Thursday, August 01, 2013

PSE detection, interlaced video and VidChecker

I've been preparing some training notes on PSE (hence last week's podcast) but noticed that VidChecker's analysis of a recent BBC recording suggested there were more flash events than were apparent to the eye. Have a look at this Off air MPEG2 from BBC News, 22nd July and pay attention to the timecodes as shown in the analysis in the second screen-grab. 

It looks like I've discovered a bug in VidChecker! The mixes to and from white that seem to provoke the PSE violation are at field rate (as you'd expect from any studio vision mixer) but VidChecker, by necessity, does a frame analysis and so the magic " more than 20 Cd/m2" prohibition on frame-frame luminance changes (as measured on a displayed calibrated at 200 Cd/m2 for peak white) are twice as likely to be triggered.

I mentioned this to the guys at VidCheck and this was the response;


You are right that the first 3 below are fades to white and back again and should not really be picked up as flashing.  

Scott has taken a look at the file, and it appears that the problem is in the interlacing artefacts during the fades where alternate lines are lighter and darker.  He has put in a fix for the next release.

Thanks again for the file and for finding this!  Attached some docs. You may already have them



The helpful document they sent was ITU rec 1702 which has a few more pointers than the OFCOM spec I used in the podcast.