TM 11-6625-3025-14/ET426-AA-MMA-010/E154 VII210/T.O. 33A1-8-902-1-1
Figure 6-14. Oscilloscope Display of the H BAR signal Waveform at Vertical Rate, White Polarity
NOTE
Adjustment of the H bar elements in the vertical plane of the pattern is based upon
15 elements spaced equally in the total vertical television field, including blanking
time. The vertical field is produced at 60 Hz for all scan rates described by present
U.S. EIA Standards; therefore, the repetition rate for H bar elements is 900 Hz,
requiring 1,111 usec between the elements. One of the 15 elements will occur
within the vertical blanking interval and will not be visible. For practical purposes,
the pattern is composed of 14 the vertical plane of the active raster. Considering
the exact amount of the raster obscured by vertical blanking, it is necessary to
generate 13.88 intervals between H bars, each interval 1,111 usec.
It is necessary, in an accurately generated pattern, for the H bar element to occur
for one complete scan line, positively locked so as to preclude drift or "run
through" that will permit intensification of partial lines, etc. Otherwise, an error of
at least one scan line width can occur in the display of the pattern.
These factors present a conflict; 1,111 usec is not an even multiple of the scanning
line time for any of the EIA Standard scan rates. Therefore, it is necessary to
accept a compromise in internal timing. The vertical interval between H bars is
allowed to vary. This provides the desired visible 14 elements of H bar, slightly off
the 15th multiple of vertical field rate.
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