Introduction

This article provides some practical advice on preparing video files for multimedia projects.

Ensure you have read the article on Digital Video File Formats & Codecs before reading this one.

What is optimising?

Because there are many different home audio visual replay systems (TV, computer, iPod etc) each with its own strengths and weakness, we have to be careful when preparing audio and video files to ensure the target system can handle them. For example, its no good posting a 4Gb DVD quality movie on a web site, it will take too long to download. On the other hand, why put a 340 x 280 pixel video clip on a DVD disc when DVD players and TVs can handle full frame wide screen and surround sound pictures?

This process of preparation (or mastering) is often called optimising.

Optimising usually involves 3 important processes ...

1. Choosing a Video codec and some related settings for the pictures

2. Choosing an Audio codec and some related settings for the soundtrack

3. Setting various other optimising parameters such as frame size and frame rate

Delivery systems that need optimised content

Web sites

If you are going to deliver a project as a web site you will need to consider ...

End-user network connection speed
File size
End-user playback software (codecs)

CD / DVD ROM

If you are going to deliver a project on an optical disc you will need to consider ...

Disc storage capacity
End-user maximum playback transfer rate from the disc (CD or DVD ROM speed)

All CD-ROMs have a maximum data size (capacity) of at least 650Mb. Audio CDs achieve a greater data size of 740Mb because unlike CDROM data discs they do not use up space with error correction data. For further reading view our CD mastering pdf.

A 4x CD-ROM drive has a maximum transfer bandwidth of 600 Kilobytes per second. Providing you ensure your data rate is below this threshold, playback should be glitch free.

Incidentally, don't assume that higher rated drive speeds will necessarily produce better data rates. The system often imposes a performance overhead so base your calculations on 4x speed drives.

Optimising applications

There are a variety of programs and processes available for optimising. Most fall into one of several broad categories ...

• Specialist commercial individual codec optimising ... such as the high end Sorenson broadcast encoder software sold by its inventors Sorenson.

• Specialist (multi codec) optimising ... such as Cleaner (the preferred choice of professionals)

• Editing programs ... such as Final Cut Pro and Adobe Premiere and Flash (Flash video format) with their "Export file as" conversion/optimising features

• DVD authoring programs such as Encore and DVD Studio Pro.

• System utilities ... such as QuickTime Pro and Windows Media Player which have authoring/optimising features.

• Conversion utilities ... usually freeware or shareware, such as Handbrake.

• iTunes ... really useful for creating MP3s.

Different settings in video & audio codecs

Most audio and video codecs allow varying degrees of compression by adjusting various settings. For example ...

MPEG-2 video codec (TV and DVD) allows for a range of compression settings to balance quality and file size.

H.264 (MPEG-4) video codec allows for a range of compression setting from the very high quality/big file size to the very low quality/small file size

Sorenson 3 codec compressor in QuickTime Pro allows quality, data rate, and keyframe duration to be determined. (There is a developer version which permits more control).

MPEG-3 audio codec allows for a range of date rate compression settings and sample rate, bit depth, stereo or mono to be decided upon. Read more here.

NOTE: An exception is the DV codec which uses fixed setting which cannot be altered. Current computers struggle to run the DV codec (QuickTime has it) and display the video simultaneously, so DV video must be "re-compressed" with another codec, such as MPEG-1 or Sorenson before the computer can play it effectively. Audio is uncompressed.

Typical compression controls

The following are the controls you will alter when optimising your video/audio files ...

Video codec settings

Choice of codec (MPEG-4, Sorenson, H.264 etc)

Bit rate (fixed or variable)

Spacial quality

Key frame frequency Codecs work by examining a frame, deciding upon the best way to compress it, and then applying the compression to it all the following frames until the next key frame is reached. The frequency with which key frames occur is important in determining the effectiveness of compression. Too often and the file size will swell, to widely apart and picture quality will suffer. As a guideline ...

High data-rate video use 1 keyframe per second for (eg frame rate = 15fps then keyframe frequency = every 15 frames)

Low data-rate video use 5 keyframes a second

Modern codecs such as Sorenson codec ... use a keyframe every 5 to 10 seconds (eg frame rate = 15 then Keyframe frequency = every 150 frames

Audio codecd settings

Choice of codec (MP3, IMA 4:1 , Qualcomm, QDesign etc)
Bit rate (fixed or variable)
Bit depth
Sample rate
Stereo or mono

General video settings

Frame size
Aspect ratio (4:3, 16:9 etc)
Frame rates

Wizards

Because the process of choosing settings for optimising can be complex, many optimising applications offer presets or "Wizards" which you can use to compress/optimise your files ready for publishing to the web or disc.

Choosing file formats & settings for optimising video

Click here for an article on how to prepare video for DVD.

Click here for an article on how to prepare video for web sites & multimedia projects.

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