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Insertion of CRC Bits into MPEG-4 Bitstream for Error Detection at Frame Level Bhumin Pathak1 , Geoff Childs2 and Maaruf Ali2 1 Airvana
2 School
Inc., Chelmsford, USA of Technology at Oxford Brooks University, Oxford, UK
It is often required for video decoders to correctly identify the type of transmission errors in order to apply the most appropriate and hence most effective error correction tools. The error resilience tool like NEWPRED in the MPEG-4 standard depends on the error detection at frame level or the video packet level. In this paper, error detection with the help of 16 bits CRC insertion in MPEG-4 bitstream is discussed and performance enhancement due to the NEWPRED demonstrated. Keywords: MPEG-4, CRC, Error detection, UMTS. 1. Introduction
A simpler way to identify errors at the application layer is required in order to implement the error resilience capabilities of the video decoder as well as for certain specific information to aid software operation. Error detection is not provided by the MPEG-4 standard at frame level. This results in the dependency of some of its error resilience tools in the measurements provided by the lower layer of the data receiving protocol stack. The mapping of errors identified by the lower layer to the application layer with the precision of a single video frame or video packet often results in a complicated process consuming a considerable amount of processing capabilities. Insertion of CRC bits in the standard MPEG-4 bitstream at frame level provides a simpler solution to this problem. However, with the insertion of extra CRC bits which are not defined in the standard, this normally results in the production of an incompatible bitstream that a standard MPEG-4 decoder somehow then has to contend with. But as mentioned in [1] and [2], this would not be the case if these bits were to be inserted at a particular place in the standard MPEG-4 bitstream.
Erroneous frame numbers identified by the CRC error detection mechanism at the decoder side can be used to send the feedback from the decoder and the encoder can then make the changes in the references of the frames yet to be encoded. This reduces further the propagation of errors in the decoded bitstream. This technique is known as NEWPRED which is described in [3]. 2. CRC Insertion
In the proposed method a 16 bit CRC is added at the end of each frame (I, P or B) of the standard MPEG-4 bitstream as shown by Figure 1.
1
2
1 - Frame Header 2 - Frame Data Figure 1.
3
1
2
3
3 - CRC bits
Insertion of CRC bits at the frame level.
In this method, 16 bits of CRC is generated using polynomial (G16) defined for the MAC layer of the UMTS architecture. While decoding, the decoder is aware of
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the total number of macroblocks (MB) in each frame. It starts searching for a new video frame header after decoding these macroblocks. It ignores everything between the last marcoblock of the frame and the next frame header as padding. If generated CRC bits are inserted in this place, after the last marcoblock and before the next header, this should preserve the compatibility of the bitstream with the standard MPEG-4 bitstream. Such insertion of CRC does not affect the normal operation of any standard MPEG-4 decoder. Also because the inserted CRC is only 16 bits in length, it is not possible for it to emulate any start sequences. This method adds an additional 16 bits of overhead to each frame but the performance improvements in the video quality with the NEWPRED implementation aided with the CRC error detection operation justifies this overhead. 3. Introduction to Newpred
The MPEG-4 “ISO/IEC 14496 [standard] provides error robustness and resilience [capabilities] to allow accessing of image or video information over a wide range of storage and transmission media. The error resilience tools developed for this part of ISO/IEC 14496 can be divided into three major categories. These categories include synchronization, data recovery, and error concealment.” [3] The NEWPRED feature falls into the category of error concealment procedures. Recovery from temporal error propagation is an indispensable component of any error robust video communication system. Errors introduced during transmission can lead to frame-store mismatch between the encoder and the decoder, which can persist until the next intra refresh (coded part of the video which is independent of any other part and can be decoded separately) occurs. When an upstream does not exist, methods in which the intrarefresh occurs at certain time intervals is the only available option. In the case where an upstream does exist from the decoder
back to the encoder, NEWPRED or demand intra refresh can be used. NEWPRED is a technique in which the reference frame for inter-frame coding is replaced adaptively according to the upstream messaging from the decoder. NEWPRED uses upstream messages to indicate which segments are correctly decoded and which segments are erroneously decoded. On receipt of the upstream message the encoder subsequently uses only the correctly decoded part of the prediction in an inter-frame coding scheme. This prevents temporal error propagation without the insertion of intra coded MBs (Macro Blocks) and improves the video quality in noisy multipath environments. The combination of the error detection by inserted CRC bits and the NEWPRED error-resilience tool of MPEG4 can result in significant improvements in the received video quality. For test purpose three different video sequences with QCIF resolution, frame rate of 10 fps and duration of ten seconds (total of 650 frames) are transmitted over simulated UMTS moving propagation environment. The received frame sequence quality is then quantified by the standard PSNR metric [4]. 4. Simulated UMTS Environment
SPW tool by CoWare was used to model an environment in which an MPEG-4 video stream is transmitted over various types of propagation conditions defined by the 3GPP standards. A 64 kbps downlink data channel and 2.5 kbps control channel were used for this UMTS simulation. These two channels were multiplexed and then transmitted over the WCDMA airinterface. Parameters configured for both channels related to the transmission time interval, transmission block size, transmission block set size, CRC attachment, channel coding, rate matching and inter-leaving were compliant with the 3GPP TS 34.108 specification [5]. The typical parameter set for reference RABs (Radio Access Barriers) and SABs
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(Signaling Access Barriers) and relevant combinations of them are presented in [5].
