‘create’
This section covers the basic formats used by mobile platforms (such as WAP mobile phones, 3G handsets, portable media centres, video iPods and others) and how to prepare your content for distribution to them.

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• How do I encode for a mobile device?

First, decide on your target audience. Theres no point encoding with stereo sound if the person watching your video only has a mobile phone with mono sound, or encoding with a high dynamic range of colours if they are viewing on a 256 colour screen. Equally, encoding your video with 25 fps (frames per second) is un-necessary if it is going to be viewed at a resolution of 176x144. A framerate of 8-12fps will be sufficient and save you subtantially on the overall filesize. (Note: Too high a framerate for the processor and it will jump frames and appear choppy or freeze)

• What format / codec do I use?

The mpeg-4 format is a widely supported standard. It is also a container standard (remember the envelope analogy?), meaning you can have an mpeg-4 file that fits the 3gpp specification for mobile phones, or the latest h.264 compression standard for video iPods and even PCs. Most encoding tools will have pre-set profiles that match against mobile devices, or common formats. If you want to target video iPods ONLY, then go for the highest quality bitrate (384kbps+) using the mpeg-4 h.264 codec. If you just want to target WAP mobile phones, go for the 3gp Simple Profile. If you want to try and reach everyone, but are happy to lose the occasional low-end devices, try for the middle ground, such as an mpeg-4 standard with AAC audio (Advanced Audio Coding) - Advanced Simple Profile. The final deciding factor is optimising the encoding to lower the file size.

• How important is the final filesize?

Quite important. Most mobile devices still have limited onboard storage space. Also, the download time will be critical in determining whether a viewer aborts the download halfway through, and never gets to see your content. Remember, mobile content is bite size, on demand. Encode at the lowest bitrates and framerates to shrink the final file size. However, it is important not to devalue the user experience by reducing quality of the video or audio, just to shrink the filesize.

• What factors do I need to take into consideration?

When encoding your video, you need to consider your target audience, the final resolution you wish to display the video at (for example, 176 pixels wide by 144 pixels high), the desired framerate (film is 24fps - frames per second), the bitrate (how much digital information is available per second) as well as the audio bitrate, how many channels (ie mono or stereo, or even surroundsound), the dynamic range of the sound (ignore low rumblings, bird noise etc) etc. Some encoding packages will do this for you automatically, however.

So the factors are: container format (ie 3gp, mp4, m4v), video height and width, video bitrate, video codec, audio bitrate, number of channels, audio sample rate, audio codec. For further explanation, check your encoding software. Experimenting with each value when encoding a small file will also help you understand the importance of each. Recommended further reading on encoding for mobile platforms includes [RealNetworks Whitepaper] (PDF).

• What can I expect?

A 3gp format file, encoded for 176x144 viewing, aimed at the basic handsets is limited to mono sound (on some handsets), and low framerates. Aiming for a fast download, especially as this clip has no fast moving action, this can be encoded at 96kbps using the mpeg4 h263 codec.

[ Sample 1 ]: 176x144, 12fps@96kbps (h263), AMR mono, 11k audio 1min = 0.9 Mb
[ Sample 2 ]: 176x144, 12fps@192kbps (mpeg4), AAC stereo, 48k audio 1 min = 1.7 Mb
[ Sample 3 ]: 320x240, 15fps@192kbps (h264), AAC stereo, 64k audio 1 min = 2.4 Mb

• What is the bitrate?

As video information is represented digitally, the bitrate is a way of measuring how much information is stored per second. The higher the bitrate, the more information is available for each second of on-screen action. The bitrate is written as, for example, 192 kb/s or 192kbps (192 kilobits per second). A slow moving scene, with little on-screen change, such as a fixed camera view of two people talking, won't require lots of information and can be encoded at a low bitrate. An action packed chase scene through a busy market, however, will require more information to be translated into an onscreen video frame and will therefore need a higher bitrate. When action is slow, the additional information will be details such as focus and sharpness, colour depth, contrast etc.

A good director or producer will try to judge the requirement for bitrate when encoding for a mobile audience. Is the detail in the background important? Will it show on a 320x240 screen. Is there something in the shadows? A higher bitrate will give more information to the viewer. Too high a bitrate and you are encoding information that the user might miss, and just results in a large filesize, or can even cause the device to skip frames because of too much information before the next frame is due. The right balance is found by judgement and experience. Remember, audio bitrate is just as important to the viewer's experience. The higher the bitrate, with audio, the wider the dynamic range. This is especially critical for shorts with a soundtrack, important sound effects or music videos. For quiet or dialogue-heavy clips, there is some scope for reducing the bitrate though.