Appraisal of H.264 Codec

Appraisal of H.264 Codec

Mrs. R. N. Mandavgane

, Dr. N. G. Bawane

1

B.D. College of Engineering Sevagram, Wardha Nagpur University.

2

Principal, S. B. Jain Institute Technology, Management and Research, Nagpur Nagpur University.

1_{rmandavgane@rediffmail.com}

Abstract- H.264/AVC, the latest video Codec standard is the most popular among Video professionals owing to its exact and unambiguous guidelines for dealing with the input stream to be transported from one place to another, having minimum loss in the transit and a very good compression ratio, both attributes highly sought after in today’s world of high quality multimedia broadcast and reception. Another parameter is the bit rate with which the data has to be transmitted. This paper is an attempt to show how various input parameters affect the output of the H.264 Codec. The input video file is a .yuv file, which after encoding becomes a .264 file, which is also called a stream file because this file has to be sent as a stream to various locations owing to its small size. At the receiving end, this stream file is decoded back into .yuv file. In the entire process, there is some loss in the quality of the video transmitted. This loss has to be minimum and is indicated by a parameter called PSNR (Peak Signal to Noise Ratio). Apart from PSNR, there are some other parameters as well like sequence parameter set, picture parameter set, information about different frames as regards slices and macroblocks, all ensconced in different files. PSNR being an important parameter will be dealt with here. There are three different input profiles namely, baseline profile, main profile and extended profile for H.264. In this work, variation of bit rate versus PSNR is studied in all the three profiles. A higher PSNR for a given constant bit rate is rated as a better performance. Here, the input sequence is a yuv file (foreman_part_qcif.yuv) with one reference frame. The quantization parameter for the I, B and P slices has been set at 30. The PSNR is plotted against bit rate and it is observed that they show a striking similarity with each other.

Keywords –, PSNR, compression, decompression, profiles of H.264

I. INTRODUCTION

Gadgets involving real time video transmission and reception for a common man, like video calling over the internet with applications such as Skype and iChat, web browsing and downloading on cell phones etc which are ubiquitous today were simply not there about 10 years ago. Advancements in hardware technology and bigger available bandwidths can be attributed to this scenario. However, the H.264/AVC and SVC Codec has further accelerated this development.

B. Paper contents –

The outline of paper is as follows. The brief introduction of H.264 codec is given in section II explaining the input output files of the encoder and decoder clearly. Section III describes graphically the performance of the three profiles. The paper is concluded in section IV.

II. BRIEF WORKING OF THE H.264 CODEC

H.264 defines the syntax of an encoded bit stream and the method of decoding it. It however, has no guidelines for the actual implementation for the same. Different companies have their own methods of implementing the hardware and software while adhering to the H.264 standards. The encoded video data (.yuv file) is in the form of a stream file with an extension of .264, which gets converted back to the .yuv format after getting decoded at the receiving end. Since video files are bulky, compression of video data is a challenge. Assuming the maximum bandwidth available to be constant, a large amount of video data to be sent demands a very high compression ratio from the encoder.

The raw video data or the original input video file is organized as a stream of frames. A frame rate of more than 25 per second is considered as acceptable. However, the encoded file is very complex and is organized as a very cryptic data about the input file. In this file, each frame is divided into slices, each slice, into macroblocks and each macroblock into blocks. All this information is stored into the .264 (encoded) file as it is the file which actually streams from one point to another. Squeezing all the information about the original video file into this streaming file makes the .264 file a very complex one. Here, the macroblock is considered as the basic unit to be communicated with by the Codec and its blocks are the subunits carrying detailed information about each pixel. The macroblock contains 26 blocks [3]. Depending upon the required output quality, H.264 has different profiles or levels namely baseline profile, main profile and extended profile. The levels define the performance limits such as sample processing rate, picture size, coded bit rate and memory size. The 4:2:0 scheme of video format is used in codec, Y the luminance component, blue and red chroma i.e. Cb and Cr are transmitted, each having half the horizontal and vertical resolution of Y. The output of the encoding process is a VCL (Video Coding Layer) which can be mapped to NAL (Network Abstraction layer) prior to transmission or storage.

Figure 1. Block schmatic for codec

In this experiment, the input file is ‘foreman_part_qcif.yuv’ and is in 4:2:0 format, the first frame of which is shown in Figure 2. The size of this file is 112 KB and has 3 frames. The .264 file generated has a size of only 4 KB. This shows a tremendous compression of the input file.

Figure 2. Frame from foreman sequence

III. PSNR CURVES

A. Baseline profile –

Figure 4. Bitrate vs PSNR curve for main profile

C. Extended profile –

For the extended profile also the observations are almost the same with the same compression. The first frame is encoded as IDR. P and B slices follow alternately for the next frames.

Figure 5. Bitrate vs PSNR curve for extended profile

D. Observation –

Figure 6. 264 file from the codec

Figure 8. Diagram showing statistics

IV. CONCLUSION

REFERENCE

[1] Iain E. G. Richardson, Wiley 2003, H.264 and MPEG 4 video compression. [2] Iain E. G. Richardson, Wiley 2010, The H.264 advanced video compression standard. [3] Iain E. G. Richardson, Wiley 2002, Video codec design.

[4] H.264/14496-10 AVC Reference Software Manual (Revised for JM 18.0).

[5] T. Wiegand, G. J. Sullivan, G. Bjontegaard, and A. Luthra, “Overview of the H.264/AVC Video Coding Standard” IEEE Transactions on Circuits and Systems for Video Technology, Vol. 13, No. 7, July.

[6] M. Fiedler, “Implementation of a basic H.264/AVC Decoder” Seminar Paper, Chemnitz university of technology, June 1, 2004. [7] D. Marpe, T. Wiegand, G. J. Sullivan, “The H.264/MPEG4 Advanced Video Coding Standard and its Applications” Standards Report. [8] Iain Richardson, “H.264 / AVC Picture Management” VCodex, White Paper, 2011.

[9] International Standard ISO/IEC 14496-10, “Part 10 Advanced Video Coding” second edition, 2004.