H.264 SVC Under the Hood
To achieve temporal scalability, H.264 SVC links its reference and predicted frames somewhat differently than conventional H.264 AVC encoders. Instead of the traditional Intra-frame (I frame), Bidirectional (B frame) and Predicted (P frame) relationship shown in Figure 3, SVC uses a hierarchical prediction structure.
(Click to enlarge)
Figure 3. Traditional I, P, and B Frame Relationship.
The hierarchical structure defines the temporal layering of the final stream. Figure 4 illustrates a potential hierarchical structure. In this particular example, frames are only predicted from frames that occur earlier in time. This ensures that the structure exhibits not only temporal scalability but also low latency.
(Click to enlarge)
Figure 4. Hierarchical Predicted Frames in SVC.
This scheme has four nested temporal layers: T0 (the base layer), T1, T2, and T3. Frames constituting the T1 and T2 layers are only predicted from frames in the T0 layer. Frames in the T3 layer are only predicted from frames in the T1 or T2 layers.
To play the encoded stream at 3.75 frames per second (fps), only the frames that constitute T0 need be decoded. All other frames can be discarded. To play the stream at 7.5 fps, the layers making up T0 and T1 are decoded. Frames in layers T2 and T3 can be discarded. Similarly, if frames that constitute T0, T1 and T2 are decoded, the resulting stream will play at 15 fps. If all frames are decoded, the full 30 fps stream is recovered.
By contrast, in H.264 AVC (for Baseline Profile where only unidirectional predicted frames are used), all the frames would need to be decoded irrespective of the desired display rate. To transit to a low bandwidth network, the entire stream would need to be decoded, the unwanted frames discarded, and then re-encoded.
Spatial scalability in H.264 SVC follows a similar principle. In this case, lower resolution frames are encoded as the base layer. Decoded and up-sampled base layer frames are used in the prediction of higher-order layers. Additional information required to reconstitute the detail of the original scene is encoded as a self-contained enhancement layer. In some cases, reusing motion information can further increase encoding efficiency.
Simulcast vs. SVC
There is an overhead associated with the scalability inherent in H.264 SVC. As can be seen in Figure 3, the distance between reference and predicted frames can be longer in time (from T0 to T1 for example) than with the conventional frame structure. In scenes with high motion, this can lead to slightly less efficient compression. There is also an overhead associated with the management of the layered structure in the stream.
Overall, an SVC stream containing three layers of temporal scalability and three layers of spatial scalability might be twenty percent larger than an equivalent H.264 AVC stream of full resolution and full frame rate video with no scalability. If scalability is to be emulated with the H.264 AVC codec, multiple encode streams are required, resulting in a dramatically higher bandwidth requirement or expensive decoding and re-encoding throughout the network.
Additional SVC Benefits
Error resilience is traditionally achieved by adding additional information to the stream so errors can be detected and corrected. SVC's layered approach means that a higher level of error detection and correction can be performed on the smaller base layer without adding significant overhead. Applying the same degree of error detection and correction to an AVC stream would require that the entire stream be protected, resulting in a much larger stream. If errors are detected in the SVC stream, the resolution and frame rate can be progressively degraded until, if needed, only the highly protected base layer is used. In this way, degradation under noisy conditions is much more graceful than with H.264 AVC.
Since an SVC stream or file remains decodable even when truncated, SVC can be employed both during transmission and after the file is stored. Parsing files stored to disk and removing enhancement layers allows file size to be reduced without additional processing on the video stream stored within the file. This would not be possible with an AVC file which necessitates an "all or nothing" approach to disk management.
The SVC stream or file inherently contains lower resolution and frame rate streams. These streams can be used to accelerate the application of video analytics or for cataloging algorithms. The temporal scalability also makes the stream easier to search in fast forward or reverse.