PSNR Improvements with NEWPRED Implementation 35
Data rate- 64 kbps
30
The typical parameters set for the conformance testing are mentioned in 3GPP TS 25.101 [6]. Also the parameters for dynamic propagation conditions for moving environment (non-fading channel model with two taps) are described by 3GPP TS 25.101. Parameters used for this simulation are listed below in Table 1.
Table 1. Parameter set for the conformance testing and moving propagation conditions.
Parameter
Typical value
Interference Received signal/ Noise (SNR) AWGN noise Eb/No (Overall) DPCH power/Tota Power Transmitted Eb/No (Traffic data channel) BER (Bit Error Rate) BLER (Block Error Rate) FER (Frame Error Rate)
−60 dB −3.0 dB 4 × 10−9 watts 6.01 dB −12.33 dB
PS NR (dB )
5. Moving Propagation Conditions
Channel Model Moving propagation conditions
25 20 15
Eb/No - 6.01 dB
10 5 0 Mother and Daughter
Highway
Foreman
With-out NEWPRED With NEWPRED
Figure 2. PSNR improvements with NEWPRED implementation. Table 2. PSNR implementation.
improvements
with
newpred
Video PSNR PSNR with ImproveClip without NEWPRED ments in name NEWPRED (dB) (dB) PSNR(dB) MD HW FM
23.8001 26.8993 17.5511
26.5746 28.9075 19.8001
2.7745 2.0082 2.249
Frame lost Improvments with NEWPRED Implementation
−0.3516 dB
160
Data Rate - 64 kbps
140 No. of frames
0.0013 0.036 0.088
Channel Model Moving Propagation conditions
120 100 80
Eb/No - 6.01 dB
60 40 20 0 Mother and Daughter
Highway
Foreman
With-out NEWPRED With NEWPRED
6. Simulation Results
Three different video clips are transmitted over moving propagation environments in the UMTS downlink simulation with and without NEWPRED implementation and results are compared. It can be seen from Figure 2 that more than 2 dB of improvement in PSNR value is achieved with NEWPRED implementation. These results are listed below in Table 2. Also a significant amount in the reduction of the frames lost is achieved with this implementation (Figure 3). Same results are listed in Table 3.
Figure 3. Frame lost improvements with NEWPRED implementation. Table 3. Frame implementation.
lost
Video No. of Clip frames name lost without NEWPRED MD HW FM
improvements
No. of frames lost with NEWPRED
with
newpred
Frames saved with NEWPRED
51 (5.1 sec) 6 (0.6 sec) 45 (4.5 sec) 90 (9 sec) 12 (1.2 sec) 78 (7.8 sec) 147 (14.7 sec) 18 (1.8 sec) 129 (12.9 sec)
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7. Conclusions
In this paper a simple method of error detection at frame level for MPEG-4 video using CRC is described. Also its potential use with one of the MPEG-4 error-resilience tools is implemented and results are analyzed with the simulation of the moving propagation conditions over UMTS networks. As can be seen from the simulation results, implementation of CRC at frame level and NEWPRED results in significant improvements on the received video quality. References 1. S.T. Worrall, A.H. Sadka, P. Sweeney and A.M. Kondoz, ‘Backward compatible user defined data insertion into MPEG-4 bitstreams’, IEE Electronics Letters, Vol. 36, no. 12, p. 1036, June 2000.
2. C. Kodikara, S. Worrall, S.N. Fabri, A.M. Kondoz, “Performance evaluation of MPEG-4 video telephony over UMTS”, 3G Mobile Communication Technologies, 2003. 3G 2003, 4th International Conference on (Conf. Publ. No. 494), ISSN: 05379989, 25–27 June 2003, pp. 73–77. 3. Information technology — Coding of audio-visual objects — Part 2: Visual, Reference number ISO/IEC 14496-2:2001(E). 4. S. Wolfe and M. Pinson: ‘Video quality measurement techniques.’ NTIA Report 02-392, U.S. Department of Commerce. 5. 3GPP, TSG Terminals, Common test environments for UE conformance testing, 3GPP TS 34.108, V4.9.0 (2003-12). 6. 3GPP, TSG Access Network, UE radio transmission and reception (FDD), 3GPP TS 25.101, V6.3.0 (200312).
