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MPEG hareketli görüntü sıkıştırma standardı

Başlık çevirisi mevcut değil.

  1. Tez No: 46284
  2. Yazar: ERHAN TELLİOĞLU
  3. Danışmanlar: DOÇ.DR. MELİH PAZARCI
  4. Tez Türü: Yüksek Lisans
  5. Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1995
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 106

Özet

ÖZET Sayısal görüntü tekniğindeki gelişmeler sonucu görüntü sıkıştırma tekniğini sayısal yayın kodlama, telekonferans, görüntülü telefon gibi çeşitli haberleşme tiplerine uygulamak mümkün olmuştur. Bu nedenle görüntü sıkıştırma tekniğindeki standartlaşma, farklı üreticilerin ürettiği ürünler arası uyumluluk problemi ve görüntü kodlamanın maliyeti nedeni ile önem kazanmıştır. İlk defa 1988 de ISO( the International Organisation for Standardisation)' da standart için çalışmalara başlanmış ve MPEG ( Moving Pictures Expert Group) ortaya çıkmıştır. MPEG-1 ile hareketli görüntü 1.5 Mbit/s civarında sıkıştırılmış sayısal veri olarak işlenebilir, varolan haberleşme şebekeleri aracılığı ile sayısal olarak taşınabilir. 1.5 Mbit/s civarında iyi bir resim kalitesi elde edebilmek için çok yüksek sıkıştırma oranlarına ihtiyaç vardır. Bu nedenle MPEG de değişik sıkıştırma teknikleri kullanılmıştır. Öncelikle kodlanacak görüntü işareti için uygun bir çözünürlük seçilmelidir. Burada insan gözünün fizyolojik özelliklerine göre bir çözünürlük azaltma işlemi yapılmaktadır. MPEG algoritması daha sonra arka arkaya gelen resimler arası benzerlik ilişkisini kullanarak ardışıl resimler arasında bulunan fazla veriyi, hareket kompanzasyonu (Motion compensation) aracılığı ile azaltmaktadır. Hareket kompanzasyonu, o andaki resmin bir önceki resimden nedensel olarak tahmini, bir sonraki resimden ( Future picture ) nedensel olmayan tahmini veya bir önceki ( Past picture ) ve bir sonraki resimlerden interpolasyon ile tahmini için kullanılır. Hareket vektörleri ( Motion Vectors ) resimdeki her 16x16 nokta matrisi için bulunur. Daha sonra tahmin edilen resimle referans resim arasındaki farka - tahmin hatası- ( prediction error ) uzaysal ( spatial ) korelasyonların atılması için ayrık kosinüs dönüşümü (Discrete Cosine Transform) uygulanır. Kuantalama işlemi yapılarak daha az önemli görüntü bilgileri atıldıktan sonra hareket vektörleri DCT' si alınmış ve kuantalanmış işaretle birleştirilip veri oranım bir kez daha azaltmak için değişken kelime uzunluklu kodlama ( Variable Length Coding ) uygulanarak sıkıştırılmış görüntü işareti elde edilir. MPEG algoritması kullanılarak yapılan bilgisayar simülasyonunda elde edilen veriler sonuç bölümünde incelenmiş, simülasyon programı sonucu elde edilen veriler bu bölümde verilmiştir.

Özet (Çeviri)

