OH I can see I'm going to be cutting and pasting this answer all over the place;
Here's the skinny and what you can do about it.
There is a slow change for the QuickTime platform coming. Right now the new OS has a built-in library which tries to re-encode your older videos. They are no longer being actively supported by Apple, since the switch is on, with the 64-bit architecture and of course the mobile platforms (all the "iThings" out there..) to make all of the supported file codecs and formats work as efficient as possible for the newer hardware.
There's also a host of security issues with these third-party developers and that only looks to be getting more stringent as the migration to the whole 'cloud' computing falderall moves into high gear.
The QuickTimePlayer X that comes with your system is trying to reformat - there is NO way to get around it as the instructions are nested down in the libraries for the application - in the QTKit.framework. It's an instruction-set that you can't get out unless you know how to program code. (I think it's in Objective C)
With that as a given, what is left to do is download and use the last available version of QuickTime Player 7 and get the codec packs you need - all of this is the 32-bit code and it's days ARE numbered so fair warning. Here is the quick list I've built as a workaround to keep your files unsullied.
Start with QT 7.6.6. - http://support.apple.com/kb/DL923
Get Perian, and install it. - http://perian.org
then VLC, - http://videolan.org
DivX - http://divx.com - and the
Flip4Mac package from Telestream - http://www.telestream.net/flip4mac/
A52/AC3 downloader: https://www.macupdate.com/app/mac/21875/a52codec - In this installer package there is an audio A52Codec.component. DO NOT USE IT! Throw it out and use the one that is linked below.
This is what I've put into my system and so far I've gotten every file to run fine, even my oldest videos.
These are codecs you should see.
AppleMPEG2Codec.component* (*optional if you've bought it)
Flip4Mac WMV Advanced.component
Flip4Mac WMV Export.component
Flip4Mac WMV Import.component
In your Home/Library/QuickTime/
AC3MovieImport.component (you may or may not want this component, in some instances it causes conflicts. In my system, it doesn't. Who knows why? I don't.)
For AC3 sound that is in most .mkv files, you need the A52Codec.component, this is the one you want, here:https://dl.dropboxusercontent.com/u/34821905/A52Codec.component.zip - unzip the file and put the component into the System/Library/Audio/Plug-ins/Components
Go back to your Perian settings and in the Audio Output button, set it to 'Multi Channel Sound' - Ignore the message Perian puts up and select it.
By doing a 'Get Info' on your files and where it says 'Open With' - default them to the QuickTime 7 program. Perian no longer will work with QuickTime Player so you must have QT 7.6.6 and set it so it is the default for all the filetypes you use.
It works perfectly with QT7. So far I've gotten ALL my old videos to play.
.avi, .mov (with the AC3 sound), .wmv, .flv, .mp4 and .m4v. all run fine as do all the older formats.
Few are happy, randerse10.
This is why the FIRST thing I did when I updated to Mavericks, was install my QT7 and Pro serial and update all the codecs I could. So far not a single file I have has been run under QTX - other than the apple trailers I've been pulling down and playing with.. but my own stuff.. nope.
I actually had no idea there were as many issues with video that there are, I just lucked out as installing my Pro key and loading up on all my audio/video programs and see what works and doesn't..I missed this grief that a lot have had to deal with.
I learned the hard way some time ago that you always experiment with the 'core apps' that Apple has - and QuickTime has been ingrained into OS X for a long, long time, so they can change how it runs at a whim.. and often do - and DID in this instance.
This is why I'm pushing so hard on the QT 7 and hopefully they'll see the Pro key still going out (It's NOT cheap!) and keep it a viable program.
I *get* that old technologies, and old codecs become obsolete. We don't have any more material on C, or Betacam, or VHS for that matter. They were digitized long ago. When we digitize, we should be using "lossless" compresson.
But I can't open my DVCproHD video in Quicktime without "conversion." And I can't choose what codec or compression settings to use. Beyond that, I don't think there should be a need to convert DVCproHD files.
No, there shouldn't be any reason to have them convert - other than Apple deciding what is best for you. Don't get me wrong, I love my computers and actualy love the OS, but what the company does with it on occasion defies logic and simple courtesy. This is one such a time and the hubris that goes with making a program to automatically re-write the format of a file without the user's permission, is breathtaking and is as egregious as changing the content of it, IMHO.
I'd wait a few weeks to let things get settled and see if any third-party companies have come up with a solution for your woes. It appears Apple is heading straight down the iOS path and their built-in applications are on track to get dumbed down.
If the issue is that a downloaded mp4 won't play without conversion...
I think that Mavericks wants certain headers that are not present in some downloaded mp4 movies. The required headers can be added by saving the Mavericks-converted mov, or alternatively this process, which will ensure the movie remains an mp4.
Open the mp4 movie in QuickTime 7. Export as mp4, selecting pass through for both audio and video. The result is an mp4 file that opens cleanly in Mavericks without requiring conversion (same audio and video quality and exactly the same file size). This works.
Cheers - ct
Does anyone know what the files are being converted to? What kind of compression? Never mind: I let one finish, and it's ProRes 4:2:2.
"we should be using "lossless" compresson."
Yes we should, but Apple inexplicably never offered a modern lossless codec. They had only the ancient "Animation", and "Uncompressed 4:2:2" which of course isn't really lossless and is widely acknowledged as a defective pile of garbage.
That basically forced users to buy a lossless third-party codec, like Sheer. Well, Sheer will now not work, and doesn't appear to be in development anymore.
So the vaunted Mac platform is left with no lossless video codec. That is pathetic.