HP 54645A User's And Service Manual page 186

If a more detailed analysis is required, then only one color field should be
selected to be the trigger. You can do this by using the oscilloscope's holdoff
control. Using the holdoff settings shown in the following table, the
oscilloscope will now trigger on color field 1 OR color field 3 when Field 1 is
selected. This is known as odd field selection. Even fields will be selected
with Field 2.
Table 6-2 Holdoff Settings
Video Standard
NTSC
PAL
SECAM
PAL-M
The holdoff can be more easily set if the sweep speed is set to 5 ms/div. Once
you have established your desired holdoff time, return to the desired time
base setting. The holdoff setting will remain unchanged.
Figure 6-6
Triggering on Color Field 1 using Holdoff
Using Option 005 Enhanced TV/Video Trigger (HP 54645A)
To trigger on odd or even fields
Fields/Picture Holdoff Range
4 33.5 ms to 50.0 ms
8 80.7 ms to 120 ms
4 40.4 ms to 60 ms
8 80.4 ms to 120 ms
6-11