Here is a circuit that will allow you to trigger your oscilloscope on specific lines in a video signal. This is a feature found in either very specialized or very expensive scopes and is a must for anyone who is developing or working with NTSC or RS170 video systems.
The circuit is based around the National LM1881 and is an enhanced version of a schematic found in the back of the data sheet. The LM1881 separates the sync signals from the raw video and generates TTL level signals. A switch is used to select the Odd or Even field and the line select logic allows use of the Digital Knobs circuit. Not shown but implied is the +5 and ground signals going to the power leads of all devices.
Use judicious layout with the video signals and keep the leads short.