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.