SUMMARY The development of digital video technology in the 1980s has made it possible to use digital video compression for a variety of telecommunication applications: teleconferencing, digital broadcast codec and video telephony. Standardisation of video compression techniques has become a high priority because only a standard can reduce the high cost of video compression codecs and resolve the critical problem of interoperability of equipment from different manufacturers. Digital transmission is of prime importance for telecommunication, particularly in the telephone network, but there is a lot more to digital video than teleconferencing and visual telephony. The computer industry, the telecommunication industry and the consumer electronics industry are increasingly sharing the same technology. In the view of shared technology between different segments of the information processing industry, the International Organisation for Standardisation (ISO) has undertaken an effort to develop a standard for video and associated audio on digital storage media, where the concept of digital storage medium includes conventional storage devices CD-ROM, DAT, tape drives, Winchesters disks, writable optical drives, as well as telecommunication channels such as ISDNs and local area networks. The effort is known by the name of the expert group that started it: MPEG ( Moving Picture Expert Group). The MPEG activities cover more than video compression, since the compression of the associated audio and the issue of audio -visual synchronisation cannot be worked independently of the video compression: MPEG-1 Video is addressing the compression of video signals at about 1.5 Mbits, MPEG -Audio is addressing the compression of the digital audio signal at the rates of 64, 128 and 192 kbits/s per channel, MPEG system is addressing the issue of synchronisation and multiplexing of multiple compressed audio and video bit streams. Two very important consequence follow: Full motion video becomes a form of computer data, i.e., a data type to be integrated with text and graphics; Motion video and its associated audio can be delivered over existing telecommunication and computer networks. The difficult challenge in the design of the MPEG algorithm is the following: on one hand the quality requirements demand very high compression not achievable with intraframe coding alone; on the other hand, the random access requirement is best satisfied with pure intraframe coding. The algorithm can satisfy all the requirements only insofar as it achieves the high compression associated with interframe coding, while not compromising random access for those applications that demand it. This requires a delicate balance between intra- and interframe coding, and between recursive and non-recursive temporal redundancy reduction. Inorder to answer this challenge, the members of MPEG have resorted to using two interframe coding techniques: predictive and interpolative. The MPEG video compression algorithm relies on two basic techniques: block-based motion compensation for the reduction of the temporal redundancy and transform domain -(DCT) based compression for the reduction of spatial redundancy.Motion compensation techniques are applied with both causal (pure predictive coding) and non-causal predictors ( Interpolative coding). The difference signal (prediction error) is further compressed with spatial redundancy reduction (DCT). The information relative to motion is based on 16 x 16 blocks and is transmitted together with the spatial information. The motion information is compressed using variable-length codes to achieve maximum efficiency. Because of the importance of random access for stored video and significant bit-rate reduction afforded by motion-compensated interpolation, three types of pictures are considered in MPEG. Intrapictures ( I ), Predicted pictures ( P ) and Interpolated pictures ( B - for bi-directional prediction). Intrapictures provide an access point for random access but only with moderate compression; predicted pictures are coded with reference to a past picture ( Intra or Predicted ) and will in general be used as a reference for future predicted pictures; bi-directional pictures provide the highest amount of compression but require both a past and a future reference for prediction; in addition bi-directional pictures are never used as reference. In all cases when a picture is coded with respect to reference, motion compensation is used to improve the coding efficiency. The organisation of the pictures in MPEG is quite flexible and will depend on application specific parameters such as random accessibility and coding delay. Among the techniques that exploit the temporal redundancy of video signals, the most widely used is motion compensated prediction. It is the basis of most compression algorithms for visual telephony such as CCITT standard H.261. Motion compensated prediction assumes that 'locally' the current picture can be modelled as a translation of the picture at some previous time. Locally means that the amplitude and the direction of the displacement need not be the same everywhere in the picture. The motion information is part of the necessary information to recover the picture and has to be coded appropriately. Motion compensated interpolation is a key feature of MPEG. It is a technique that helps satisfy some of the application dependent requirements since it reduces the effect of errors while at the same time contributing significantly to the image quality. In the temporal dimension, motion compensated interpolation is a multiresolution technique; a subsignal with low temporal resolution (typically 1/2 or 1/3 of the frame rate ) is coded and full resolution signal is obtained by interpolation of the low resolution signal and addition of a correction term to a combination of a past and future reference. Motion compensated interpolation (also called bi-directional prediction in MPEG terminology ) presents a series of advantages, not the least of which is thatthe compression obtained by interpolative coding is very high. The other advantages of bi-directional prediction ( Temporal interpolation ) are:. It deals properly with uncovered areas, since an area just uncovered is not predictable from the past reference, but can be properly predicted from the future reference.. It has better statistical properties since more information is available; in particular, the effect of the noise can be decreased by averaging between the past and the future reference pictures.. It allows decoupling between prediction and coding (no error propagation). The trade off associated with the frequency of bi-directional pictures is the following: increasing the number of B-pictures between references decreases the correlation of B-pictures with the references as well as the correlation between the references themselves. Although this trade off varies with nature of the video scene, for a large class of scenes it appears reasonable to space reference at about 1/1 Oth second interval resulting in a combination of the type IBBPBBPBB IBBPBB. There is a trade-off between the coding gain provided by the motion information and cost associated with coding the motion information. The choice of 16 x 16 blocks for the motion-compensation unit is the result of such a trade-off, such motion compensation units are called Macroblocks. In the more general case of a bidirectionally coded picture, each 16 x 16 macroblock can be of type intra, Forward predicted, Backward predicted or Average. As expressed below, the expression for the predictor for a given macroblock depends on reference picture (past and future) as well as motion vectors: x is the coordinate of the picture element, mvoi the motion vector relative to the reference picture I», mva the motion vector relative to the reference picture I,. Macroblock Type INTRA FORWARD PREDICTED BACKWARD PREDICTED AVERAGE Predictor Iioo =128 Ii(X) =Io(x+mvoi) IiGO = Io (x+mv2l) Prediction Error Ii

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