The television vertical blanking interval has become an increasingly valuable resource to the broadcaster. Used as a means of incorporating special services such as closed captioning for the deaf, technical quality control through use of test signals, and various forms of data transmission including computer generated imagery, this portion of the conventional television signal allows a significant expansion of the services that may be provided to the public or to private institutions.
One area of increasing importance is the field of distance education. Although television has been used for this purpose since the 1950's the high costs involved have limited the use of this media. As an alternative still image video has been explored by over 100 institutions as a relatively low cost means of effectively delivering instruction to remotely located students.
It is estimated that over 3000 schools and institutions in the United States and Canada are using still video for educational purposes. While a large majority of these use telephone lines for both picture and voice communication purposes, Utah State University has pioneered in the use of the television vertical blanking interval for rapid, low cost, transmission of full color still pictures to students throughout the state.
Early in 1993 SCOLA and Colorado Video collaborated in a series of tests to determine the feasibility of nationwide broadcast of instructional still imagery by satellite. Basic considerations were that there be no change or interruption of SCOLA's normal 22 hour a day schedule of foreign language programs, simplicity in installing the required equipment, and a minimum of special effort by the hard working SCOLA staff.
The effectiveness and reliability of the vertical blanking interval system having been proven by seven years of operation at USU, it was wanted to find out if this form of broadcasting would work well as a means of low cost national and international coverage via satellite. Transmission technical quality was the first consideration, but the study was expanded to include subjects such as program input sources, video tape recording, system requirements, and the investigation of two forms of still picture encoding.
Test material for the four month experiment was derived from SCOLA's normal programming which consisted primarily of videotaped news programs from all parts of the world. Not only did this provide a great deal of diversity relating to production techniques and variations in technical quality, but it also allowed a means of conveniently comparing the original "real time" images with the recreated still picture transmissions. As a rough estimate over one million images were transmitted by the vertical blanking interval system while tests were being conducted.
Hardware installation at the McClelland, Iowa transmission site involved two devices, a Colorado Video model 240 VBI Transmitter and a Tektronix 149A NTSC Test Signal Generator which was used to insert the still picture signal on line 20 of the regular program. For monitoring purposes a Colorado Video 241 VBI receiver was also connected to a downlink satellite receiver.
Receiving equipment was also located at Boulder, Colorado and used a conventional VHS video cassette recorder with the output fed to a model 241 receiver. A 19" receiver/monitor was used for picture viewing and could be switched between the live signal and the VBI frozen image for comparison. Images were both viewed directly and tape recorded for later playback. Satellite signal reception was from a dish at the University of Colorado and was retransmitted on channel 21 of the Boulder cable system.
A simple protocol was employed for transmission. A single field of video was "frozen" from SCOLA's normal program and the VBI conversion and transmission process initiated. Using a single line in the vertical interval the still image was transmitted in a period of eight seconds following which the next picture was automatically acquired and transmitted. Although the equipment being used was capable of full frame operation, the single field mode was chosen in order to eliminate the picture distortion created when motion occurs in the original program material.
Picture transmission is in a left to right mode and a vertical white cursor appears on the transmission monitor screen to show the operator the degree of picture completion. A receiving location normally displays a new image in the form of a left to right "wipe." This provides a bit of dynamic action as a new picture replaces the old one during the 8 second update period. Optionally, a second memory may be used in the receiver to hide the picture build up thus providing a "slide show" presentation with instant transitions.
Noise considerations:
Noise or interference in still picture systems may produce different subjective effects than in the case of conventional television. This is true whether the noise is present in the original video input signal or occurs some place in the transmission path. A single frozen field or frame taken from a noisy video source can exhibit a fixed pattern of "graininess" which may be more noticeable than the shifting characteristic of noise in real time video. Similarly, "herringbone" interference will produce a series of fixed stripes in a still image while in a real time display these will usually move through the picture allowing the observer to "see through" because of persistence of vision.
As in normal good television practice it is desirable to have a clean video input signal to the still picture transmitter and keep the VBI injection levels as high as practical. No problems with high level impulse noise interference were encountered during the tests, but it should be noted that if such would occur the result would be seen as one or more vertical "streaks" in the received picture due to the orthogonal nature of the image reproduction. In the case of NTSC color transmission severe impulse noise may also cause a hue shift to occur after a noise burst if line sync is lost.
No problems were noted from adjacent line interference during direct broadcast reception, even though the vertical interval might be heavily loaded with other data channels. Maintaining the quality of the transmitting station master sync is important and it was found that an occasional abrupt change in phase or timing could produce a vertical displacement of the received still image as well as a hue shift in the case of color.
Two different types of picture encoding were evaluated during the four month test period. The first of these involved a proprietary conversion technique which reduced the NTSC color subcarrier component to a much lower frequency, while the second involved a straightforward orthogonal conversion. Both worked very satisfactorily in direct reception situations but the system which modified the color subcarrier proved to be important in the playback of VHS recordings of VBI signals.
The ability to record and reproduce VBI program material as well as main channel content is an interesting and potentially useful aspect of the system. Several observations were made in relation to this:
For further information please contact:
Glen Southworth
Colorado Video, Inc.
P.O. Box 928
Boulder, CO 80306
or COMPUSERVE 71234,2601
Return to GLOSAS News Contents for this issue.
URL: http://library.fortlewis.edu/~instruct/glosas/still42.htm
June 1994
GLOSAS NEWS was orinally posted to the WWW at URL: http://library.fortlewis.edu/~instruct/glosas/cont.htm by Tina Evans Greenwood, Library Instruction Coordinator, Fort Lewis College, Durango, Colorado 81301, e-mail: greenwood_t@fortlewis.edu, and last updated May 7, 1999. By her permission the whole Website has been archived here at the University of Tennessee server directory of GLOSAS Chair Dr. Takeshi Utsumi from July 10, 2000 by Steve McCarty in Japan.