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[ In Focus ] |
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Deliver HDV at its DVD best
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by Wayne Cole
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The popularity of HDV camcorders and the lack of availability of blue laser disc recorders leave many shooters with the desire to deliver HDV content as widescreen DVD video. Online video forums, however, indicate lots of confusion and questions about the proper process to use to maintain the best quality.
Unrealistic expectations are the first hurdle. The process is, after all, downconverting compressed HD video to compressed SD video, so losses are inevitable. Compression technology and formats are still "black magic" to many videographers, yet they mistakenly think that by making some combination of esoteric encoder settings the HD quality can be preserved in the SD MPEG-2 file.
Converting MPEG-2 To MPEG-2
The best quality will likely be obtained by transcoding HDV to DVD-compliant MPEG-2. This is because HDV video is MPEG-2 High 1440 @ Main Profile transport stream or packetized elementary stream, while DVD requires MPEG-2 Main Level @ Main Profile program or elementary stream (with appropriate sequence headers and GOP closures for conversion to DVD-resident video object files).
What many online discussions recommend (needlessly, in my opinion) are the use of all manner of intermediate formats between HDV and the DVD MPEG-2 encoder. The illusion is that you can somehow improve the quality by feeding the DVD MPEG encoder a full res or higher res format than the HDV original. However, this only introduces more digital "generations" with corresponding losses (or at least differences) due to round-off and truncation errors.
The point is that an HDV picture has already undergone the spatial and temporal compression of MPEG-2 in the camcorder. It is specified to contain a 1440x1080 or 1280x720 raster that for HD display will be reconstituted as 1920x1080 or 1280x720 with a 16:9 picture aspect ratio. For SD, the display will be 720x480 with a 4:3 picture aspect ratio, period.
To get to the SD display, proper settings for the encoder will merely scale the 16:9 HDV picture to an anamorphic 4:3 picture and re-organize the input to main level from High 1440 level. This can be done mathematically with less loss than using an expanded intermediate format.
Aspect Ratio Control
MPEG-2 for standard definition is always encoded to a 4:3 display aspect ratio. Metadata in the encoded stream tells whether the picture aspect ratio is also 4:3 or 16:9. Think of the display aspect ratio as the picture frame and the picture aspect ratio as that of the visible image itself.
Players can (but may not always) use this metadata to adjust the display at output time. If this data designates the picture aspect ratio as 16:9, the player may be able to crush the image vertically, thereby producing an undistorted 16:9 letterboxed image within the 4:3 display area. On the other hand, if the player "knows" it's connected to a widescreen display, it will simply stretch the horizontal dimension of the encoded picture to fit the 16:9 display area.
This means that the DVD author or compressionist must tell the encoder to designate the HDV input as a 16:9 picture, even though it will be stored as a (distorted) 4:3 picture. Many encoders don't have the intelligence to set this data field in the encoded DVD MPEG-2 file automatically. So you, as the compression tool user, have to determine if your encoder takes care of this for you or if you have to set this parameter yourself.
Part of the confusion is that the output picture aspect ratio setting, when it exists, is often either hard to find or hard to understand. The result is that many videographers go through unnecessary cropping and scaling operations prior to the final encode to SD MPEG-2. These extra processing steps compromise the final output quality.
Generally speaking, most encoders will correctly assess the field dominance of the source and set it correctly for the chosen output format. However, some users don't seem to want to leave these encoder settings alone and end up hurting the quality of the final output.
In the land of NTSC, HDV is upper field dominant and so is MPEG-2 for DVD. Even if you shoot 720p HDV, it is upper field dominant.
While some people think setting the encoder to create progressive or frame mode MPEG-2 improves quality, it actually degrades it. MPEG-2 for DVD was optimized for interlaced displays, so turn off any de-interlacing in the encoding process and leave the field dominance settings at upper field first.
Playback Confusion
Most software DVD players will pick up the widescreen metadata in the video you encoded from HDV and display undistorted 16:9 imagery automatically. However, that may not be the case for hardware DVD players in a home theater or conference room setup.
There are generally setup menus in the player where you must tell the player the aspect ratio of the display to which it is connected. Some displays also have to be manually set for 4:3 or 16:9 picture output. If either setting is incorrect, the output will appear distorted even though you have done a proper job of encoding your HDV for SD DVD playback.
So, for the output to look its best, leave your HDV in its "native" format and use an MPEG-2 encoder or DVD authoring program that can understand HDV as a source or input format. Be sure to set the encoded output for widescreen picture aspect ratio even though the MPEG-2 encoding dimensions will show 720x480 for SD DVD. Then, it's up to the viewer to set the player and display devices correctly for widescreen playback.
Wayne M. Cole, CCV, CLVI, and member of the AGCV Board of Advisors, is also the owner of IHP, a video production company located Santa Barbara, CA. Contact him at wcole@ihpweb.com.
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