On the IO Board why is the VGA designed in this way?

[andrezheng]

Hi Guys

Could you please let me know why the VGA display port designed in this way?
(1) Why there's an capacitor C6 on the G color line?
(2) What is the functionality of the circuit highlighted in red rectangle? R1, R2, Q1, SOG?

Thank you so much.

[eflanery]

C6 appears to AC couple the green signal, likely to provide DC isolation between it and the sync signals.
R2 drops VSync to an appropriate level for Q1's base.
Q1 appears to modulate the HSync signal with the VSync signal, to create a composite sync signal.
SOG (the dashed box) is a switch, to select the composite sync signal generated by Q1 ('Override'), or a 'SOG' composite sync signal generated (or left floating) elsewhere ('Auto').
R1 drops the selected composite sync to an appropriate level for injection onto the green signal.

[andrezheng]

Thank you so much for your detailed explain. I'll digest these comments one by one.

C6 appears to AC couple the green signal, likely to provide DC isolation between it and the sync signals.
R2 drops VSync to an appropriate level for Q1's base.
Q1 appears to modulate the HSync signal with the VSync signal, to create a composite sync signal.
SOG (the dashed box) is a switch, to select the composite sync signal generated by Q1 ('Override'), or a 'SOG' composite sync signal generated (or left floating) elsewhere ('Auto').
R1 drops the selected composite sync to an appropriate level for injection onto the green signal.