need to use output buffers on the RGB outputs or can I really drive it directly
out of the chip as stated in the datasheet?
The ADV7123 has been designed to drive doubly terminated 75 Ohm video lines (as
shown in figures 4a and 4b in the datasheet, RevB pg 14) and explained further
in the attached application note. All the curves in the datasheet are generated
with such a double terminated load.
On page 15 of the datasheet there is a section describing the conditions where
an external buffer amplifier is required "Video Output Buffers". Broadly
speaking, this applies to cable lengths greater than 10m where attenuation and
variable group delay for high frequencies can be a problem.
Since the line is terminated, it appears as a pure resistance, the capacitive
element disappears. If there is stray parasitic capacitance at the output this
will certainly degrade performance, cause ringing etc.