Probably compression artifacts. What kind of footage? What codec used? If you can get it to work at 100% scale in PPro then I'd suggest transcoding whatever the source file is to a production codec. We can not help you very well without knowing some details about your project.
Thanks a lot Rick. (I'm sorry for not being specific. It's my first post)
I'm working with .mts files (AVCHD) from a Lumix GH2.
I'm not sure about the codec I'm using (I'm a beginner in AE). When I get the info of the file it says MPEG-2 Transport Stream.
What codec would you suggest transcoding them to?
I have always edited in PPro without paying attention to these things and I found it very strange that this doesn't happen in PPro... I haven't found any tutorials about these things.
Thank you again.
I would try any production codec. Jpeg QuickTime or PNG compressed QT would be a good place to start. Depending on your system there are lots of lossless codecs you could try. All MPEG formats use intra frame compression which means that in your MPEG stream you have full frames and then frames where the intermediate color and luminance values are estimated by the codec. This is what is causing the temporal color artifacts. Premiere Pro's Playback engine is designed to compensate for this intra frame compression, but AE can only look at a single frame at a time so the CPU is decoding the estimated values and giving you your artifacts.
When you are shooting use the lowest ISO setting you can to minimize noise, watch out for slow camera moves over highly detailed backgrounds, and minimize the sharpness and contrast settings in the camera. This will give you an image that is easier to work with in post. You can do your color grading there to get the look you want.
I really appreciate your explanation. It was very helpful. Now I understand how this works a little better. And thanks for the advice when shooting with my camera.
One last thing. What would be a good way to transcode my footage. Some software like Xylisoft or similar? I'm not familiar with this process yet.
OK. Thanks again. I'll tell you when I finally get it right.