Low-Resolution TV: Subjective Comparison of Interlaced and Noninterlaced Pictures

The lower limit of the picture repetition rate for television pictures is dictated by the critical-fusion frequency ( C F F ) . 1 ' 2 The C F F is approximately proportional to the logarithm of the luminance over a wide range. It is also approximately proportional to the logarithm 199 200 T H E B E L L SYSTEM T E C H N I C A L J O U R N A L , J A N U A R Y 1967 of the size of the flickering area. The C F F is on the order of 60 pictures per second for present day television luminances and picture sizes. The television engineers of the 1930's experimented with two-fold line-interlaced pictures as a means of saving bandwidth. In two-fold line-interlaced pictures, alternate lines are scanned during successive vertical deflection cycles. Engstrom'' found that the vertical deflection cycle should be greater than .50 Hz and should be a multiple of the power line frequency. In 1941 the National Television System Committee (NTSC) 4 adopted a vertical deflection frequency of 60 Hz for two-fold line-interlaced commercial broadcast systems. Two-fold line interlace has since been adopted by virtually all television systems, regardless of the application. One-half of the lines in a line-interlaced television picture are scanned during alternate half-cycles of the frame rate which is 30 Hz. The result is essentially two light pulses for each frame period, i.e., an apparent rate of 60 light pulses per second. Thus, large-area flicker is negligible if present at all. When all of the lines except one of a line-interlaced television raster are masked that line appears stationary and nonflickering.