proven to be too time consuming for many teleconferencing applications. Therefore, various data compression methodologies were developed in order to reduce the time to transmit an image.
The prior art discloses several attempts to reduce this transmission time by decreasing resolution. For example, the Slow Scan Receiver/Transceiver Model 285C manufactured by Colorado Video, Inc. provides a resolution of 256 lines by 512 lines with 8 bits per pixel. At 9600 bits/second, this device requires 111 seconds to transmit a video image. Another technique is run-length encoding. In run-length encoding, the source image is raster scanned and a message is transmitted containing the value of a particular pixel and the length of the serial run of pixels with that value. However, for certain images, run-length encoding will increase the amount of data to be transmitted. See L. Grim, "Reversible Image Compression" 86-89 (1980) (dissertation).
According to the prior art, it is also possible to dispose of or "throw away" certain less informative bits of data and transmit the remaining data to a receiving station where replacement data is calculated and added back to form a complete picture. This shortens the time to produce a single frame. U.S. Pat. No. 4,222,076, entitled "Progressive Image Transmission"--Knowlton, discloses a method for calculating a 16.times.16 matrix to describe the gray level intensity produced by adjoining pixels. However, the technique disclosed requires extensive calculations and exceeds the capabilities of all but the most expensive computers. Moreover, Knowlton does not disclose any means for selectively improving the resolution of a portion of the image.
The data compression/expansion techniques taught in the prior art are generally time consuming and provide only a low resolution capability which is unsuited for graphics video standards requiring high resolution. Noreover, these techniques often require that the sending and receiving stations share the same video standard. It is desired that a teleconferencing system be independent of video standards. Also, while the prior art does disclose certain techniques for automatically updating an output image, the prior art does not teach the use of dynamic external inputs by the viewer to concentrate the resolution power of the invention. For example, it would be desirable for an operator to be able to signal the machine as to the portion of the screen on which the resolution power should be focused. This feature would allow a viewer to determine which of the many portions of a figure are illegible or need further resolution. Those areas could then be designated, thereby permitting greater viewer satisfaction.
The prior art also discloses few uses of color in teleconferencing. The reason is that existing technology requires a total of six million bits of information in order to accommodate red, blue and green images. These six million bits would require up to six hundred seconds to transmit. This is too long a time to be useful in most teleconferencing applications. What is needed to satisfy teleconferencing requirements is a timely means for providing transmission and display of an analog-type color picture using digital data.
It is thus a principal object of the present invention to generate a low resolution video image of sufficient quality to permit immediate review and analysis by viewers.
It is also an object of the invention to provide a means for automatically updating the low resolution image.
It is a further object of the invention to automatically add higher resolution to selected portions of the image.
It is yet another object of the present invention to allow the user to select those portions of the initial image where high resolution will be performed first.
It is yet another object of the present invention to provide an image generated by digital data which resembles an analog output.
It is yet another object of the present invention to allow color as well as black/white images to be communicated rapidly.
It is yet another object of the present invention to permit color and non-color stations to interact.
It is yet another object of the present invention to permit a sending station to communicate with a plurality of receiving stations.
It is yet another object of the present invention to provide a teleconferencing system which is independent of video standards.