Optimising (compressing) video files for DVD by Matt Ottewill

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DVD and CD do not have enough storage capacity to hold long uncompressed PAL or NTCS TV and DV video files. Files must therefore be compressed using a codec (compression/decompression algorithm).

What is a codec?

A codec is a piece of software, sometimes called an algorithm, which is used to compress a media file (reduce its size) and then later decompress it before it is played. This process is useful for 2 primary reasons ...

  1. To allow more data to be stored on a given storage device (such as a digital video camcorder hard drive or memory card).
  2. To increase the speed of transmission across a network (such as the internet).

The term "codec" is usually used in reference to audio and video media files, but still image compression methodologies, such as jpeg, can also be called codecs. A codec may be lossy (cause some deterioration in quality) or lossless.

A "hard codec" is integrated into the electronics of a hardware processor/device such as a camcorder, MP3 player or video capture card. A "soft(ware) codec" is installed onto a system and runs on the systems CPU. QuickTime mostly comprises a collection of soft codecs which run on a PCs CPU and can be used by any application installed on the system, such as a web browser or video editing application.

Examples codecs are ...

  • .jpg (images)
  • .mp3 (audio)
  • .mp4 (video)

DV codec

DV video is compressed using the DV codec which squashes the huge amount of data produced by a camcorders CCD in order to record it onto DV tape. Even after compression DV files need 216Mb of storage for every minute of video. That means that a DVD disc could only contain about 20 minutes of video.

MPEG codecs

To fit video onto optical discs (DVD and CD) it must first be compressed using a codec. Although the web, hi-def TV broadcast and Blu-Ray uses the newer MPEG-4 codecs, such as H.264, most DVD discs are encoded with the older MPEG-2 format.

The Motion Pictures Expert Group (MPEG) has also developed 2 popular video codecs ...

MPEG-1

The MPEG-1 codec was developed for CDROM video discs. It has a low bit rate of up to 1.8 Mbps (Mega bits per second). 1.4Mbps is a more typical bit rate.

Because of this low bit rate, frame sizes is restricted to 352 x 288 for PAL and 352 x 240 for NTSC.

MPEG-1 is commonly used for CD-ROM and the Internet and although supported by the DVD standard is not normally used for DVD video discs.

MPEG-2

The MPEG-2 codec is the first choice of DVD video authors. It allows bit rates of up to 9.8 Mbps.

This bit rate permits the full frame sizes of PAL (720 x 576) and NTSC (720 x 480).

Therefore, MPEG-2 creates video files of full broadcast quality.

What do I need to encode video to MPEG-2?

Video files (such as DV) can be encoded with several programs ...

QuickTime Pro

Apple's collection of system codecs, QuickTime, which includes a playback utility called the QuickTime player. By purchasing an unlock code from Apple (about $35) the QuickTime player becomes a powerful authoring program. Among its features is file/codec conversion. QuickTime pro can convert DV video files into MPEG-2.

Find out more from the QuickTime website.

Find out how QuickTime can replay MPEG-2

You can export video edits straight from Apple's Final Cut Pro (or Express) into (QuickTime) MPEG-2 format.

Final Cut Pro / Compressor

Final Cut Pro has a companion program called Compressor which you can launch from Final Cut Pro. Compressor is a great MPEG-2 video encoder. As well as creating encoded MPEG-2 video files, compressor prepares uncompressed audio soundtrack files ready for processing to AC3/Dolby audio.

PC encoders

I've no doubt that there are a multitude of encoders available for the PC that work alongside Premiere and Encore etc on that platform.

Choosing MPEG-2 bit rates

The bit rate you choose will determine ...

  • Picture quality
  • Frame size
  • File size
  • How much strain will be put on a computer's CPU when it is using a "soft MPEG codec" to play and display DVD on a monitor

What bit rate settings should I use?

When choosing a bit rate you need to consider ...

  • Total running time of your video files. Will too high a bit rate make them too big to fit on a disc?
  • Will too high a bit rate put too great a strain on computer CPU's or DVD video players? Older computers may strain to handle 6 or 7Mbps.

Maximum practical bit rate

The MPEG-2 codec allows for encoding between 1Mbps and 9.8Mbps. In practice 8Mbps is the maximum bit rate you should use, for 2 reasons ...

  1. You need to allow some bandwidth for audio and subtitles
  2. Some DVD-video players will struggle to decode bit rates above 8Mbps

To recap ...

  • 1Mbps = minimum data rate allowed by DVD specification
  • 9.8Mbps = maximum data rate allowed by DVD specification
  • 8Mbps = maximum data rate you should use (to allow for poorly specified DVD players
  • 5.5 - 6.5Mbps = typical commercial DVD data rates

Constatnt & variable bit rates

MPEG-2 encoders normally give 2 additional options ...

Constant-bit-rate encoding (CBR) This setting means that your bit rate will remain constant whatever is happening in the video. This means that scenes of high on screen activity and movement may look noticeably worse than more static scenes.

Variable bit rate encoding (VBR) This setting takes account of the amount of on screen activity and adjusts the bit rate to ensure good quality throughout. When there is low activity the bit rate lowers and when there is high activity it increases.

Try our video
data rate calculator

A utility to help you determine the
highest possible video data rate
for your DVD.
download icon OSX Mac (1.6Mb Zip)

download icon PC (1.7Mb Zip)

download icon Shockwave (96Kb)

Suggested settings

Encoding for stand alone DVD players

If you are authoring a DVD video disc you should do the following ...

  • Use the MPEG-2 compression codec

  • Encode files using Cleaner rather than QuickTime if possible

  • Use VBR encoding

  • Use a maximum bit rate of 8Mbps but consider using between 5.5 and 6.5Mbps if your video does not contain lots of activity or you have limited disc space.

Encoding for computer DVD drive playback

  • Use the MPEG-2 compression codec

  • Encode files using Cleaner rather than QuickTime if possible

  • Use VBR encoding

  • Encode some small video edits containing screen high activity and test them on the lowest spec target computer you want them to playback on.