avcC

avcC is a box in the MP4 (ISO/IEC 14496-15) file format that contains the AVCDecoderConfigurationRecord for H.264/AVC video streams. It is stored inside the sample description of a video track (stsd) when the track uses the avc1 or avc3 codec. The avcC box provides the necessary decoder configuration to initialize a decoder before processing the video data.

The primary purpose of avcC is to convey the essential parameters and the initial parameter sets required by decoders. It includes profile and level information, the length of NAL units, and the actual parameter sets in a compact form. Specifically, the record begins with configurationVersion, AVCProfileIndication, profile_compatibility, and AVCLevelIndication, followed by lengthSizeMinusOne (with reserved bits). It then specifies the number of Sequence Parameter Sets (SPS) and, for each SPS, its length and data. After that, it includes the number of Picture Parameter Sets (PPS) and, for each PPS, its length and data. The SPS and PPS NAL units are the critical parameter sets that describe how the video stream should be decoded.

Usage of avcC is to allow devices and software decoders to configure themselves and validate compatibility before decoding any frames. By providing SPS/PPS and related parameters in advance, it enables prompt and correct initialization of the decoder, avoids per-frame parsing delays, and helps ensure synchronized playback across different containers and players. If avcC is missing or corrupted, playback may fail to initialize the decoder or may require the player to extract SPS/PPS from the elementary stream, which can complicate playback.

Creation and maintenance of avcC are typically handled by video encoders or muxers when producing MP4 files. Some containers and tools may update or replace avcC if SPS/PPS change during a session, though such changes are relatively uncommon for standard AVC streams. See also: H.264/AVC, MP4 container formats, AVCDecoderConfigurationRecord.