Now that we have an idea of how MPEG-2 compression works, what can be done on a practical level to ensure that the quality of a film or video is not noticeably compromised when the work is released on DVD-Video? The first thing is to recognize that VBR encoding can only do so much, and to plan content accordingly. The longer the main feature, the more action scenes, and the longer the featurettes (these are often encoded at lower bit-rates than the feature), the more likely that a title will need to be released on a DVD-9. If that extra disc manufacturing cost isn't in the budget, plans for added-value material may have to be scaled back to allow acceptable video quality for the main feature. The video bit-rate may also be increased by cutting back on the number and type of audio streams.
Another crucial factor is the quality of the material before it gets to the encoder. Just because the material will be compressed doesn't mean that compromises made upstream won't show up in the final result. "The quality you get out of MPEG depends what you start with," says David Deelo, compression manager at Crest National in Hollywood, California. "It's the old adage about garbage in, garbage out."
Best results will be achieved from video that either is shot in a component digital video format such as D1 or Digital Betacam, or is transferred from film directly to these formats (higher resolution such as HDTV is even better). The highest possible resolution should be maintained at each editing and processing step the video goes through before MPEG-2 encoding.
If a program has been off-line edited on a non-linear system that uses compression, Deelo suggests that it be edited on-line to create the source master for the DVD release. At the least, the NLE output should be evaluated before the project is compressed. "Look for smooth gradations from dark to light," Deelo says, "such as in the falloff from a light shining on a wall. You don't want to see any banding. And watch for 'mosquito-ing' around titles—weird spider effects where the edges are not clearly defined."
Even when an NLE system does not use compression, Deelo says outputting to D1 or Digital Betacam may still be preferable to "transcoding" directly from the edited NLE file to MPEG. That's not due to any weakness in software-based encoding, which he says can yield results comparable to hardware encoding. Instead, Deelo is concerned about pre-processing, which is used to optimize the video signal before compression. "DVNR [digital video noise reduction] is not just grain removal," he says. "There is also a series of filtering and enhancements we do, such as brick-wall filtering to take out high frequencies before they hit the encoder." Deelo has not found software-based pre-processing that he feels gives as good results as hardware DVNR.
Perhaps the most welcome observation that Deelo has to offer is that film makers can set aside some of the concerns they have acquired from seeing their work prepared for NTSC broadcast or VHS release. He says he has had cinematographers tell him that "they like bright, saturated colors, but they know that when they are in telecine they can't use the colors they want, because they are making a master for television. But that's not true for DVD. Go for the color, go for the sharpness; make the master look how you want it to look. If you like what you see coming out of telecine, we can preserve those qualities when we encode it for DVD."
Related articles:
— Software MPEG Encoders: Heuris MPEG Power Professional
— Software MPEG Encoders: Ligos LSX-MPEG Suite
— Software MPEG Encoders: Cinema Craft Encoder Lite
Freelance writer Philip De Lancie covers technology, techniques, and markets for professionals in media production and distribution.
