In view of perfect compatibility with the standard source and channel codec, temporal sampling-based multiple description coding (MDC) has become a better choice for practical applications. However, for the frames change from one scene to another temporal correlation may be destroyed by temporal sampling extremely, which results in the false estimation when the related frames are lost at the side decoder. Therefore, in this article the frames containing scene change are detected and duplicated before temporal sampling, which maintains better temporal correlation in each description. Furthermore, for better rate distortion performance temporal sampling is employed adaptively, that is, frame skipping or up-sampling according to the motion characteristics in original video. The experimental results exhibit better performance of the proposed scheme than other schemes whether in the on–off MDC environment or packet lossy network, especially about 15 dB improvements for the frames with scene change. Therefore, it may be a promising choice for video transmission over error-prone channels, especially over wireless networks.
Keywords:Video coding; Multiple description coding; Temporal sampling
In the recent years, the increasing demands for multimedia communication have generated a lot of research interests in developing novel image or video compression technologies. Due to network congestion and delay sensibility, it is always a great challenge for video transmission over lossy network. Multiple description coding (MDC) is an attractive approach to solve this problem. It can efficiently combat packet loss without any retransmission thus satisfying the demand of real-time services and relieving the network congestion .
MDC encodes the source message into several bit streams (descriptions) carrying different information which then can be transmitted over multiple channels. In its simplest form, two parallel channels are assumed to connect the source with the destination. If only one channel works, the descriptions can individually be decoded to sufficiently guarantee a minimum fidelity in the reconstruction at the receiver. However, when both channels work, the descriptions from the channels can be combined to yield a higher fidelity reconstruction.
In the past years, many approaches for realizing MDC have been proposed, such as those using interleaved scalar quantizer , lattice vector quantizer [3,4], pairwise correlating transforms , FEC , and so on. Although these methods have shown good performance, they are incompatible with widely used standard codecs, such as H.26x and MPEG-x.
To overcome the limitation, pre- and post-processing-based MDC may be a good choice. In pre-processing, the original source is split into multiple sub-sources before encoding and then these sub-sources can be encoded separately by the standard codec to generate multiple descriptions. The study of  is a typical example, which employs hierarchical B pictures in the H.264/AVC scalable extension  for a pre-processing-based MDC. Furthermore, sub-sampling technique can also be applied in pre-processing to realize multiple sub-sources, such as the MD video coder based on spatial sampling  and the MD video coder based on temporal sampling . In the method of spatial sampling, through zero padding inside each individual frame, only the correlation of intra-frame is considered to improve side distortion and the temporal correlation of inter-frame is neglected completely. In , the method of temporal sampling is proposed to make motion compensation interpolation (MCI) more efficient for lost frames, which turns to better rate distortion performance. However, considering the video sequences containing different scenes, simple MCI method may lead to false interpolated reconstruction at the decoder. Recently, for robust transmission over wireless network, in [11,12] some new MD methods like distributed MDC have been proposed to achieve better rate distortion performance by optimized redundancy allocation for the splitting video sub-sequences.
To address this problem from scene change, a preliminary scheme is presented in . Here, in this article an improved MD coding based on adaptive temporal sampling is proposed to make sure the decoder can work correctly when scene changes. According to the temporal correlation in the original video, the frames containing scene change can be detected before temporal sampling. Then, these frames are duplicated and transmitted over both channels for better side reconstruction. Furthermore, adaptive temporal sampling is applied, that is, frame skipping and up-sampling in terms of motion characteristics between the frames, which can achieve better rate distortion performance.
The rest of this article is organized as follows. In the following section, the proposed MD video coding scheme is presented including pre- and post-processing stages and improvement for scene change. The performance of the proposed scheme is examined against some other relevant MD coders in Section “Experimental results”. We conclude the article in Section “Conclusions”.
