Producing Content for Mobile Delivery
- Temporal Compression: Successive frames of video are compared to one another, and a series of keyframes (I-Frames)—containing all the data that comprises the image—are created to establish a baseline quality. Delta frames (P- and B-Frames) fall between I-Frames and are more highly compressed, holding only the pixel information that has changed from the previous frame and requiring a decoder to reference other frames to present a complete image.
- Color Space Subsampling: This process exploits the fact that human eyes are more sensitive to brightness than to color, so it reduces the amount of color information.
- One-Pass: One-pass encoding is speed-optimized, whereby the codec analyzes the need for bitrate slightly ahead of the actual encoding. It is used in applications requiring real-time encoding, such as live streaming.
- Two-Pass: Primarily used in nonreal-time applications such as video-on-demand, two-pass encoding analyzes the bitrate requirement in a complete pass through the video, and then it applies the bitrate map to the video in a second pass.
- Constant Bitrate (CBR): Video is encoded at a bitrate below a ceiling, usually specified by the mobile network operator. Video bitrates above the ceiling would cause interruptions in delivery.
- Variable Bitrate (VBR): Bitrate varies with demand of the content, producing higher-quality output. Dynamic, high-motion scenes require more bitrate than talking heads. Spikes in encoded video bitrate are acceptable depending on the delivery medium because they provide higher quality, but they don’t work well in narrowband networks.
Multiple factors determine encoding bitrate and bit optimization (where bits are allocated). The delivery channel likely has a maximum available bitrate, further limited by RF variability and headroom allocated to buffer variability. The playback device is limited by which codecs are supported and the power of the CPU processor used to perform the decoding. The content type and length also affect bitrate requirements. Dynamic action, for example, requires a higher bitrate to look good. Long run times may need less bitrate for small file sizes in a video-on-demand application.
Production
In order to effectively deliver video content to wireless and mobile users, content providers must move from an "MTV" mind-set to a narrowband mind-set. Every aspect of production needs to be examined from this viewpoint. Not all content will work well in narrowband. Preprocessing repurposed content for mobile delivery will reap significant benefits. The best scenario is to deliver content purposely designed to encode well for narrowband delivery. Whether repurposing or creating new content, be sure to return to the fundamentals of production:
- Rely on strong storytelling and well-composed shots.
- Place less emphasis on visual eye candy.
- Pace edits less frenetically.
The narrowband mindset requires a shift in production values. First, the rate and amount of change should be carefully controlled. Both contribute significantly to higher bandwidth requirements. Second, a jump to a higher visual quality level is difficult to achieve. Instead, one must accumulate small improvements in each step of production. Therefore, no opportunity to improve quality is too small. Always choose the quality option. Do not fall for the "it’s so small it doesn’t matter" trap. Minimized video requires maximized quality. The encoding process magnifies shortcuts on quality.