To do this, the encoders first need to determine the best matching block in an area surrounding the macroblock in the previous frame (motion estimation). On the basis of the match, the encoders next will determine whether the macroblock is unchanged (encoder can skip) or if some differences exist and can be encoded. (The encoding of the differences will yield fewer bits than the encoding of the macroblock itself.)

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Each of these two steps can be implemented in various ways. Some are more efficient, like compute cycles and memory utilization. Others provide much greater quality. The second step can be invoked in a variety of ways, according to an encoding strategy and the context of the application—constant versus variable bit rate, offline-mode versus real-time transcoding, etc. Hence, when it comes to video, adherence to a standard is not a measure of quality.

Real-time or on-demand transcoding is challenging because the transcoder has to convert the media stream from one codec to another while maintaining appropriately short input to output latency. The transcoder does not have the benefit or luxury of multiple passes over the incoming bitstream to transcode while maintaining bandwidth efficiency using information from a time later in a clip. The real-time transcoder has to employ coding strategies that predict the changing bandwidth needs of video frames and encode them accordingly. Failure to do so leads to highly variable video quality and inefficient bandwidth utilization. 

Bandwidth variations are common in networks in general, though they are more severe in mobile and wireless networks. In today’s 2G to 3.5G networks, quality of service is rarely in place or enabled, and best-effort approaches are used. Available bandwidth to a 3/3.5G user typically varies from 64 to 300 kbits/s. Fluctuations in available bandwidth result in severe degradation in the customer experience.

Dynamic bit-rate adaptation (DBRA) is a general term used to describe the ability of a delivery system to adjust its transmitted bandwidth to match network fluctuations. DBRA approaches are different for real-time streaming protocol (RTSP) and hypertext transfer protocol (HTTP) progressive download (PD).

In RTSP, infrastructure providers and handset vendors are increasingly adopting a standardized approach. For the RTSP DBRA scheme to be of value, handset support and the support of RTSP streaming and relay servers are essential.

For HTTP PD, there is no standardized way today to implement DBRA. Different terminals and media-player technologies implement different strategies. Some, like Apple’s iPhone (OS 3.0), require a multiplicity of bit rates to be listed in the content asset, and the decision of which one to select is left to the iPhone media player. Other streaming and media-player technologies use different approaches.

Moving Forward with the Essentials

As mobile video delivery technologies have emerged, designers are facing new challenges in delivering video to any screen/terminal. Smart encoding, real-time transcoding, and dynamic bit-rate adaptation strategies are available today and are essential ingredients for scalable software-based deployments delivering a high-quality customer experience at a reasonable cost.