Proposed MDC scheme for scene change
Figure 1 illustrates the proposed MDC scheme for scene change. In the pre-processing stage, scene change is first detected, and then considering the different motion information between frames, if the inter-frame motion is enough smooth some frames are skipped to obtain high compression efficiency. On the other hand, if the abrupt motion occurs, the original video sequence is up-sampled to generate a new-length video with adaptively redundant frames. In the post-processing, the corresponding decoders should be designed with two main functions. One is down-sampling the decoded video stream to erase the redundant frames. The other is using error concealment methods to estimate lost frames. The details of adaptive temporal sampling and the improvement for scene change are shown in the following subsections.
Figure 1. Block diagram of proposed scheme.
Adaptive temporal sampling in pre-processing
In terms of the principle of MDC, higher quality of side decoded video will result from more correlations in descriptions because better error concealment is available, but more redundancy introduced will also bring about lower efficiency to the central decoder. Obviously, it is a better solution that the redundancy added should make a tradeoff between the reconstructed quality and compression efficiency. As a result, in the pre-processing stage, the sampling with flexible frame rate is employed to introduce the adaptive redundancy. Since unstable motion appearance of inter-frames will affect the performance of error concealment at the side decoder, the rate of sampling should be various according to the motion information between any two neighboring frames. More interpolated frames or higher rate of up-sampling will be utilized to smooth the abrupt motion between the frames. On the other hand, if no abrupt motion happens, no redundant frames are needed. Furthermore, if the motion information between continuous frames is enough stable, some middle frames can be skipped to guarantee the high compression efficiency. Such method of pre-processing mainly aims to generate descriptions with regular motion which can make better estimation of lost frames available at the side decoder.
For any two neighboring frames denoted by Fk and Fk+1, all the motion vectors for each macroblock are computed and their maximum value can be obtained. Here, it is denoted by and ((x, y) is the coordinates of the maximal motion vector). Then, the module is compared with the threshold T1. If , some frames may be skipped. But, the number of skipped frames depends on the module of maximal motion vectors between Fk and the following frames of Fk+1. At the same time, considering the balance of two channels and better evaluation at side decoder, even frames in the middle are skipped. For example, if , , and , then two frames Fk+1 and Fk+2 are skipped and Fk and Fk+3 are retained for better side reconstruction. Furthermore, if , the motion between the two frames is considered unstable so redundant frames are interpolated to smooth such motion information. In view of the balance of two channels, even frames are interpolated to maintain equal frame number of two descriptions. Consequently, the number of interpolated frames is computed by and the redundant frames can be generated using the general algorithm of motion compensated interpolation, such as . It is noted that for an original video with N frames if the mean value of all can be computed as follows.
Then the thresholds T1 and T2 are set empirically as T1 = T and T2 = 2T.
In the pre-processing, new interpolated frames and skip some other frames may increase the encoding computational complexity to some extent. However, the increasing complexity mainly comes from the decision of interpolated or skipped frames. After the simple decision using motion vector, only a few frames need to be interpolated or skipped. Therefore, it cannot lead to higher complexity.
Additionally, the labels (‘O’, ‘I’, or ‘S’) are set for each frame to distinguish the original frame, interpolated one, or skipped one, then indexed with odd or even numbers and transmitted over two channels, respectively. Here, the labels are coded by entropy encoder, which nearly can be neglected compared with the total bit rate.
Improvement for scene change
It is noted that the above temporal sampling method is proposed assuming any two neighboring frames are correlative in the video. However, considering the video comprised by different scenes, there is no correlation between the last frame in previous scene and the first frame in subsequent scene even if the two frames are neighboring. Here, we design the video sequence “merged” with three scenes, that is, the first 30 frames of “coastguard.qcif”, “bridge close.qcif”, and “highway.qcif” as an example, which is shown in Figure 2a–c. If using the simple comparison between and the threshold T2, the interpolated frames will be inserted between two neighboring frames where scene change happens. Obviously, this will lead to substantial quality degradation, as shown in Figure 2d–e.
Figure 2. Three different scenes in the test video sequence “merged”. (a) Scene 1, (b) Scene 2, (c) Scene 3, (d) Frame interpolated between Scenes 1 and 2, (e) Frame interpolated between Scenes 2 and 3.
