Hybrid Codec Scalable Video Backwards Compatibility
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Solution Overview
Problem
Hybrid codec scalable video bitstreams that combine AVC and HEVC NAL units are not backwards compatible with legacy decoders, as existing methods fail to correctly handle the mixed encoding standards, leading to incorrect parsing and decoding.
Innovation Solution
The method involves determining the presence of both AVC and HEVC encapsulation units in a bitstream and adding a prefix syntax element to HEVC units with specific values that legacy decoders will discard, allowing legacy decoders to ignore HEVC units and ensuring compatibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If HEVC NAL units are added to an AVC bitstream to enable hybrid codec scalability, then video coding efficiency and scalability are improved, but backwards compatibility with legacy AVC decoders deteriorates because they cannot correctly parse HEVC NAL units
Solution Approach 1:
An intermediary byte is inserted between the AVC NAL unit header and HEVC NAL unit payload. This intermediary byte acts as a separator that allows AVC decoders to correctly identify the boundary between AVC and HEVC data, enabling them to discard the HEVC portion without corrupting their parsing of AVC data. This resolves the compatibility issue while maintaining the ability to transmit both AVC and HEVC streams in the same bitstream.
2Adaptability or versatility
If a single bitstream contains both AVC and HEVC NAL units, then scalability across different decoder capabilities is achieved, but the complexity of bitstream parsing and decoding increases
Solution Approach 1:
The bitstream is segmented into distinct AVC and HEVC portions using the intermediary byte as a delimiter. This segmentation allows decoders to clearly identify where AVC data ends and HEVC data begins, simplifying the parsing process by providing explicit boundaries rather than requiring complex analysis to distinguish between the two coding standards.
Solution Approach 2:
Different parts of the bitstream are marked with different characteristics: AVC NAL units maintain their original AVC syntax while HEVC NAL units are preceded by a unique intermediary byte. This local differentiation allows decoders to apply appropriate parsing rules to each segment based on its type, reducing overall parsing complexity.
3Reliability
If legacy AVC decoders encounter HEVC NAL units in an AVC bitstream, then decoding errors occur, but modifying the bitstream structure to prevent errors reduces the efficiency of HEVC encoding
Solution Approach 1:
The intermediary byte serves as a protective mediator that prevents legacy AVC decoders from attempting to parse HEVC NAL units. By placing this distinctive marker before HEVC data, AVC decoders can reliably identify and skip HEVC portions without corruption, ensuring decoding reliability while preserving the integrity and efficiency of HEVC encoding.
Data Source
AI summary
A hybrid codec scalable video bitstream comprises first encapsulation units carrying video data of a base layer and encoded according to a first video coding standard and second encapsulation units carrying video data of at least one enhancement layer and encoded according to a second video coding standard. A prefix syntax element is added to each second encapsulation unit. The prefix syntax element has a value triggering a legacy decoder to discard or ignore the second encapsulation units. A non-legacy decoder selects, based on the value of a first portion of each of the first and second encapsulation units, whether to decode the encapsulation unit according to the first video coding standard or remove the first portion from the encapsulation unit and decode the remaining portion of the encapsulation unit according to the second video coding standard.