Some Thoughts on Using Comb Filters in the Broadcast Television Transmitter and at the Receiver

The existing NTSC black-and-white television system forced compatibility requirements on the later introduced color television system. One of these requirements was resolved by interleaving color information in the upper band of frequencies already apparently occupied by luminance information. Present methods employed by television receivers to separate luminance and chrominance from each other result in pictures plagued with limited horizontal resolution and distractions in the form of dot crawl and crosscolor. The frequency spectrum of video pictures in general is such that luminance information clusters around the even harmonics of one half the horizontal line scanning frequency, while color information clusters around the odd harmonics of one half the horizontal line scanning frequency. This suggests that such methods could be improved upon by the use of comb filters. The advantages and disadvantages of a combing scheme in both the receiver and transmitter will be discussed. Emphasis will be placed on pictures with both high and low vertical correlation, the trade-offs in vertical resolution for a reduction in color and luminance interference effects, the higher horizontal resolution that is now possible, and the consideration of the combing scheme response in both frequency and time domain.