To address this problem, we must find out where scene change occurs in the video first. Here, we utilize the sum of compensated absolution difference (SCAD) as the criterion. The value of SCAD can be calculated as follows. Let denote the pixel at the coordinates (x,y) in the current frame Fk-1. And its previous frame Fk-1 can be considered as the reference frame. Then, the best match block for each block in frame Fk can be searched in its reference frame Fk-1 and all the searched block from frame Fk-1 can be produced as the motion compensated frame . Therefore, the SCAD can be calculated by , where is the pixel at the coordinates (x,y) in the motion compensated frame and the resolution of the video is M × N pixels. Figure 3 shows the statistics of SCAD values for the video “merged”. From Figure 3, the obvious scene changes can be found out at the sharp changes of the curve.
Figure 3. The SCAD values in the test video sequence “merged”.
To distinguish the scenes in the video, new marks are needed, that is, ‘A’ represents the first frame of a scene and ‘Z’ represents the last frame. When scene change appears, both the first and the last frames of the current scene will be transmitted over all channels to ensure the intact rebuilding of frames. Here, take two channels as example. At the encoder frame Fk-1 and Fk with scene change will be duplicated and the ‘ZZAA’ will be labeled at the same time. Then adaptive temporal sampling will be applied, which is introduced in Section “Adaptive temporal sampling in pre-processing”. At the decoder, if two channels can work the duplicated frames will be deleted to obtain the central reconstruction. If only one channel works, side reconstruction can be accepted according to the labels.
Adaptive temporal sampling in post-processing
Decoder design for the on–off environment
In the post-processing stage, two situations for decoding should be taken into account, that is, the design of central or side decoder. Since the two descriptions are generated by odd and even splitting, at the central decoder the video streams from the standard decoder can be interleaved and realigned in the same way firstly. According to the labels, the interpolated frames can be down-sampled and the skipped frames can be interpolated to obtain the central reconstruction. If only one channel works, that is, the side decoder is employed, all the skipped frames with label ‘S’ should be interpolated first and reset their labels with ‘O’. Then, four possibilities exist at the side decoder.
(1) If the current label is ‘O’ but its following label is ‘I’, the represented frame is just the reconstructed one.
(2) If the current label is ‘I’ but its following label is ‘O’, the represented frame is the interpolated frame and it can be regarded as the reconstructed one.
(3) If the current label is ‘I’ and its following label is also ‘I’, the continuous frames represented by ‘I’ should be merged to a reconstructed frame.
(4) If the current label is ‘O’, and its following label is also ‘O’, a new frame should be interpolated between the two frames denoted by ‘O’.
In Figure 4, a simple example illustrates the pre- and post-processing. The original video sequence has ten frames denoted by F1 to F10. After pre-processing, the motion-modified video has 16 frames. From the figure, we can see even frames are interpolated adaptively, such as two frames interpolated between F1 and F2, four frames interpolated between F4 and F5. At the same time, even frames are skipped adaptively, such as two frames F6 and F7 skipped. After splitting by odd and even frames, the generated descriptions are denoted by video on channel 1 and video on channel 2 and the labels are ‘OIOIIOSIO’ and ‘IOOIISOIO’, respectively. At the receiver, the skipped frames with label ‘S’ are reconstructed first and reset the labels with ‘O’, so the labels can be updated as ‘OIOIIOOIO’ on channel 1 or ‘IOOIIOOIO’ on channel 2, respectively. When only channel 1 works, the reconstruction from side decoder 1 is achieved like the figure. The two interpolated frames between frames 3 and 5 will be merged into a new reconstructed one while a new frame is interpolated between frames 5 and 7 to estimate the lost frame 6. On the other hand, if two channels work, the lossless frames except two skipped frames F6 and F7 can be achieved without the processing by H.264 codec.
Figure 4. A simple example for the pre- and post-processing stages.
Decoder design for packet lossy network
In packet lossy network, due to both descriptions received with packet losses, only central decoder should be designed in post-processing stage and it is different from the approach in on–off MDC environment.
Figure 5 shows the block diagram of central decoder in packet lossy network. After H.264 decoding, the two generated video sequences are interleaved by odd and even frames to produce a new video data. Then, such video data are processed to obtain central reconstruction.
Figure 5. Block diagram of central decoder for packet lossy network.
In post-processing stage, temporal error concealment is first used to reconstruct those lost packets. Since the redundant frames exist, more motion information can be preserved than the conventional scheme, which turns to better reconstruction quality. After temporal concealment, those redundant frames are down-sampled. However, with the increasing of packet loss rate, there are still errors which are difficult to be concealed only by motion compensation, such as Figure 6a. Furthermore, error propagation will happen when such frames are up-sampled to interpolate skipped ones. As a result, before frame interpolation, spatial error concealment is employed to improve the performance from temporal error concealment. Figure 6b shows obvious visual improvement to substantiate the performance of spatial concealment. Here, the standard test video “coastguard.qcif” is coded at 80 kbps per channel with packet loss rate 15% per channel.
Figure 6. Error concealment in post-processing. (a) without spatial concealment: 26.742 dB; (b) with spatial concealment: 27.868 dB.
Performance in on–off MDC channels
Here, there are mainly two experiments taken into account to present the efficiency of adaptive sampling in temporal domain. The first one is shown the better performance of the proposed scheme than the conventional scheme without pre-processing stage. In the second experiment, the advantage of the proposed scheme is illuminated compared with the scheme using spatial sampling .
In the first experiment, the standard test video “coastguard.qcif” is used with 30 frames per second. For a fair comparison, the same mode and parameters are chosen in H.264 encoder and decoder .
Figure 7 shows the central and side distortion of the proposed scheme against the conventional scheme at the bit rate from 50 to 300 kbps per channel. From the figures, we can clearly see that our proposed scheme can still consistently improve around 0.8–1.5 dB in central distortion and 0.5–1.7 dB in side distortion. Obviously, this is just a global comparison for the whole video. In fact, some individual frames may achieve more advantages over the conventional schemes. From Figure 8, the side reconstructed frames by the two compared schemes are presented to illustrate the visual improvements of the proposed scheme at the bit rate 140 kbps.
Figure 7. Comparison between the proposed and conventional scheme. (a) Rate/central distortion performance; (b) Rate/side distortion performance.
Figure 8. Comparison of side reconstruction for 72nd frame. (a) The conventional scheme: 22.999 dB, (b) the proposed scheme: 32.014 dB.
Figure 9 shows an example of the pre-processing for the standard video sequence “Coastguard.qcif”. From Figure 9, it is shown the maximum values of motion vectors, that is, ∥MV∥s. Here, the threshold T = 1.6003. In the original video “Coastguard.qcif”, according to the pre-processing mentioned in Section “Adaptive temporal sampling in pre-processing”, only 30 frames need to be interpolated and 18 frames need to be skipped, which can also be found in Figure 9.
Figure 9. An example of pre-processing for the test video “coastguard.qcif”: maximal motion vector between frames and the number of interpolated or skipped frames.
Furthermore, Figure 10 shows the performance comparison between the proposed scheme and other schemes, such as the scheme in  using spatial sampling and the schemes in [11,12] using distributed MDC. From Figure 10, we can see that compared with the scheme in  better rate and central/side distortion performance achieved by the proposed scheme, that is, at the same bit rate, 1–2 dB improvement in central distortion and 1–3 dB in side distortion. Additionally, compared with the schemes [11,12] the proposed scheme has obtained 0.5–1.5 dB improvement in central distortion and 1–2 dB in side distortion.
Performance in packet lossy network
Here, the standard test video “coastguard.qcif” is chosen to examine our proposed scheme with 30 frames per second. For a fair comparison, the same mode and parameters are chosen in H.264 encoder and decoder. Furthermore, the organization of packets is a checkerboard type for all the cases.
First, we validate the effectiveness of the proposed decoder with temporal-spatial concealment. In Figure 11, at the different bit rate 30, 80, and 180 kbps, the decoder with temporal-spatial concealment has better performance than the decoder only with temporal concealment. Especially at the larger packet loss rate, the proposed scheme has improved around 0.8–1 dB.
Figure 11. Comparison between the temporal-spatial concealment and temporal concealment. (a) 30 kbps, (b) 80 kbps, (c) 180 kbps.
In Figure 12, at the same bit rate 180 kbps, the proposed scheme has achieved better performance than the conventional ones. The improvements are brought from 0.6 to 1.1 dB. Note that for low packet loss rate, the improvement is greater. With high packet loss rate, both the proposed scheme and the conventional one cannot do well in error concealment with so many packets lost.
Figure 12. Comparison between the proposed scheme and the conventional one.
Performance for scene change
Here, the test video “merged” in Section “Proposed MDC scheme for scene change” is used to exhibit the better performance of the proposed scheme for scene change. In the proposed one the three scene changes have been taken into account when using adaptive temporal sampling while in the compared one scene changes have also been processed only through adaptive temporal sampling. For fair comparison, the same method of adaptive temporal sampling has been adopted for the above two schemes.
Figure 13 shows the central and side distortion of the proposed scheme against the compared scheme at the bit rate from 50 to 450 kbps per channel. Here, the bit rate means the total bit rate from both channels, the central quality denotes the reconstruction from both channels and the side quality is the average PSNR values from two side decoders. From the figures, we can clearly see that our proposed scheme can still consistently improve around 2–5.5 dB in side distortion although it has a slight decline in the central distortion. Obviously, this is just a global comparison for the whole video. In fact, some individual frames with scene change may achieve more advantages over the compared scheme. In Figure 14, the proposed scheme has achieved 15 dB improvement performance at 240 kbps for the frames with scene change.
In view of perfect compatibility with the standard codec, the MD video coding based on pre- and post-processing may be a better choice in the practical applications. In this article, adaptive frame skipping or up-sampling is employed in the pre-processing to obtain better tradeoff between the compression efficiency and reconstructed quality. Furthermore, when scene change happens in the original video, the pre-processing are improved to avoid error frame interpolation, even severe quality degradation. As a result, the proposed MD video coding scheme has demonstrated superior rate-distortion performance to the conventional MD video coder and spatial sampling-based scheme, which may be a promising choice for video transmission over error-prone channels, especially over wireless networks.
In this article, a novel scheme of multiple description coding and transmission has been proposed based on adaptively temporal sampling, especially for scene change. The experimental results have shown better performance of the proposed scheme than other corresponding schemes whether in the on–off MDC environment or packet lossy network, especially for the frames with scene change.
This study was supported in part by the 973 program (2012CB316400), the National Natural Science Foundation of China (Nos. 61103113, 60903066), the Beijing Natural Science Foundation (No. 4102049), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20090009120006), the Fundamental Research Funds for the Central Universities (No. 2011JBM214), the Jiangsu Provincial Natural Science Foundation (BK2011455) and PHR (IHLB) (PHR201008187).
VK Goyal, Multiple description coding: compression meets the network. IEEE Signal Process. Mag. 18(5), 74–93 (2001). Publisher Full Text
AR Reibman, H Jafarkhani, MT Yao Wang, AR Reibman, H Jafarkhani, W Yao, MT Orchard, R Puri, Multiple-description video coding using motion-compensated temporal prediction. IEEE Trans. Circuits Syst. Video Technol 12(3), 193–204 (2002). Publisher Full Text
S Chang, PC Cosman, LB Milstein, Performance analysis of channel symmetric FEC-based multiple description coding for OFDM networks. IEEE Trans. Image Process. 20(4), 1061–1076 (2011). PubMed Abstract | Publisher Full Text
C Crave, B Guillemot, O Crave, C Guillemot, B Pesquet-Popescu, C Tillier, Distributed temporal multiple description coding for robust video transmission. EURASIP J. Wirel. Commun. Netw. (2008) Article ID 183536 (2008)
ITU-T Rec. H.264 | ISO/IEC 14496-10 AVC Draft ITU-T Recommendation and Final Draft International Standard of Joint Video Specification (JVT Doc JVT-G050, Joint Video Team (JVT) of ISO/IEC MPEG & ITU-T VCEG(ISO/IEC JTC1/SC29/WG11 and ITU-T SG16 Q.6) 7th Meeting, Pattaya, Thailand, 2033) 7-14 March