Long-GOP editing
Jul 1, 2009 12:00 PM, By Steve Mullen
Today's enhanced NLEs support various types of MPEG-2, resulting in a faster workflow.
Current NLE operation
When MPEG-2 sources are used by a project, some of the processes previously described are the same. One, or more, MPEG-2 sources are decoded on the fly to 4:2:2 or 4:4:4 YCrCb data. One data stream, one data stream combined with one or more graphics, or the computed result of combining several data streams (plus one or more graphics) become a real-time video stream that is sent to a monitor window on the computer's display and/or to an external monitor. The computed result is used only as a real-time preview. (See Figure 2.)
In this cycle, long-GOP sources are decoded only once. And, it makes no difference when there are decoded — during ingest or at the time of playback. Moreover, if, as recommended by some, our 4:2:0 video had been decoded and recompressed, this 4:2:2 intermediate format would also have had to be decompressed on the fly to 4:4:4 YCrCb data. Upsampling 4:2:0 to 4:2:2 and then later upsampling from 4:2:2 to 4:4:4 opens the window to more conversion rounding errors.
The same logic applies to recommendations to convert 8-bit sources to an intermediate format that uses 10-bit data. Placing 8-bit samples within 10-bit words, of course, does not increase the precision of the digital data. It simply needlessly increases the size of all source files.
To increase the accuracy of preview or render calculations, the NLE itself need only be set to use a high-precision mode. Final Cut Pro, for example, can compute effects at 8 bits, 10 bits or 32-bit floating-point. Media Composer works at either 8 bits or 16 bits. In all cases, 8-bit data are on the fly fed into higher precision calculations.
Figure 3. Apple’s Final Cut Pro enables editors to select the 4:2:2 render codec.
As the number of layers increases and effects become computationally more complex, real-time previews are no longer practical. In this case, the editor can choose to render all, or portions, of a project. While making this choice might seem to open the door to generation loss, it does not.
Final Cut Pro offers an editor the option of selecting Apple's “uncompressed quality” ProRes 422 HQ as the render codec. (See Figure 3.) Generation loss caused by re-encoding to a long-GOP codec is prevented. A render file can be played back for a smooth preview, or combined with new video and/or effects.
Transfer from and to NLEs
While video transfer to Apple's Color application can be long-GOP MPEG-2, during color correction, data are decoded to 4:2:2 video. Color correction is never performed with MPEG-2. Color corrected results are returned to Final Cut Pro using ProRes 422 HQ.
Figure 4. Export is done without the use of renders.
Select figure to enlarge.
This same procedure can be used to transfer segments from Final Cut Pro (or Media Composer) to another application. Working this way saves the needless conversion of hundreds of hours of source material to an intermediate codec. Transfer back is via ProRes 422 HQ to Final Cut Pro or Avid's mastering quality DN×HD codec to Media Composer.
Export
Long-GOP generation loss during export is prevented because Final Cut Pro always generates each export frame from source files. Render files are not used. (See Figure 4.) With an Avid Media Composer, deleting precomputes before export prevents render files from being used during export — even though they use DNxHD or DVCPRO HD codecs.
Given the fear, uncertainty and doubt that has too long existed around long-GOP MPEG-2, is there a scenario in which generation loss can occur from natively editing long-GOP MPEG-2? Yes. Were an editor to not export an HD master to HDCAM SR, HDCAM, XDCAM 422HD, D5 or tape-based DVCPRO HD and instead export to a tape format that employs long-GOP MPEG-2, then the master would indeed be second-generation long-GOP MPEG-2. However, formats such as HDV are rarely accepted as master tapes.
Steve Mullen is owner of Digital Video Consulting, which provides consulting and conducts seminars on digital video technology.
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