May 27, 2009 Educational Radio Net, PSRG 53rd Session
Television is ubiquitous… vision at a distance receiving equipment surrounds us completely. In today’s world television is a very mature technology however, if you were born before 1950, you can very likely remember when there was no television. I remember vividly the moment that I first viewed a television set as a youngster and it changed my life in the sense that I wanted to be a part of something very exciting. I was raised in a very small rural Ohio farming village and, while delivering papers late one evening, I peeked through the front door of a house rumored to have a television. This strange box was the talk of the town, the only one in the town, and the only external evidence of something going on in that house was the strange antenna structure attached to the chimney. Looking through the front door I could see a large wooden box labeled RCA and I was instantly captivated by the moving images on the small screen. From that moment in 1951 and forward I was intent on becoming an electrical engineer specializing in television technology.
The early wooden box TV’s were very expensive devices and only the well to do enjoyed the early television experience. The Chicago Worlds Fair in 1933-34 demonstrated a device for electronically reproducing a moving image and one or two of my older family members had actually seen this device. RCA, Dumont, and Zenith… among others… had worked hard in those eighteen years between 1933 and 1951 to produce these first consumer television sets. They were, of course, black and white sets which operated with vacuum tubes since any solid state device was years away in development. Bulky and deep chassis units with point to point wiring. A three dimensional wonderland when you stop to think about it.
Without doubt there were amateur radio operators who salivated at the thought of transmitting images via the ham bands but generating the images for transmission was also a very expensive proposition. There were no small battery operated camcorders or inexpensive security cameras as we know today rather still images were generated by flying spot scanner and enormous studio cameras were required to produce moving video signals. Video recorders using two inch tape eventually enabled storage of video images but, for the most part, the early programs were live events filled with memorable errors.
Technology does march on and changes came rapidly. The physical size of television components started to shrink and performance was better and better. Mass production resulted in falling prices and, eventually, low voltage solid state devices replaced the vacuum tube. Well to do amateurs could actually afford quality video equipment.
Let me preface this next part by saying that I am not going to present a detailed account of how the various slow scan and fast scan signals are developed. Any ARRL Handbook has excellent sections which are easily read. I will just hit the high points and direct the listener to these other sources.
Amateur efforts at transmitting images can be broken into two parts. Slow scan television and fast scan television. The NTSC fast scan video standard developed in the late 40’s and early 50’s remained intact until updated by color television requirements many years later. A consequence of NTSC fast scan video is the requirement of about 4.5 Mhz of data bandwidth for a high quality display and, given that the signal is transmitted as a double sideband amplitude modulated signal with the lower sideband passed through a vestigial filter the actual bandwidth required is about six Mhz. Clearly, transmitting fast scan NTSC based video in the high frequency amateur bands is out of the question. The only region with available bandwidth is UHF and up in the 70 cm wavelengths and shorter.
Enter slow scan image transmission. If you have a non moving image such as a photographic slide it is possible to sequentially sample the slide in a line by line fashion and develop an analog signal which can modulate some carrier wave and be passed to a remote receiving device where the modulation is undone… so to speak… and reconstruct the original image. The scheme uses audio frequencies which fit within the normal voice audio range of the average amateur radio transmit/receive system. The penalty for using low data rates is time. A complete image might consist of 120 or 240 lines and take several tens of seconds to transmit. So, moving images are out. A number of schemes for processing and transmitting slow scan images have evolved over the years and today’s schemes are very robust and yield excellent results with minimal equipment. Digital cameras, computers, and scan converters can be combined to make slow scan color television an exciting pursuit and within the budget of most radio enthusiasts. Listen around 14.230 Mhz to hear the characteristic warble of slow scan signals.
In contrast to slow scan techniques, fast scan television based upon the NTSC video standard has changed very little over the years. The idea is to process sequential images so fast that the persistence of the eye renders them continuous in appearance. The frame rate in analog fast scan television is 30 frames per second for black and white and about 29.94 frames per second for color. Both black & white and color frames consist of 525 lines of information which are divided into two fields each containing 262.5 lines. Bright scenes at 30 frames per second tend to flicker so the two interlaced fields double the flicker rate and flicker is generally unnoticeable to the average eye. The huge bandwidth requirement for fast scan techniques is a result of large amounts of information processed in a short amount of time.
Today’s market offers a plethora of fine video equipment perfectly suited to the amateur radio operator. Ebay is loaded with excellent video cameras from the security sector and a couple of manufacturers offer wide band video transmitters. Receiving fast scan video is as simple as capturing the radio signal and down converting it to a standard channel which is available on your home analog television set.
Analog television, especially color television, based on the NTSC standard was the first technology triumph of the 20th century in my view. Television changed our society in ways we never imagined. The digital computer is important but the television came first.
In summary, slow scan television is used principally for narrow band still image transmission in contrast to fast scan television which is associated with wide bandwidth high quality moving image transmission.
This concludes the set up discussion for Amateur Television. Are there any questions or comments with regard to tonight's discussion topic?
This is N7KC for the Wednesday night Educational Radio Net
Wednesday, May 27, 2009
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