Offline Proxy editing for High Definition Video
When working with uncompressed YUV video files like we do at HomeDVD, we use a technique called proxy editing. Why do this? Uncompressed HD video files by nature are large and as such present a data handling problem on most computer platforms particularly when editing them. Did I mention slow as molasses in January.
It simply means that rather editing the large uncompressed files on the NLE timeline, use a lower resolution clone. Then an average powered computer platform can work in real-time to preview and help when editing and correcting your work. It is faster and less frustrating to the video editor.
Once the edit is done – this includes the regular raw edits, any compositing, special effects, transitions, titles, synchronizing audio tracks, etc., the chopped up and modified low resolution file is replaced by the higher quality “unedited” equivalent on the time line. All of the edit attributes (stored as meta tags and other functions) of the “edit”, is applied to the uncompressed file, which can then be rendered in its final form with all of the edits, or further tweaked with finer edits.
Of course you can’t escape the final render time for the HD file which can be many hours. A 400ft real of film for example, digitized in either 8bpp or 10bpp HD YUV 4:2:2 AVI form, rendered after edit can take up to 12 hours (Windows 7 64bit, multi-core 3GHZ, 12GB RAM) to complete. Hint, don’t use AVI, use Quicktime with the same 8bit or 10bit YUV 4:2:2 attributes and v210 flavour, goes two to three times faster. Same quality and same file size. Go figure.
The proxy technique will work in Adobe Premiere CS4 and CS5 (CS3 is possible but needs attention). I did not check if the other more main stream consumer NLE’s like Sony Vegas Pro, Pinnacle Studio etc., have this feature.
Note (update Aug 2014):
If you have CS5 operating on a fast computer platform in Win 7 and above, you don’t need to do this proxy stuff. Just do your edits on the native uncompressed YUV HD file and export/render/transcode the file format of choice directly. Crunch time is still and issue, but the editing cycle is a lot faster, which is why the proxy thing is great if you have the clunky CS4 version. In CS5 this is not a problem. Hardware assist using Adobe Mercury playback Engine or an approved CUDA based graphics card makes rendering very fast.
Here’s how its done, assuming the raw hi-res uncompressed file is created and ready for edit:
- create a working folder for the low res cloned video file(s)
- create the low res equivalent cloned file – use Adobe Media Encoder to do this. Import the original hi res file into the Media Encoder and set in low res values for export. It doesn’t matter the final frame size or the codec used, just choose one that allows frame size changes and variable bit rates. Quicktime will work. It would be a good idea to use a smaller resolution like a quarter size and a bit rate that gives reasonable video quality – 6Mb/s to 8Mb/s would do. You would like to see the color correction and compositing changes at least for the bit rate setting chosen.
- Export the low res clone file in the Media Encoder Queue.
- Save the the cloned file into its working folder just created. Use the same filename as the hi res file – just to keep things manageable for large projects
- Now lets edit. Create a new project for the HD project in Premiere – use the actual full frame preset. The smaller frame size of the cloned file will show up as a small window in the preview screens. That’s ok, just expand the video to fit if you wish.
- Import the low res HD file to edit and place in the project window
- Drag the file to the time line and do your “edits”
- When checked and done, we are ready for the magic,
- In the project window right click on the low res file and choose Select Clip > Replace Footage. Browse for the hi res uncompressed equivalent and click Select.
What will happen is that all of the associated edits done on the clone file will be inherited by the new uncompressed file. Now, just render the “edited” hi rez file using Adobe Media Encoder. Use the same compression codec/parameters as the original file to yield the final edited result. At which point the YUV file can be further transcoded to H.264, MPEG2 or AVCHD for Blu ray or DVD authoring respectively.