Environment And Situation
- Control room of an apartment
- Number of installed product : 3 (PC-based NVR, dual-LAN supported)
- Remote support : X (I actually went to the site and diagnosed)
Reported Issue
- In viewer, when user changes play speed while playing back the recorded data, it randomly plays the data in hyper speed(almost 30x~60x)
- For example: 4x play means 4 seconds in video per a second. But in the site, it played 30~60 seconds per a seconds, showing the video stutturing.
Diagnosis
- Checked the overall environment.
- System(CPU / RAM usage), network environment(bandwidth), resoulution, stream configurations, etc. -> Nothing suspicious.
- Some of the installed cameras had unusual fps and gop values
- Normally, fps and gop values are set to be equal(for exmaple, if fps is 30 then gop is also 30 so that iframe can appear every second)
- But the cameras' set up values were fps 15, gop 60(iframe per 4 seconds)
Assumption
- Somehow the viewer keeps failing to find iframe to play.
- And it's maybe because iframe appears with a long gap.
Quick note: iframe is kind of a key-frame. Since the viewer starts decoding from an iframe, it's necessary when it comes to playback.
What I Tried
- Set all the cameras' gop value to 15(same as fps)
Result
- Ran a test with data before changing the gop values and after.
- During interval before changing the gop, the issue occurred almost every time I tried.
- But after chaning the gop, the issue no longer occurred.
Concolusion
- The issue was triggered by large GOP value (GOP 60 with FPS 15).
- With only one iframe every four seconds, the viewer sometimes failed to find an appropriate iframe after changing the playback speed, causing abnormal playback behavior.
- According to the viewer developer, this is likely related to the viewer's iframe searching logic, which is still under investigation.
Keep This In Mind
- Check camera settings(especially gop and fps) first when it comes to playback issue.
- Always check before/after data to confirm assumption.