Each complete frame contains all the information necessary to enable the JPEG

Each complete frame contains all the information

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Each complete frame contains all the information necessary to enable the JPEG decoder to identify each field in a received frame and then perform the corresponding decoding operation.
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Multimedia System 030728.ppt 157 DC level shifting Image Component Tiling DWT on each tile Tiling, DC level shifting and DWT on each image tile component JPEG-2000 Image Tiles Multimedia System 030728.ppt 158 JPEG2000 encoding JPEG2000 encoding JPEG2000 encoding C1 C2 C3 DC level shifting DC level shifting DC level shifting R G B Color Transformation Compressed Image Data Colour Image JPEG 2000 Multiple-component Encoder Multimedia System 030728.ppt 159 MPEG Bidirectional Coding Bit rate (Mbit/s) 50 40 30 20 10 I IB IBBP Figure: Constant Quality Curve Better quality Worse quality I=Intra or spatially coded ‘anchor’ frame P= Forward predicted frame. Coder sends difference between I and P. Decoder adds difference to create P B=Bidirectionally coded picture. Can be coded from a previous I or P picture or a later I or P picture. B frames are not coded from each other. I B B P GOP-Group of picture Multimedia System 030728.ppt 160 An MPEG-2 Coder Figure: An MPEG-2 Coder Buffer Differential encoding Entropy and Run-length encoding MUX Video in Bidirectional coder Demand clock Elementary Stream out Slice Quantizing table Rate control Spatial data Motion vectors
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Multimedia System 030728.ppt 161 The psychoacoustic models associated with the various MPEG coders control the quantization accuracy of each subband sample by computing and allocating the number of bits to be used to quantize each sample. Since the quantization accuracy that is used for each sample in a subband may vary from one set of subband samples to the next, the bit allocation information that is used to quantize the samples in each subband is sent with the actual quantized samples. This information is then used by the decoder to dequantize the set of subband samples in the frame. Forward Adaptive Bit Allocation (MPEG): Multimedia System 030728.ppt 162 This mode of operation of a perceptual coder is known, therefore, as the forward adaptive bit allocation mode and, for comparison purposes, a simplified schematic diagram showing this operational mode is given in figure. PCM output AFB PCM input Subband samples Quantized subband samples Bit allocations QB FF PM SMRs + bit allocations DQ SFB Demux Quantized samples + bit allocations Encoded bitstream Dequantized subband samples Figure: Perceptual coder schematics: forward adaptive bit allocation (MPEG) Forward Adaptive Bit Allocation (MPEG): Multimedia System 030728.ppt 163 It has the advantage that the psychoacoustic model is required only in the encoder. It has the disadvantage, that a significant portion of each encoded frame contains bit allocation information which, in turn, leads to a relatively inefficient use of the available bit rate.
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