Missing Coded Pictures

Missing frames / coded pictures could cause a real headache.

This is a preliminary post as my research appears to have a long way to go. I first started to notice some strange frame counts in original native files some months ago. The usual scenario is that a file will arrive, after being exported from a DVR, usually with a non specific file extension such as .264, .h264 or more commonly .dav.

If you have read a few of my posts you will be aware that, in the majority of cases, these raw streams can be re-wrapped into a usable container, such as .mp4 or .avi.

I had started to witness a few error messages appearing during the re-wrapping process within FFmpeg and, as such, decided to dig a little deeper.

Using FFprobe to analyse each individual frame displays each coded picture number.

CPNCan you spot it? Where is number 4161?

Throughout this hour long file (it had a hard encoded timecode – thank you developer), There was 10 missing Coded Pictures.

I have started to run a number of debug tests to identify exactly what is going on, but felt it important to highlight the issue.

As this is the original file, and not a re-wrap or transcode, a possible explanation is that the frames have been dropped during the export process from the DVR Hard drive to the USB Device.

Most of the DVR’s that export their files in the manner seen here, store all their recorded footage together, they are not seperated by camera. If you scanned through the raw data on the Hard Disk Drive, you might possibly see a set of images (usually an Mpeg Group of Pictures) from Channel 0, then a set from channel 1, etc etc…. It’s only when they are exported do all the GOPS for a single channel get joined up.

Is it possible that during this process, a number have gone adrift?

If you have come across something similar, and identified a cause, please let me know.

I will update after a few more tests.

By Spreadys Posted in EEPIP

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