DVD Authoring or How to Burn a DVD Without Getting Burned, Part 2

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DVD Authoring Part 2 by Bob Hudson
Or How to Burn a DVD Without Getting Burned

MPEG ENCODING IS CRITICAL STEP

The most critical stage of DVD authoring is compressing the video with an MPEG encoder.

The DVD Video specification requires that the video be encoded as MPEG-1 or MPEG-2, but MPEG-1 is almost never used for DVD's. The specification also requires that the video be a certain size: for NTSC, such as is used in North America, the frame size (resolution) must be 704x480, 720x480, 352x480 or 352x240 and the frame rate must be 23.976 or 29.97 frames per second.

For PAL video (the European standard) it's 720x576, 704x576, 352x576 or 352x288 at 25fps.

If you work in the popular DV format (which includes mini DV, Digital 8 and DVCAM), your source video will be 720x480 at 29.97 fps for NTSC and 720x576 at 25fps for PAL. This is something to keep in mind during editing, because some editing programs work in other resolutions, for instance NTSC 640x480, and in order to encode non-standard resolutions to MPEG-2 the source video would have to be converted and that often results in a loss of quality. Some video editing programs produce video that is 720x486: an advanced MPEG encoder will crop off the extra lines to make it 720x480 and there is no loss of quality.

Any loss of quality in your source video is likely to be multiplied after encoding to MPEG-2, which is a highly compressed format. You may have noticed that when you capture an hour of DV video (which is considered to be compressed 5:1) to your computer, it takes up almost 14GB of hard drive space. When you author a DVD project and burn that same hour to a DVD it has to fit in less than 4.37GB of space: that's a lot of compression. If you have two hours of video on a DVD it is compressed to just two percent the size of so-called "uncompressed video."

We often see the phrase "DVD quality," but that is somewhat misleading. Encoding your video to MPEG-2 and burning it to DVD will not make it better than the original. At best it will preserve much more of the quality of the original than would, for example, a VHS tape copy, but if there are flaws in the original they may get noticeably worse. The notion of "DVD quality" is due in large part to those Hollywood movies which cost millions to produce on the highest quality 35mm film equipment, the film is digitized (scanned) at extremely high resolutions as progressive scan images and processed and encoded on some very expensive systems. It's a far cry from using a $1,500 camcorder shooting interlaced video that is compressed even before it gets to the MPEG-2 encoding stage.

But even the Hollywood productions would not look so impressive had they not been encoded with a good MPEG encoder and the quality of the MPEG encoder you use can make a vast difference in the final quality of your burned (or replicated) DVD. If you are shopping for DVD authoring software, you should consider getting a DVD authoring program that allows you to decide which MPEG encoder you use. Some DVD authoring programs, such as Apple's popular iDVD, have a built MPEG-2 encoder and you cannot use other encoders. The built-in encoders may be okay for home movies or, depending upon the software, some simple commercial projects, but they generally don't do a very good job if you want to get more than 60 minutes of video on one DVD and some, quite frankly, don't do a very good job on any length of video.

The highest quality encoding is usually "two-pass, variable bitrate (VBR)" encoding. That means the MPEG encoder goes through your video twice: on the first pass it analyzes each frame to determine how much data should be allocated to each frame, and then on the second pass it encodes it (there are VBR encoders that can do more than two passes for even higher quality).

The bitrate is also referred to as the data rate or bandwidth and in DVD encoding it is expressed as megabits per second, or Mbps. A typical bitrate would be 6Mbps. In VBR encoding there would be an average of six megabits of data allocated for each one-second of video, but some complex sections might be encoded at 8Mbps, while some easier sections (for instance those with little movement) might be encoded at 4Mbps, but for the whole video the average would work out to 6Mbps. VBR MPEG encoders usually let you set the average bitrate as well as minimum and maximum rates.

There is also constant bitrate (CBR) encoding. The bitrate never varies, so the tricky stuff (fast motion, transitions, etc.) gets encoded with the same amount of data as the easy stuff. CBR encoders can be considerably faster than VBR encoders, but the speed comes at the expense of quality and you might have to use a higher bitrate than you would with a two-pass VBR encoder.

In addition to software MPEG encoders there are also hardware encoders, some of which will only work from videotape sources and others which will also encode files from your hard drive. Hardware encoders can vary in price from a couple of hundred dollars to several thousand. The growth in MPEG encoding for things like Tivo has led to development of a new generation of inexpensive MPEG encoder chips that can deliver very good results. While a software encoder might take, for instance, 10 hours to encode one hour of video using VBR, a hardware MPEG encoder would do it one hour, in "realtime." Some hardware encoders have inputs for your VCR's and camcorders so you can capture and do MPEG encoding in realtime. However, because the video is captured as MPEG, you cannot do any real editing to it because of the nature of MPEG files. You can cut out some parts of the video you don't want, but that's about it. If you want to do any fancy editing, do that before you feed your video to an MPEG encoder.

SELECTING AN MPEG-2 BITRATE

With both hardware and software MPEG encoders, the bitrate you select for your video will determine not only the quality of the video, but also how much you can fit on a DVD. If you are going to burn your project to DVD-R or +R, it may also affect how compatible your disks are with various DVD players.

Video bitrates usually fall into the range of 4Mbps to 8Mbps. At 4Mbps you could get 135 minutes of video on a single DVD, but at 8Mbps you could only get 70 minutes on a DVD. The lower the bitrate the harder it is to get good quality and some encoders simply cannot deliver acceptable quality at 4 or even 5Mbps.

The amount of video you can get on a DVD also depends on the audio bitrate.

On Video DVD's the audio uses one of three basic formats: compressed Dolby Digital (AC3), uncompressed PCM, or compressed MPEG audio. For NTSC DVD's, MPEG audio is an optional part of the specifications, so not all DVD players will play the MPEG audio. Unfortunately this is often the only type of compressed audio provided in many DVD authoring programs, so if you are creating DVD's for other than personal use, you should use uncompressed PCM audio or Dolby Digital. There is also a "DTS" audio format which uses very expensive encoders.

Stereo PCM audio has a bitrate of over 1.5Mbps, while compressed Dolby Digital and MPEG audio can have a bitrate that is as low as 10% of PCM audio. This can make a big difference in how much video you can get on a disk or how high of a video bitrate you can use. In the example above I said that at 4Mbps you could get 135 minutes of video on a DVD - that's by using audio compressed to 224Kbps (.224Mbps). If you used PCM audio and a 4Mbps video bitrate you would only be able to get about 115 minutes of video on the DVD.

The best choice for audio is Dolby Digital. It can have very good quality at low bitrates and is compatible with all NTSC and PAL DVD players. It requires a special encoder and authoring software that will support it.