Opus audio data packets begin with a "table of contents" (TOC) sequence which defines the frame duration, audio bandwidth and coding mode of the packet, as well as describing how individual frames are packed into the data packet. [RFC 6716 Section 3.1]. Other types of data packets are used with Opus in various containers are designed to start with a sequence which is not a valid TOC. This simplifies sorting such data for muxing implementations and ensures they will be rejected by the decoder if they are accidentally passed as Opus audio data.
Below is a list of such alternate sequences, to avoid duplication.
List of invalid Opus audio data sequences
- `Op` is used as a prefix for metadata headers in .opus files. RFC 7845
- '0x3FF' in the first 11 bits marks an `opus_access_unit` in mpeg-ts. OpusTS
Constructing invalid sequences
The only restriction that doesn't depend on the number of bytes in the packet is [R5], which is, "Code 3 packets contain at least one frame, but no more than 120 ms of audio total."