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Discover the benefits H.264 scalable video codec

Posted: 16 May 2008 ?? ?Print Version ?Bookmark and Share

Keywords:H.264 scalable video codec? network bandwidth? tutorial design?

This layered approach allows the generation of an encoded stream that can be truncated to limit the bandwidth consumed or the decode computational requirements. The truncation process consists simply of extracting the required layers from the encoded video stream with no additional processing on the stream itself. The process can even be performed "in the network". That is, as the video stream transitions from a high bandwidth to a lower bandwidth network (for example, from an Ethernet network to a handheld through a WiFi link), it could be parsed to size the stream for the available bandwidth. In the above example, the stream could be sized for the bandwidth of the wireless link and the decode capabilities of the handheld decoder. Figure 2 shows such an example as a PC forwards a low bandwidth instance of a stream to a mobile device.

Figure 2: Parsing levels to reduce bandwidth and resolution.

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.

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.

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 3.75fps, only the frames that constitute T0 need be decoded. All other frames can be discarded. To play the stream at 7.5fps, 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 15fps. If all frames are decoded, the full 30fps 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.

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