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Editing long-GOP video

Nov 1, 2008 12:00 PM, By Steve Mullen

Perform MPEG export using smart GOP splicing.

Figure 4. Simulated average data rate as a function of removing two, four, six, eight and 10 frames from two six-frame GOPs. The average data (in purple) is 0.622MB per frame.
Select image to enlarge.

Third, smart GOP splicing works only when the export codec is the same as the source codec. Obviously, this is the case when HDV-sourced projects are exported to HDV tape. With productions now needing to be burned to optical discs, it might seem the technology would be of less value. However, it continues to be used in several ways.

Some NLEs use smart GOP splicing to burn AVCHD-based red-laser discs from AVCHD source files that were recorded to solid-state media and hard disks. Given the huge computing load imposed when AVCHD is encoded, it helps make this long-GOP format practical to use. Moreover, because smart GOP splicing has value only when the source files are AVCHD, it promotes the support of native AVCHD editing. This in turn eliminates the need to convert AVCHD to a far less storage efficient codec when importing source files. In this way, AVCHD is replicating the history of long-GOP, native MPEG-2 editing.

Professionals who create long-form work, where multiple rough cuts are produced before effects are applied, can also benefit from smart GOP splicing. Rather than distribute rough-cuts on standard-definition tape or DVDs, high-definition MPEG-2 can be burned to optical discs. Red-laser DVD discs can be burned using the BD-5 and BD-9 (double layer) Blu-ray option. (In this case, the MPEG-2 data rate is limited to 25Mb/s, rather than 40Mb/s.) Alternatively, Blu-ray Discs (BDs) can be burned. Both options make use of MPEG-2 as the media codec. And, both types of discs can be played on any BD player.

Steve Mullen is owner of Digital Video Consulting, which provides consulting and conducts seminars on digital video technology.

MPEG-2 editing myths

The all too frequent claim that during the capture or import of long GOP MPEG-2, it must be converted to a “better” codec is false. All MPEG-2-based formats can, and generally should, be edited natively. Interframe source files require the least storage space and the least disk bandwidth.

Expanding file size during import in no way can improve or preserve image quality. Moreover, native editing typically enables more streams of video to be edited in real time. NLEs obtain YCrCb frames from uncompressed source files, on-the-fly decompressed intraframe source files, and on-the-fly decoded HDV, XDCAM HD, XDCAM EX and XDCAM HD 422 source files. (As part of a decode, 4:2:0 MPEG-2 video is upsampled to 4:2:2.) Therefore, no matter the source format on disk, exports are always made from 4:2:2 uncompressed video.

When you play back a timeline, you are viewing 4:2:2 uncompressed video that is output directly via HD-SDI or HDMI, or converted to RGB. When effects are rendered in real time, the render engine outputs 4:2:2 uncompressed video. When you manually render effects, the render engine's uncompressed 4:2:2 output may either be sent directly to a file or compressed. Obviously, compressed files have the advantage of requiring far less space and typically do not require a RAID-based editing system.

For example, with HD MPEG-2 source video, Avid Xpress Pro HD and Media Composer can render effects to compressed DN×HD files. And, beginning with Apple Final Cut Pro 6, one can request that effects applied to MPEG-2 source video be rendered to ProRes 422 files. Moreover, you can request Apple's Color application to render to ProRes 422 files.

Therefore, clips with applied effects will not be re-encoded to MPEG-2. Likewise, graphics are never encoded to MPEG-2 during editing. The only time MPEG-2 source files will be re-encoded to MPEG-2, or any interframe format, is when you request an export to MPEG-2 or H.264 to create DVD or Blu-ray optical discs.