f" to return to the top of the screen before painting the next frame. These 24 lines represent "dead air" time, in which no image information is sent.5 Each line of the VBI is capable of transmitting 256 bits (32 bytes) of data. Since the VBI appears once per frame, or 30 times per second, this means that each line of the VBI is capable of sustaining a bit rate of 7680 bits per second.5 The tentative plan for implementing the V-chip is to add the program rating information to line 21 of the VBI, along with the closed-captioning information. The difficulty is that line 21 is also being used for newer "extended data services" (XDS) that will be capable of providing such things as scheduling information and station call letters to the viewers. Fitting all three of these data signals into the 7.68 kbps of line 21 is one of the primary difficulties in designing the V-chip implementation.12 The magnitude of the problem will be determined by the complexity of the rating system chosen by the broadcasters. If a relatively simple rating scheme is used, small modifications could be made to the existing closed-captioning decoders to receive the rating data and block the programs. This would require no change in the architecture of the televisions, and would be almost free of cost to install. Electronic Industries Association6 (EIA) engineer, Tom Mock, says that the existing closed-captioning chips have enough memory to support a system of up to three content categories, such as "sex", "violence", and "mature content," with four levels of blocking each.12 If the broadcasting industry selects a system of more complexity, it would be far more difficult to implement. Each television would require additional circuitry to handle the decoding of the ratings. This would mean that television designers would have to alter the internal layouts of the television components, adding up to $40 to the cost of the television, depending on the manufacturer and model...
