UCConfig Learning Notes: UCConfig Mode 0

In Reference Document 1, UCConfig Mode 0 is defined as: “Non‐scalable single layer AVC bitstream with Simulcast (number of simulcast streams >= 1).” In other words, UCConfig Mode 0 does not require SVC scalability support, and standard AVC encoder bitstreams generally fall into this category. However, it should be noted that although Mode 0 does not require SVC scalability, it still has explicit requirements for the NAL Unit structure of the output bitstream, most notably the inclusion of SVC-specific Prefix NAL Units.

Layering Structure

Since the encoding mode corresponding to UCConfig Mode 0 is AVC, the concept of layering does not exist. Therefore, the encoding structure contains only a single layer.

• In fact, this encoding structure is the most common one for H.264 encoders: the decoding of the first P-frame after an I-frame depends only on the preceding P-frame, while subsequent P-frame decoding depends on the I-frame and preceding P-frames. Therefore, if any P-frame in the entire GOP is lost, all subsequent P-frames in that GOP cannot be successfully decoded.

Mapping this to SigmaStar’s webcam solutions (such as SSD268G, SSC9351, etc.), this corresponds to the so-called NormalP encoding mode, which is also the default encoding mode for these solutions:

Bitstream Structure Requirements

In addition to the encoding structure mentioned above, UCConfig Mode 0 has clear requirements for the bitstream output by the encoder:

• Under UCConfig Mode 0, although SVC scalability is not required, the output bitstream must include a Prefix NAL Unit immediately preceding the NAL Unit of each compressed picture. The priority_id within this Prefix NAL Unit is used to indicate the relative priority of the bitstream, where 0 indicates the highest priority, 1 indicates the second highest priority, and so on.
  • This way, if a node sends multiple UCConfig Mode 0 streams simultaneously to a central server, the server can use this priority_id to determine the priority of each stream upon reception;
• In each Prefix NAL Unit, dependency_id, quality_id, and temporal_id must all be 0; no_inter_layer_pred_flag, discardable_flag, and output_flag must be 1; and use_ref_base_pic_flag must be 1.
• For decoders that do not support the SVC mode, if a Prefix NAL Unit is encountered in the received streaming data, the decoder will simply discard that NAL Unit. Therefore, non-SVC-aware decoders can still successfully decode bitstreams complying with the UCConfig Mode 0 specification.

Reference Documents:

• H.264 AVC/SVC UCConfig Mode Specification V1.1