Encrypted Video Stream Trick Play Control Word Synchronization
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Solution Overview
Problem
Existing methods for generating encrypted video streams face difficulties during abnormal access, such as trick play, where only partial access to the stream occurs, disrupting the regular supply of control words and making it challenging to maintain consistent playback speed in both forward and reverse directions.
Innovation Solution
Synchronizing the occurrence of decryption information, specifically ECMs, to sections containing I-frames, and changing control words midway through crypto periods to ensure availability of both current and next control words for an equal duration, allowing for maximum playback speed in both directions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If control words are supplied regularly in ECMs every tenth of a second, then decryption can commence quickly after switching to a stream, but during trick play access only part of the stream is accessed which disrupts the normal regular supply of ECMs and control words
Solution Approach 1:
The video stream is segmented into sections containing I-frames that can be decoded independently. ECMs are synchronized to these sections rather than being supplied at fixed time intervals. This segmentation allows the decryption system to reliably obtain control words whenever I-frames are accessed, regardless of whether the access pattern follows normal sequential playback or abnormal trick play patterns.
Solution Approach 2:
ECMs containing control words are placed in advance within sections containing I-frames. By synchronizing ECM occurrence with these independently decodable sections, the system ensures that control words are already available when needed for trick play operations, eliminating the need for additional ECM retrieval operations during abnormal access.
2Stability of the object's composition
If control words are changed at the border of crypto periods, then the regular supply pattern is maintained, but during reverse play the required next control word is not available because it assumes forward play
Solution Approach 1:
The control word change timing is made dynamic by synchronizing it with the occurrence of I-frames rather than using fixed crypto period borders. This allows the system to adapt to different access patterns - whether forward or reverse playback - because I-frames are naturally distributed throughout the stream and provide reliable synchronization points regardless of playback direction.
Solution Approach 2:
Different parts of the stream have different properties regarding control word availability. By placing ECMs specifically at sections containing I-frames rather than uniformly throughout, the system creates local zones where control words are guaranteed to be available. This local quality enhancement ensures that wherever I-frames occur in the stream (whether accessed in forward or reverse order), the necessary decryption information is present.
3Adaptability or versatility
If three control words per ECM are supplied (current, next future, and next past), then trick play access is supported, but additional information not provided for in standard streams is required
Solution Approach 1:
The system uses the existing I-frame structure already present in standard video streams to provide the synchronization information needed for trick play. Rather than adding separate control structures or metadata, the invention makes the stream self-sufficient by utilizing the inherent I-frame distribution to naturally provide the timing and synchronization information required for reliable control word delivery during abnormal access patterns.
Data Source
AI summary
An encrypted stream of video information (20) contains first video frames (22) and second video frames which are accessible and not accessible during trick play respectively. From a source stream encrypted that is for decryption with repeatedly changing control words sections of the stream are identified where respective first ones of the frames occur in the stream. Control words for decryption (24) are included in the stream. At least part of the control words are included in the stream at positions selected synchronized to the identified sections.