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Compressed Video Communications - Sadka A.

Sadka A. Compressed Video Communications - John Wiley & Sons, 2002. - 283 p.
ISBN: 0-470-84312-8
Download (direct link): compressedvideo2002.pdf
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A content-based error resilience tool is the adaptive INTRA refresh (AIR) defined in Annex H of the MPEG-4 video coding standard. AIR is an application-layer error-resilience tool that is also based upon an estimation of the amount of motion in a video scene. Once the motion MBs are identified, they are entered into
5.7 QUALITY OPTIMISATION FOR VIDEO TRANSMISSIONS
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a motion map and a fixed number of these motion MBs is INTRA coded in every video frame (Section 4.7.1). In order to establish the best trade-off between error resilience and throughput, the number of AIR MBs per frame could be adapted to the amount of motion in the video scene (Worrall et al., 2000). As illustrated in Section 5.6.1, there is a strong correlation between the activity of the video scene and the size of the motion information required to encode it. During periods of high scene activity, a large number of AIR MBs would cause a large drop of video quality. This quality deterioration is mainly attributed to the low efficiency of the INTRA coding mode. To optimise the video quality, it is necessary to increase the MB refresh rate after periods of high motion. This could be accomplished by observing the change in the variable A as described in Figure 5.10 for the Suzie sequence. If A increases then the number of AIR MBs encoded within a frame also increases, and vice versa. Therefore, this adaptive scheme achieves the fast updating of the video scene following periods of high motion. On the other hand, it reduces the number of AIR MBs during periods of high activity, thereby preventing an unnecessary drop in video quality. Figures 5.12 and 5.13 show the subjective and objective quality improvement, respectively, achieved by the adaptive scheme when the MPEG-4 coded Suzie sequence is transmitted over a GPRS channel using CS-1 and a C/I of 12 dB. The Suzie sequence is encoded at a bit rate of 64 kbit/s, a frame rate of 10 f/s and then encapsulated in RTP packets for real-time transmission over GPRS using the packetisation scheme of Figure 5.7. For the fixed quality control scheme, the size of each RTP packet is set to 700 bits, while the number of AIR MBs encoded per video frame is set to 8. It can be seen that a significant improvement in quality is achieved by the adaptive content-based quality control technique, both subjectively and objectively. In Figure 5.13, the PSNR recovers more rapidly with the adaptive scheme, after the high motion section, due to the increase in AIR MBs after the motion peak.
(a) (b)
Figure 5.12 Frame 28 of Suzie sequence coded with MPEG-4 at 64 kbit/s, 10 f/s, after transmission over a GPRS channel (CS-1, C/I = 12 dB): (a) fixed quality control scheme (RTP packet size = 700 bits, 8 AIR MBs/frame), (b) adaptive content-based quality control scheme
200
VIDEO COMMUNICATIONS OVER MOBILE IP NETWORKS
Suzie (Adaptive) Suzie (700 bit pk, 8AIR/frm)
Frame Number
Figure 5.13 PSNR values for 50 frame of Suzie sequence coded with MPEG-4 at 64 kbit/s, 10 f/s, after transmission over a GPRS channel (CS-1, C/I = 12 dB) for both the fixed and the adaptive content-based quality control schemes
5.8 Prioritised Transport for Robust Video Transmissions over Mobile Networks
In addition to its scaleability benefits, the layered video coding discussed in Chapter 3 has inherent error-resilience benefits, particularly when the base layer can be transmitted with higher priority and the enhancement layer(s) with lower priority. The layered video coding is usually accompanied by the use of UEP (Unequal Error Protection) to enable the high-priority base layer to achieve a guaranteed service quality and the enhancement layers to produce quality refinement, as examined in Section 4.4.1. This approach is known as layered coding with transport prioritisation, and is used extensively to facilitate error resilience in video transport systems (Wang and Zhu, 1998). An improvement to conventional layered coding algorithms used for error-resilience purposes introduces the rate-distortion optimisation factor (Gallant and Kossentini, 2001) in the multi-layer video coder. In this technique, the rate-distortion optimisation of each layer is performed in both the error-free and error-prone cases in accordance with the available bandwidth and the particular network conditions. The error-free case involves determining the optimal allocation of bit rate among the source-coding elements, and the error-prone case involves the optimal allocation of bit rate between the source-coding and channel-coding elements with emphasis on the priorities assigned to the generated video layers (UEP).
A similar method for improving the quality of video transport over networks is the prioritisation of different parts of the video bit stream by sending data as two separate streams (refer to Section 3.10). This enables the video encoder to demand that the network send the data using channels with different priorities, allocating
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