I have recently discovered that some MP4 video clips do not play back correctly on my (ancient) MacPro running OSX 10.6.8.
I am seeing the same issue for clips sent to me to play back on my desktop and for those that I have viewed online. The MP4s created on the older machine look fine on the older machine. I am assuming that the problem is with an old codec or with Quicktime itself being unable to interpret the MP4.
I'm attaching two images, one from the older Mac and one from the newer, for reference.
From the older MacPro, the blacks look jagged and washed out:
On a newer Mac Mini, a 2014 running macOS 10.12.6, the video looks fine:
Anyone else see this, and if so, what causes it (aside from "ancient Mac syndrome")?
Also - I have changed the hardware for the Mac 10.6.8 - it is now running on a 2010 MacPro 3.33 GHz 6-core Intel Xeon with 32 GB memory , rather than the 2008 Quad-Core.
I see no one has answered and I was waiting because I have no personal experience with MP4 files having problems on 10.6.5 but your assumption about the codec is most likely correct. MPEG4 is a large and ever changing specification with multiple profiles and new ones being created all of the time. I'm guessing that the video uses an MPEG4 Profile that was not supported by the version of Quicktime from 10.6.5.
There is a utility called MediaInfo that will give you information about video files. You might want to use it with a video that works correctly with one that doesn't and see what's different about the encoding. Here is a sample from my iPhone just to show you what Media Info can tell you: