www.elsevier.com/locate/jvci

J. Vis. Commun. Image R. 18 (2007) 186–190

Short Communication

Video transmission using advanced partial backwarddecodable bit stream (APBDBS) q

Yong Fang a,*, Cheng-ke Wu a, Lu Yu b

a National Key Laboratory on ISN, Xidian University, Xi’an, Chinab Institute of Information and Communication Engineering, Zhejiang University, Hangzhou, China

Received 25 October 2005; accepted 23 October 2006Available online 27 December 2006

Abstract

An analysis on the error robustness of partial backward decodable bit stream (PBDBS) presented by Gao and Tu shows that bits atboth ends of video packets are safer than those in the middle when the PBDBS is used. According to this, a new structure of video packetcalled advanced partial backward decodable bit stream (APBDBS) is proposed in this paper. First, different types of syntax elements invideo packets are reordered according to their syntactical importance. The more important (such as motion vectors) are placed at bothends of video packets, while the less important in the middle. Second, MBs in video packets are reordered according to their spatialimportance. MBs near to the centers of images are placed close to both ends of video packets while marginal MBs are placed in the mid-dle. The simulation shows that with the APBDBS used, up to 2 dB luminance peak signal noise ratio (PSNR) improvement can be foundcompared with the PBDBS.� 2006 Elsevier Inc. All rights reserved.

Keywords: Error resilience; PBDBS; APBDBS; Resynchronization; Video transmission

1. Introduction

Recently, many video coding standards have been devel-oped such as H.26· series and MPEG series [2–4]. On theone hand, these video coding standards achieve high com-pression rate. On the other hand, compressed video bitstream is more sensitive to channel errors when transmittedover noisy channels such as wireless networks because ofspatial and temporal propagation due to using variablelength coding (VLC) and motion compensation, respective-ly. In spite of the usage of channel coding (such as forwarderror corrected, FEC), errors in bit stream are stillunavoidable, so robustness against channel errors is animportant property of video bit stream and a good over-view on this topic can be found in [6]. Many methods have

1047-3203/$ - see front matter � 2006 Elsevier Inc. All rights reserved.

doi:10.1016/j.jvcir.2006.10.003

q This work was funded by National Natural Science Foundation ofChina under Grant Nos. 60532060 and 60333020.

* Corresponding author.E-mail address: yfang79@gmail.com (Y. Fang).

been presented for this purpose, such as layer coding [7],error resilient entropy coding (EREC) [8], multiple descrip-tion coding (MDC) [9], error concealment [12–15], and soon. These methods do improve the quality of reconstructedimage at decoders. However, layer coding and multipledescription coding will decrease coding efficiency andEREC will increase computational complexity. To avoidthe loss of consequent MBs, resynchronization markersare usually inserted which synchronizes bit stream at thecost of the decrease of coding efficiency [10]. In addition,reversible variable length coding (RVLC) [11] method isalso proposed to decode bit stream from two directions.This method can retrieve bit stream after errors and limiterror propagation, but decreases coding efficiency andincreases decoding complexity.

A new method, called PBDBS, which is similar toRVLC but substantially different, is proposed in [1]. Thismethod reverses the bit stream of some coded MBs so thatthese coded MBs can be decoded in a backward direction.It makes the resynchronization markers synchronize bit

Y. Fang et al. / J. Vis. Commun. Image R. 18 (2007) 186–190 187

stream in two directions and can limit error propagation.Therefore it improves the quality of reconstructed imageswhen bit stream is transmitted across noisy channel.

Based on the PBDBS, in this paper, a new structure of vid-eo packet is proposed which is called advanced partial back-ward decodable bit stream (APBDBS). In the APBDBS,different parts in video packets are reordered according totheir syntactical and spatial importance, i.e., importantinformation is placed as close as possible to both ends of vid-eo packets, while the less important near to the middle.Therefore, error resilience of video bit stream is improved.

This paper is organized as follows. After introducing theframework of the PBDBS, we theoretically analyze how theposition of bits in video packet affects their decodability inSection 2. Then in Section 3, we proposed our improve-ment on the PBDBS based on the analysis. Simulationparameters and results are presented in Section 4, and con-clusions are given in Section 5.

2. Analysis on error resilience of PBDBS

Gao and Tu [1] propose a novel video transmissionscheme called PBDBS. In the PBDBS, the bit stream ofsome MBs in a video packet (or a GOB in H.263+) isreversed so that these MBs can be decoded in a backwarddirection. In Fig. 1, the standard structure of video packetsis compared with the structure when the PBDBS is used. Inthe PBDBS structured video packet, MBs before theswitching point j are forward decoded, while MBs after j

are backward decodable. This scheme makes resynchroni-zation markers synchronize bit stream in two directionsand can limit error propagation, therefore it improves thequality of reconstructed images when bit stream is trans-mitted across noisy channel.

Because of the usage of variable length codes (VLC), theoccurrence of random bit errors corrupts not only the cur-rent syntax element but also succeeding bits until to nextresynchronization markers (in that succeeding bits will beundecodable or be decoded erroneously). Therefore, the

Fig. 1. Structure of vide

probability of bits to be decoded correctly depends on theirposition in video packets. From this, it is obvious thatwhen the PBDBS is used, the decodability of bits at bothends of video packets is the highest and decreases when bitsare closer to the center.

To obtain better error resilience, important informationshould be placed as close as possible to both ends of videopackets when the PBDBS is used, which is the topic of Sec-tion 3.

3. Advanced partial backward decodable bit stream

(APBDBS)

Based on the analysis in Section 2, an improvement onthe PBDBS is proposed in this section which is referredto as advanced partial backward bit stream (APBDBS).In our proposed structure, video packets are reordered fur-thermore according to the syntactical and spatial impor-tance of different parts of bit stream. Our proposed videopacket structure is shown in Fig. 2.

3.1. Modified structure of video packet

First, the difference of syntactical importance of differ-ent parts in video packets is utilized. This is similar to datapartitioning [5] in some sense. Usually, mode information,coded block patterns (CBP) and motion vectors (MV) ofMBs are more important than transform coefficients.Therefore, header and motion information of all MBs invideo packets is placed at the beginning of the video packetwhen data partitioning is used. Between header and textureinformation, motion marker is inserted. However the dif-ference between the APBDBS and data partitioning layson that part of motion and header information is reversedand placed at the end of video packets. As shown in Fig. 2,headers and direct current (DC) components (for intra cod-ed MBs) or motion vectors (for inter coded MBs) of MBsj + 1 to k are backward decodable and placed just beforethe next resynchronization marker. Between two halves

o packet (or GOB).

Fig. 2. Advanced PBDBS structure of video packets.

Fig. 3. Prediction directions of motion vector coding.

188 Y. Fang et al. / J. Vis. Commun. Image R. 18 (2007) 186–190

of motion and header information is texture information.Just as header and motion information, texture informa-tion of MBs j + 1 to k is reversed and backward decodable.

Second, the difference of spatial importance is utilized.Usually, audience pay more attention to the central fieldof pictures, so central MBs have greater subjective impor-tance In addition, in common video sequences, picturesare composed of a static background and several objectswith high motion activity located at the center, which indi-cates that central MBs have greater objective importance.This difference is exploited in the APBDBS. Assuming thateach video packet contains just a row of MBs (e.g., oneGOB in H.263), the positions of MBs in bit stream will cor-respond to their spatial positions. As shown in Fig. 2, MBs1 to j are reordered. It deserves to be pointed out that MBs1 to j are reversed MB by MB, while MBs j + 1 to k arereversed bit by bit. Therefore, MBs 1 to j are still forwarddecodable. The only difference is that central MBs willalways be decoded before marginal MBs.

Given below is the rough description on the decodingprocess. First, header and motion information of MBs j

to 1 is decoded. The decoding order is from MB j to MB1. At the same time, header and DC/motion informationof MBs j + 1 to k is decoded independent of that of MBsj to 1. Then, texture information of MBs j to 1 and MBsj + 1 to k is decoded separately.

3.2. Backward predicted coding of motion vectors

and DC/AC components

Just as described above, the decoding order of MBs 1 toj is reversed, so any intra prediction technique must bechanged. These techniques include differential coding ofMVs and intra DC–AC prediction. Below modified predic-tion technique for motion vectors is described and similarmodification can be applied to intra DC–AC prediction.

In the existing standards, the motion vector componentsare intra predicted by using a spatial neighborhood of threemotion vectors already transmitted. For each component,

the median value of the three candidate motion vectorsfor the same component is computed as the predictor.The selection of the three candidate motion vectors isshowed in Fig. 3a.

