The MPEG - 1 audio / video digital compression standard was approved by the International Organization for Standardization / International Electro technical Commission (ISO / IEC) MPEG group in November 1991 for coding of moving pictures and Associated - Audio for Digital Storage Media at up to about 1.5 Mbit / s. Common digital storage media include compact discs (CDs) and video compact discs (VCDs), Out of the specified 1.5 Mbps, 1.2 Mbps is intended for coded video, and 256 kbps can be used for stereo audio. This yields a picture quality comparable to VHS cassettes and a sound quality equal to CD audio.
In general, MPEG - 1 adopts the CCIR601 digital TV format, also known as Source input Format (SIF). MPEG - 1 supports only noninterlaced video. Normally, its picture resolution is 352 x 240 for NTSC video at 30 fps, or 352 x 288 for PAL" video at 25 fps. It uses 4:2:0 chroma subsampling..
The MPEG - 1 standard, also referred to as ISO / IEC 11172, has five parts: 11172 - 1 Systems, 11172 - 2 Video, 11172 - 3 Audio, 11172 - 4 Conformance, and 11172 - 5 Software. Briefly, Systems takes care of, among many things, dividing output into packets of bit - streams, multiplexing, and synchronization of the video and audio streams. Conformance (or compliance) specifies the design of tests for verifying whether a bitstream or decoder complies with the standard. Software includes a complete software implementation of the MPEG - 1 standard decoder and a sample software implementation of an encoder.
The need for bidirectional search
Motion Compensation in MPEG - 1
As discussed in the last chapter, motion - compensation - based video encoding in H.261 works as follows: In motion estimation, each macroblock of the target P - frame is assigned a best matching macroblock from the previously coded I - or P - frame. This is called a prediction. The difference between the macroblock and its matching macroblock is the prediction error, which is sent to DCT and its subsequent encoding steps.
Since the prediction is from a previous frame, it is called forward prediction. Due to unexpected movements and occlusions in real scenes, the target macroblock may not have a good matching entity in the previous frame. The above figure illustrates that the macroblock containing part of a ball in the target frame cannot find a good matching macroblock in the previous frame, because half of the ball was occluded by another object. However, a match can readily be obtained from the next frame.
MPEG introduces a third frame type -B - frames - and their accompanying bidirectional motion compensation. The following figure illustrates the motion - compensation - based B - frame coding idea. In addition to the forward prediction, a backward prediction is also performed, in which the matching macroblock is obtained from a future I - or P - frame in the video sequence. Consequently, each macroblock from a B - frame will specify upto two motion vectors, one from the forward and one from the backward prediction.
If matching in both directions is successful, two motion vectors will be sent, and the two corresponding matching macroblocks are averaged (indicated by "%" in the figure) before Comparing to the target macroblock for generating the prediction error. If an acceptable match can be found in only one of the reference frames, only one motion vector and its corresponding macroblock will be used from either the forward or backward prediction.
B - frame coding based on bidirectional motion compensation
The inevitable delay and need for buffering become an important issue in real - time network transmission, especially in streaming MPEG video.
The following figure illustrates a possible sequence of video frames. The actual frame pattern is determined at encoding time and. is specified in the video's header. MPEG uses M to indicate the interval between a P - frame and its preceding I - or P - frame, and N to indicate the interval between two consecutive I - frames. In the figure, M = 3, N -9. A special case is M -1, when no B - frame is used.
Since the MPEG and decoder cannot work for any macroblock from a B - frame without its succeeding P - or I - frame, the actual coding and transmission order is different from the display order of the video (shown above).
MPEG frame sequence
Table The MPEG - 1 constrained parameter set
Other Major Differences from H.261
Beside introducing bidirectional motion compensation (the B - frames), MPEG - 1 also differs from H.261 in the following aspects:
Source formats. R261 supports only CIF (352 x 288) and QCIF (176 x 144) source formats. MPEG - 1 supports SIF (352 x 240 for NTSC, 352 x 288 for PAL). It also allows specification of other formats, as long as the constrained parameter set (CPS), shown in the above table is satisfied.
Slices. Instead of GOBs, as in H.261, an MPEG - 1 picture can be divided into one or more slices, which are more flexible than GOBs. They may contain variable numbers of macroblocks in a single picture and may also start and end anywhere, as long as they fill the whole picture. Each slice is coded independently.
Slices in an MPEG - 1 picture
Table Default quantization table (Q) for intracoding
For example, the slices can have different scale factors in the quantizer. This provides additional flexibility in bitrate control.
Moreover, the slice concept is important for error recovery, because each slice has a unique slice - start - code. A slice in MPEG is similar to the GOB in 11.261 (and H.263): it is the lowest level in the MPEG layer hierarchy that can be fully recovered without decoding the entire set of variable - length codes in the bitstream.
Quantization. MPEG - 1 quantization uses different quantization tables for its intra - and inter - coding. The quantizer numbers for intra - coding vary within a macroblock. This is different from H.261, where all quantizer numbers for AC coefficients are constant within a macroblock.
The step size [it j] value is now determined by the product of Q[i, j] and scale, where Q1 or Q2 is one of the above quantization tables and scale is an integer in the
Table Default quantization table (Q2) for inter - coding
range [1, 31]. Using DCT and QDCT to denote the DCT coefficients before and after quantization, for DCT coefficients in intra - mode,
and for DCT coefficients in inter - mode
where Q1 and Q2 refer to above tables.
The following table lists typical sizes (in kilobytes) for all types of MPEG - 1 frames. It can be seen that the typical size of compressed P - frames is significantly smaller than that of I - frames,
Table Typical compression performance of MPEG - 1 frames
Layers of MPEG - 1 video bitstream
because inter - frame compression exploits temporal redundancy. Notably, B - frames are even smaller than P - frames, due partially to the advantage of bidirectional prediction. It is also because B - frames are often given the lowest priority in terms of reservation of quality; hence, a higher compression ratio can be assigned.
MPEG - 1 Video Bitstream
The above figure depicts the six hierarchical layers for the bitstream of an MPEG - 1 video.
Table Profiles and Levels in MPEG - 2
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Introduction To Multimedia
Multimedia Authoring And Tools
Graphics And Image Data Representations
Colour In Image And Video
Fundamental Concepts In Video
Basics Of Digital Audio
Lossless Compression Algorithm
Lossy Compression Algorithms
Image Compression Standards
Basic Video Compression Techniques
Mpeg Video Coding I – Mpeg 1 And 2
Mpeg Video Coding Ii- Mpeg-4, 7, And Beyon
Basic Audio Compression Techniques
Mpeg Audio Compression
Computer And Multimedia Networks
Multimedia Network Communications And Applications
Content-based Retrieval In Digital Libraries
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