“Offset Carrier” Success

One of the most welcome scientific advances in television broadcasting is a method, developed jointly by RCA and NBC, for improving reception in areas between stations operating on the same channel.

This method, known as “offset carrier,” was first demonstrated by RCA in March of 1949. It resulted from studies begun soon after the first reports of interference between co-channel stations. The interference, noted particularly in so-called fringe points, was one of the reasons for the FCC’s 1948 “freeze” on the building of new stations.

The offset carrier technique now is being used successfully wherever there is co-channel interference, and it is the basis for allocations by the FCC. Employing “offset” are such important stations as WNBT, New York, WRGB, Schenectady, WNBW, Washington, and WBZ-TV, Boston, all assigned to Channel 4 in the East; and between WWJ-TV, Detroit, WNBK, Cleveland, and WNBQ, Chicago, in the Mid-west.

The tell-tale sign of co-channel interference, is a “venetian blind” effect, a moving pattern of horizontal bars shifting up and down the picture screen of receivers whenever signals from the two stations are picked up simultaneously.

RCA investigation revealed that interference of this type is created when the signal of either or both of the stations deviates from normal. As this occurs, dark bars, varying in number according to the difference in frequencies of the conflicting stations, generally appear on television receivers in the outlying program service areas.

This interference may occur when the receiver is well beyond the range at which it can receive programs from the more distant station. Instances of such interference have been reported up to 150. miles, and beyond.

RCA tests disclosed that the dark bars became less noticeable as their number was increased. This laboratory solution of the difficulty, it was then ascertained, could be put into practical operation in the field by changing intentionally, by a specified amount, the normal transmitting frequency of one of the interfering stations.

Actually, in the case of any station, a frequency shift of 10,500 cycles higher or lower than the normal frequency of that channel (67,250,000 cycles) completely eliminated all visual trace of the venetian-blind effect. So slight is this “offset,” compared to the total width of the frequency band transmitted, that the tuning of receivers is not affected by the move.

Under the arrangement now in experimental operation in the East, stations WNBW, Washington, and WBZ-TV, Boston, are offset 10,500 cycles below WNBT, New York; station WRGB, Schenectady, is offset by that amount above New York. In a similar situation in the Midwest, station WWJ-TV, Detroit, operates without frequency change, while WNBK, Cleveland, and WNBQ, Chicago, are offset plus and minus, respectively.

RCA/The Story of Television RCA,1951