NAEB Newsletter (Feb 1957)

NAEB NEWSLETTER SUPPLEMENT LOW POWER TELEVISION TECHNICAL CONSIDERATIONS Several manufacturers of television equipment market low power television station packages which sell for around $50,000. This price includes all equip¬ ment necessary for live and film pick up, audio and lighting equipment as well as the transmitter and an¬ tenna. Some packages also include the cost of a 200 foot tower and its installation. Obviously such a package contains only essential items, although with the equipment provided it is possible to program a television station. There are. a number of technical considerations which should be pointed out lest you think this is thf ultimate answer for educational television. For cei tain situations this type of installation is entirely ad equate; for others it cannot hope to do an adequate job. One of the first technical considerations is coverage. The area covered is dependent upon ef¬ fective radiated power (ERP). This is dependent upon the power output of the transmitter, loss in the transmission line to the antenna and the gain (or loss) in the antenna. Coverage is also dependent upon an¬ tenna height and, hence, it is very important that the transmitting antenna be placed as high as economical¬ ly possible in order to insure maximum radiation from it. Tall buildings are an advantageous location for the antenna of a low power TV station since they or¬ dinarily are near the center of population to be covered, save the cost of a high tower and permit the transmitter to be located closer to the antenna, and thus save the cost and power loss of a long trans¬ mission line. Most low power packages are offered with trans¬ mitters of 100 to 200 watts power output. The FCC requires at least 100 watts ERP regardless of the size of the city and also requires that a certain minimum signal strength be maintained over the city limits of the principal city served. This limitation will deter¬ mine whether low power equipment may be used. The following table will give an idea of the coverage pro¬ vided in the various TV bands with 100 watts ERP and various antenna heights above average terrain. The radii shown have been predicted by FCC methods. Another technical consideration involved is that of vidicon cameras versus image orthicon cameras. The low power package uses vidicon cameras while the standard television broadcaster uses an image or- FEBRUARY, 1957 thicon as a live pick up tube. The cost of a vidicon camera chain is approximately $5,000 while an image orthicon chain will cost three times this amount or in the vicinity of $15,000. The vidicon equipment pro¬ duces very acceptable pictures. However, it requires much higher light levels than image orthicons which may necessitate studio air conditioning. Excellent picture quality may be obtained with image orthicon cameras with 75 foot candles of incident light while vidicon cameras will require double this amount or more. Consequently this additional requirement of light will restrict the area which can be covered by a vidicon camera unless a high capacity air conditioning unit is employed for studio cooling. In general it may be said that lenses, camera tripods and other accessory equipment for image orthicon equipment is larger, of better quality and thus more expensive. However, skilled handling is required and it is more costly to maintain and operate. Space requirements for a low power television station are not severe. The transmitter itself requires very little space. Studio space will be dependent upon the type and extent of the live programming con¬ templated. Large class rooms may be converted into TV studios. Adequate ceiling height is a prime re¬ quirement. Since most television flats used as back drops are 10 feet high the preferred ceiling height should be 14 feet to allow for lights and the grid from which they are hung. This height also contributes to lower room temperatures and comfort of performers and permits proper back lighting. Another consideration for studio space is sound isolation. Since it is not customary to show micro¬ phones on television they must necessarily be operated farther from performers. Studio noise thus becomes a problem; however inexpensive sound treatment may be applied to walls and ceiling. As a result TV studios are usually less live than radio studios. Extraneous noise filters in through walls, floors, windows and other openings. Consequently a relatively quiet studio location should be selected' or sound isolation may be necessary. Floors should be smooth and solid so that cameras can be dollied smoothly while on the air. Accessibility is important, thus ground floor studios have many advantages. Closed circuit television requires no FCC con¬ struction permit. However the communications Act This material was presented by Cecil S. Bidlack, NAEB TV Engineer, at the low power television session of the Region III meeting held January 24-25 at Lafayette, Indiana. It is based in part on a pamphlet prepared by Cyril M. Braum of the Joint Committee on Educational TV, Washington, D. C. Similar information to the above has been prepared on "Establishing a 10-watt FM Educational Broadcasting Station." It will be mailed to you free upon request to NAEB, 14 Greg¬ ory Hall, Urbana, Illinois.