Producing Content for Mobile Delivery
Limiting creates a fixed "amplitude ceiling" to prevent overloading and distortion of audio codec due to excess amplitude. Reducing -3dB to -6dB down from full-scale audio will give the audio codec enough headroom to prevent clipping.
Once you have your final audio track, convert the sample rate to match the sample rate of the codec to be used at final encode. This increases the efficiency and accuracy of the encoding. You will get better results using "audio-tools quality" sample rate conversion instead of a video encoder’s. Remember that noise is the archenemy of any encoding process. Noise robs bits that should be allocated to cleanly encode the subject of the content. Noise reduction can be overdone, however, and should be used judiciously with a critical ear.
You may choose to convert stereo to mono audio. Mono is half the amount of data as stereo since the audio bitrate is not divided between two channels (128Kbps stereo = 64Kbps per channel, whereas 128Kbps mono = 128Kbps per channel).
Mono audio quality will be much better because there are twice as many bits per channel dedicated to reproducing sound. Not all handsets have stereo playback. Some content, usually music videos and movie trailers, may require stereo audio, but if you don’t need to use stereo, don’t.
Preprocessing Video
There are several things to remember when preprocessing your video. You should crop video edges. Clean, well-defined edges encode best, while edge jitter robs bits needed to cleanly encode the subject. There’s no reason to encode video overscan. This prevents wasting encoding bits on borders instead of on content.
Also, remove letterboxes where possible. If you cannot remove it, make sure it is Super Black. Since handsets do not use interlaced displays, you should deinterlace content shot on interlaced video, which will convert the video scanning from interlaced to progressive. This will result in smoother motion and smoother edges.
For content shot on 24 fps film and delivered on 30 fps videotape, apply an inverse telecine filter, which will remove the 3:2 pulldown added in the telecine process. This will return the video to 24 fps, which, when frame rate subsampled for mobile delivery, will result in better image quality.
Optimize black level for the best codec performance. Establish Super Black as the reference, but do not push midtones and greys into black. Push saturation and contrast to make the images pop. Adding brightness will add noise to video, so it is not recommended.
Remember, handsets are not NTSC devices, so you can push saturation and color beyond "legal NTSC." Be sure to check the results on the handsets!
When subsampling, calculate frame rate using whole numbers: -30 fps —> 15 fps; -24 fps —> 12 fps. The linear reduction in file size is 30 to 15 fps = .5 file size.
Scaling up is not recommended as it will negatively affect the quality. Maintain the pixel aspect ratio of the source throughout the preprocessing and encoding chain.
Video noise is very inefficient to encode. It eats up a huge amount of bits, which are diverted from the subject. Reducing noise makes the codec’s job easier to create higher visual quality content. There are many different noise reduction solutions available ranging from complex hardware to simple software. Hardware systems offer complex algorithms and a greater variety of techniques, and they provide the most amount of control. Software is often more easily integrated into the NLE workflow. <>
Despite the many challenges of producing and delivering content to mobile handsets, the opportunity is immense. We are at the beginning of an exciting time in the evolution of how people consume and enjoy video. Proper production techniques will be vital to ensuring its success in the wireless marketplace.