Hsync and Vsync occur in both active-low and active-high forms. (X11 modelines refer to those as "-Hsync" and "+Hsync") I've only ever seen Csync in the active-low form, but there's no reason you couldn't have the opposite.
I've only ever looked as RS170/NTSC csync; When I still had access to a Sun (which uses csync instead of separate) or SGI (which uses sync-on-green) I never thought to stick an oscilloscope on their output, but here's what's true for RS170/NTSC csync:
* The falling edge represents the start of the hsync pulse
* The low-passed value represents vsync.
Here's an ASCII diagram:
Code: Select all
__---------------------------
__--------------------------- <- upper margin (back porch)
__--~~~~~~~~~~~~~~~~~~~~~~---
__--~~~~~~~~~~~~~~~~~~~~~~---
__--~~~~~~~~~~~~~~~~~~~~~~--- <- active field
__--~~~~~~~~~~~~~~~~~~~~~~---
__--~~~~~~~~~~~~~~~~~~~~~~---
__---------------------------
__--------------------------- <- lower margin (front porch)
___________________________-- <- vsync
^^ ^ ^ ^
| | | right margin (front porch)
| | active field
| left margin (back porch, also colorburst)
hsync
Alternatively, some video sources generate what's called "Serration vsync", which looks like this:
Code: Select all
_---------------_------------
__---------------------------
__--~~~~~~~~~~~~~~~~~~~~~~---
__--~~~~~~~~~~~~~~~~~~~~~~---
__--~~~~~~~~~~~~~~~~~~~~~~---
__--~~~~~~~~~~~~~~~~~~~~~~---
__--~~~~~~~~~~~~~~~~~~~~~~---
__---------------------------
_---------------_------------ <- so-called "equalization" pulse
_______________-____________- <- vsync is also serrated
NTSC nominally has 3 scanlines of equalization before and after the 3 scanlines of vsync. I think the extra pulses are there to make sure the highpass filter doesn't stop hsync's edge detector from working.