Delivering MPEG-4
Aug 1, 2009 12:00 PM, Are Olafsen
A dedicated DSP chip means more onboard processing in a familiar environment.
Asymmetry
These are just some items from a long list of the added functionality in MPEG-4 AVC, and they are a real tribute to the visionary engineers who worked on the standard. Most important, they are all necessary if the target of equivalent or better quality in half the bit rate of MPEG-2 is to be achieved.
As mentioned earlier, MPEG compression algorithms are asymmetric. Here is another example of asymmetry. To achieve what is in effect a two-fold improvement in compression efficiency demands at least a 10-fold increase in processing power.
The traditional architecture of a transmission encoder uses standard DSP chips to provide the processing power. However, this is unlikely to deliver sufficient performance in an AVC encoder, due not only to the large number of complex processes required, but also because of the amount of memory reads and writes required.
The use of multiple reference frames for temporal compression, and the use of multiple motion vectors, calls for many frames to be held in memory and clocked through as required, sometimes at different rates.
In simple terms, handling the computational requirements and memory management demands would necessitate multiple DSPs. After a certain point, the overhead of the communication between DSPs and the bottleneck of transferring large amounts of data — video frames — around becomes a limiting factor, and further performance improvements become increasingly difficult.
The key
The solution is to develop a dedicated digital signal processing chip. Building a dedicated chip means that key processes can be handled onboard in application-specific hardware. Other processes can be undertaken by DSPs inside the chip. This gives the application designer the best of both worlds: the familiar programming environment and access to proven routines, with maximum speed from the direct access to other internal processes.
With high-speed I/O tailored to the requirements of high-bandwidth video streams, a single chip can support access to the full MPEG-4 AVC toolkit. And because it remains a software-driven device, it can be updated as improvements in algorithms appear, making the encoder or transcoder future-proof.
HDTV is being rolled out around the world, and its mass take-up by consumers will depend on them perceiving that HD in the home is a visibly better and more satisfactory experience than before. That is largely dependent upon the quality delivered by the encoder, which means it is vital for everyone to ensure that the best is made of the MPEG-4 codec.
Are Olafsen is the director of satellite DTH segment for Grass Valley.
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