Why optimise video files?
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, it's 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 hi-def pictures?
This process of preparation (or mastering) a file for delivery is often called optimising and usually involves 3 processes ...
- Choosing a Video codec and some related settings to compress the data rate of the pictures
- Choosing an Audio codec and some related settings to compress the data rate of the soundtrack
- Setting various other optimising parameters such as frame size and frame rate
Delivery systems that need optimised content
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 Red book audio CD article. 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.
There are a variety of programs and processes available for optimising. Most fall into one of the following categories ...
|Conversion utilities||Miro Video Converter.com/ / Handbrake / MPEG Streamclip||Often freeware and shareware|
|Video editing programmes||Final Cut Pro / Adobe Premiere|
|Browser extension/plug-in||firefogg.org||Extension for FireFox|
|Specialist commercial codec optimising||Sorenson / Flash video encoder||No longer widely used|
|Specialist (multi codec) optimising||Cleaner||No longer widely used|
|Desktop DVD authoring programs||Encore / DVD Studio Pro|
|System utilities||QuickTime Pro / Windows Media Player|
Different settings in video & audio codecs
Audio and video codecs permit different controls in compression by adjusting various settings. Here are a few examples ...
|MPEG-2||Wide range of compression settings to balance quality and file size|
|H.264 (MPEG-4)||Range of compression setting from the very high quality/big file size to the very low quality/small file size|
|WebM||Wide range of settings.|
|Ogg Theora||Wide range of settings.|
|Sorenson 3||The 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||Range of date rate compression settings and sample rate, bit depth, stereo or mono to be decided upon. Read more here.|
|DV codec||Fixed settings for PAL and NTSC (4:3 and 16:9) which cannot be altered|
Typical compression controls
The following are the settings you can alter when optimising your video/audio files ...
|Codec choice||MPEG-4, Sorenson, H.264 etc|
|Bit rate||Fixed or variable|
|Frame rate||24, 25, 30fps etc|
|Aspect ratio||4:3, 16:9 etc|
|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 keyframe's a second
- Many codecs use a keyframe every 5 to 10 seconds (eg frame rate = 15 then Keyframe frequency = every 150 frames
|Choice of codec||MP3, IMA 4:1 , Qualcomm, QDesign etc|
|Bit rate||Fixed or variable|
|Bit depth||variable or fixed (16-bit, 24-bit etc)|
|Sample rate||32KHz, 44.1KHz (CD), 48KHz etc|
|Channels||Stereo or mono|
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.