With the APBDBS applied, for MBs 1 to j, the leftmotion vector (MV1) becomes unavailable. To get aroundthis problem, the prediction direction is reversed corre-spondingly. Fig. 3b shows the modified prediction direc-tion. Because of the random property of motion, thismodification will not introduce any decrease in coding effi-ciency. Experimental results also confirm that the differencebetween forward and backward predictions is trivial.

4. Results and discussions

4.1. Error concealment

Although our proposed structure can obtain betterimage quality, degradation will still be perceived becausesome MBs are lost. To mask these effects, error conceal-ment at decoders is necessary. Many methods for errorconcealment have been proposed, such as maximallysmooth image recovery (MSR) proposed by Y. Wang[13], spatial interpolation using projections onto convexsets (POCS) [15] proposed by H. Sun and so on. In oursimulation, error concealment is simple. For intra MBs,concealment is realized by spatial interpolation using pix-els of neighboring MBs. Only correctly received MBs areused for concealment (if there is no correctly receivedneighboring MB, pixel values are set to 128). For inter

Y. Fang et al. / J. Vis. Commun. Image R. 18 (2007) 186–190 189

MBs, the motion compensated reference blocks are copieddirectly to conceal damaged MBs if motion vectors areavailable. If motion vectors are unavailable, they are setto zeros.

4.2. Results

Three standard video sequences are used in our simula-tion: Foreman, Coastguard and Container. All thesesequences are QCIF format, 25 frames/s. The size of groupof pictures (GOP) is 30, i.e., for every 30 frames, one Iframe is inserted. The length of each tested sequence is300 frames. Quantization step is fixed to 8. Every videopacket contains a row of MBs (11 MBs for QCIF format).Six WCDMA error patterns [16] are applied.

The objective comparisons are listed in Table 1. In Table1, STD stands for standard video packet structure. We use

Table 1Objective comparison between three different structures (PSNR, in dB)

BER Foreman Coastguard

STD PBDBS APBDBS STD

Error free 33.89 32.93WCDMA-1 18.86 20.29 22.01 18.46WCDMA-2 20.17 21.73 23.13 20.17WCDMA-3 18.50 20.45 22.86 20.09WCDMA-4 28.13 28.62 29.01 29.89WCDMA-5 28.87 28.89 29.43 27.42WCDMA-6 28.90 29.92 30.41 29.03Average 23.91 24.98 26.14 24.18

Fig. 4. The 1th reconstructed frame for three structures (the top three are reconconcealment).

Fig. 5. The 16th reconstructed frame for three structures (the top three are reerror concealment).

average luminance PSNR (in dB) to evaluate objectivequality of images. It is easy to find that our proposed struc-ture is superior to the original PBDBS structure. Up to2 dB improvement can be found in the APBDBS comparedto the PBDBS.

Figs. 4 and 5 show the subjective comparisons of threestructures. It is easy to find that the APBDBS has betterimage quality compared to the PBDBS. Because DCs areplaced close to both ends of video packets, the 1th fame(I frame) of our proposed structure is much better thanthat of other structures (Fig. 4). In addition, becauseMVs are placed more safely, temporal propagation of theAPBDBS is less serious than that of other structures(Fig. 5).

To sum up, from both objective and subjective qualitycomparisons, our proposed structure is superior to the ori-ginal PBDBS structure.

Container

PBDBS APBDBS STD PBDBS APBDBS

35.2120.25 21.60 25.03 26.47 27.1220.56 22.16 28.12 28.61 29.5421.85 22.49 29.97 29.75 30.3529.52 30.07 33.45 34.00 34.2928.70 29.59 34.88 34.95 34.9729.63 30.27 34.40 34.63 34.7725.09 26.03 30.98 31.40 31.84

structed images without error concealment and the bottom three with error

constructed images without error concealment and the bottom three with

190 Y. Fang et al. / J. Vis. Commun. Image R. 18 (2007) 186–190

5. Conclusions

In this paper, we propose a new video packet structure(i.e., APBDBS) which reorders contexts of video packetso that more important information are put closer to bothends of video packets. The APBDBS outperforms thePBDBS due to it exploits the syntactical importance of syn-tax elements and the spatial importance of MBs. This supe-riority is proven by theoretic analysis and experimentalresults. The APBDBS greatly improves the image qualitytransmitted over error-prone channels while does notincrease the complexity and coding overhead.

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