Picket fence edge effect: One spurious effect which can creep into video game displays results from using the chroma subcarrier as a video clock. Since the defined relationship between Fsc and line frequency Fh is
Fh = 2/445 Fsc
and there are 262.5 horizontal lines per TV field, the number of cycles of subcarrier per field can be determined:
- SC cycles per field = 445/2 x 262.5 = 58,406.25
The ratio chosen results in a subcarrier phase pattern of 4 TV fields ( 2 TV frames ) period. That is, for a given subcarrier phase at some line beginning 2 frames must elapse before this phase repeats itself.
Refer to figure 3 showing this pattern in abbreviated form. Since the subcarrier phase changes 180 degrees from line to line any given edge will be displaced by h cycle of subcarrier time, about 140 nsec for NTSC. The lines are numbered in sequential order as they actually occur in time for an interlaced system.
With the convention of the figure a true vertical edge of a given location is shown for the 5th 0–1 edge on the first 4 scan lines. Notice the alternating displacement both from the line-to-line basis and the frame-to-frame basis. The latter will induce a slight 30 Hz flicker component to the edge.
While the displacement of a 3.58 Mhz rate is admittedly small, the edge effect can ripple down to lower frequency clocks derived from the subcarrier itself. Its presence is not noticed from typical playing distances but will definitely reduce the smoothness of vertical edges in the picture.
The subcarrier frequency was purposely chosen to produce the shifting described in order to visually cancel any residual 3.58 component in the picture by averaging it out over 4 fields. Thus, it is not actually possible to eliminate by a direct frequency divider. While dividing Fsc by 2 or 4 will reduce the flicker component a given 0–1 edge will still toggle from line to line producing the "picket fence edge".
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