2012-10-25, 09:46
I too get lip sync issues (including recent nightlies), and I'm convinced it's specifically a vsync issue. I've noticed that XBMC's GUI is very responsive when vsync is disabled --- but when I enable it, there's a noticeable mouse lag. If mouse input is visibly lagging, then it follows that video might be delayed systematically. And sure enough, when I disable vsync, I go back to instantaneous input response and have no lip-sync issues at all. I've also noticed similar lag issues in games (both mouse and audio) when vsync is enabled, but not all.
It's still curious that everyone doesn't experience this, as I doubt it's a matter of simply not seeing it. It's way beyond reasonable tolerance levels on my setup --- my wife noticed it, unprompted. Thankfully I get very little tearing with vsync disabled (practically none at 24hz), so I just leave vsync off. But what's going on? Netflix in a browser uses Aero vsync (the one built into Windows's GUI), and its lip-sync is close to spot on. Same thing with MPC-HC and its various custom vsync modes. MadVR, with its fullscreen exclusive rendering paths and such, is flawless. So what does XBMC do differently that might bog down its vsync routines, but only on some video setups? Does it initiate triple buffering, and only some drivers/cards/OSes actually respond to those calls?
EDIT : Come to think of it, the mouse lag looks and feels exactly like a heavily buffered video mode that, in older computer games, was usually referred to as 'mouse smoothing,' 'input smoothing' or 'frame rate smoothing.' Older nVidia drivers used to have a similar setting referred to as "max frames to render ahead." Is there a flip queue in XBMC's rendering engine that isn't setting a commensurate delay in audio processing? But again, why wouldn't everyone on a Windows OS have the same problem?
It's still curious that everyone doesn't experience this, as I doubt it's a matter of simply not seeing it. It's way beyond reasonable tolerance levels on my setup --- my wife noticed it, unprompted. Thankfully I get very little tearing with vsync disabled (practically none at 24hz), so I just leave vsync off. But what's going on? Netflix in a browser uses Aero vsync (the one built into Windows's GUI), and its lip-sync is close to spot on. Same thing with MPC-HC and its various custom vsync modes. MadVR, with its fullscreen exclusive rendering paths and such, is flawless. So what does XBMC do differently that might bog down its vsync routines, but only on some video setups? Does it initiate triple buffering, and only some drivers/cards/OSes actually respond to those calls?
EDIT : Come to think of it, the mouse lag looks and feels exactly like a heavily buffered video mode that, in older computer games, was usually referred to as 'mouse smoothing,' 'input smoothing' or 'frame rate smoothing.' Older nVidia drivers used to have a similar setting referred to as "max frames to render ahead." Is there a flip queue in XBMC's rendering engine that isn't setting a commensurate delay in audio processing? But again, why wouldn't everyone on a Windows OS have the same problem?
