By Asheesh Bhardwaj, Software Chip Architecture and Specifications, Texas Instruments dspdesignline.com (January 30, 2009)
Video transcoding was once a fairly straightforward process of decoding and then re-encoding, but some applications can avoid the brute force method and save system resources as a result.
Audio-video transcoding has historically been considered a straightforward, traditional affair in which the encoded source video was decoded to produce a not-quite-perfect version of the original uncompressed content and then re-encoded into the format required for transport or viewing.
As the number of video compression algorithms has grown and more low-cost consumer systems have begun using digital video, the brute force approach has come under scrutiny by engineers who have been tasked with designing low cost systems with good video performance. Although the brute force approach produces a high quality video result, quality decreases with each encode/decode cycle because artifacts are included in the coding as if they were valid data. Other drawbacks include:
Processor demands: As the algorithms have become more efficient, they have also become more complex and require greater processing capability, especially if the conversion is expected to be executed in real time. Even if a chip used for transcoding can handle the brute force approach, a less computationally-intensive method would allow the same chip to handle more channels and reduce overall system cost.
Memory resources: The decode/encode operation typically requires the decoded data to be stored in memory and extra memory increases the system bill of materials. Particularly in price-sensitive consumer devices, this can mean the difference between market success and failure.
Video playback on mobile devices provides a good example of an application in which it is desirable to reduce processor loading and save memory resources. Although transcoding does not take place in the cell phone itself but in the Video on Demand (VoD) server or video gateway, Mobile TV/mobile video telephony is still restricted by five considerations beyond the VoD server and video gateway itself, including:
- Network bandwidth
- Processing power in the mobile phone
- Display resolution
- Memory size
- And -- a parameter not normally thought of in video playback -- the mobile phone's energy consumption.
Even though mobile video devices are becoming more powerful with each generation, processor speed of a typical device is between 300 MHz and 600 MHz and memory capacity is about 64 Mbytes. Even a cursory evaluation indicates that not all of the information encoded in HD or SD video for a large screen has to be processed for viewing on a mobile device. On the other hand, the HD or SD video captured using handheld devices needs to be processed for viewing on the end equipment by the network servers.
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