When I create a BD (h.264 AVC), I usually use Sorenson Squeeze for creation of h.264-Video and ac3-audio files. Recently I observed some strange results in a project (HD 720p50). The project has a trailer, which ends with a view over the city of Cologne. The last frame of the trailer is used as a basis for the subsequent menu. The menu starts with the individual buttons flying into their position from the left. When the buttons have reached their positions (after 4 seconds), the chapter selection menu becomes active.
Now the problem: The last clip in the trailer showed slightly different color values compared to the following menu. The menu itself had terrible video quality with artefacte all over the screen. So I have two issues:
1. bad video quality of the chapter selection menu.
2. visible color level differences between end of trailer and start of menu.
First I checked the color values of the QuickTime Reference Files in Media Composer. For both sequences (trailer and chapter selection menu) the color values are equal and there should be no visible difference in the transcoded h.264 files.
Now I analysed Sorensen Squeeze and found the reasons for both issues.
With issue 1 (bad video quality) the reason is, that the default parameters for the h.264 codec (MainConcept) is "2-pass variable bitrate" with key-frames every 10 seconds. This works fine for video, however it is bad for stills. I changed the setting to "1-pass constant bitrate" with key-frames at every 2 seconds. This has solved the video quality issue.
With issue 2 (visible color differences), I have finally found the reason. It is again in the default codec settings. On the right side of the codec settings window, there is a field for "color range and coefficient" (in my case correctly set to the range 16-235, ITU Rec. BT.470-2). However, just above this field is a check-box "Set Black Normalization Level to 16". When this box is checked, the input-video signals are checked for their black level, and the codec adjusts all color values to the darkest black. In my case the trailer has some black values below 16 which are then taken as reference. They are shifted to 16 and hence all other color values are modified accordingly. When I unchecked this checkbox, the colors of the last clip in my trailer and the chapter selection menu were identical with no visible color difference.
I just want to make this known to our MC community. May be it helps the one or other to overcome similar problems.
Joachim