Windows Media Video 9 Tarun Bhatia Multimedia Processing Lab University Of Texas at Arlington 11/05/04

Introduction 1

Introduction 2

Introduction 3 Latest video codec which gives state-of-art video quality from very low bit rates (160x120 at 10 Kbps for modem) to very high bit rates (1920x1080 for HDTV) 8-bit, 4:2:0 format Uses block based transform and motion compensation with quantization and entropy coding.

The Block Diagram :-

Block Transforms(Integer DCT) 8x8 blocks can be encoded using 1_8x8 2_8x4 2_4x8 4_4x4 -Frame / Macroblock/Block signaling -Block level for coarse and fine level specification -Frame level for overhead reduction -Only 8x8 used for I frames

8x8 Integer DCT Matrices WMV 9H.264 HP

Key features of the Transforms The norms of vectors of the ratio 288:289:299 The variation in the norm accounted for in the encoder itself At the decoder inverse transform(rows) -> rounding-> inverse transform (columns) ->rounding (to operate in the 16 bit range)

Quantization Same rule applied to all block sizes Both types with (bit savings at low bit rates) and without dead zone ( available Type used signaled at the frame level to the decoder At the encoder side automatic switch from uniform quantization to dead zone quantization as Q – parameter increases Other factors like noise and rate control can be used to control this switch

Loop Filtering Done to remove blocky artifacts and thus quality of current frame for future prediction Operates on pixels on the border of blocks The process involves a discontinuity measurement Checks are computationally expensive so done only for one set of pixel per boundary

Motion Estimation and Compensation Max resolution of ¼ pixel (i.e. ¼, ½, ¾) allowed 16x 16 motion vectors used by default but 8x8 allowed Bicubic filter with 4 taps/ Bilinear filters with 2 taps to generate subpixel precision. 4 combined modes 1.Mixed block size (16x16 and 8x8), ¼ p,bicubic 2.16x16, ¼ p, bicubic 3.16x16, ½ p, bicubic 4.16x16, ½ p, bilinear Bilinear filters for chroma components

Advanced entropy coding Simple VLC codes Multiple code tables for encoding each particular alphabet out A possible set of code tables is chosen (based on frame level quantization parameter) and signaled in the bitstream Additional information e.g. motion vectors resolution coded using bitplane coding

Interlaced coding Supports field and frame coding

Advanced B frame coding B frames:- employ bi-directional prediction Fractional position definition with respect to the reference frames for better scaling of motion vectors Intra coded B frames for scene changes Allow inter field reference

Overlap smoothing The deblocking filter smoothens out the block as well as true edges and it may be disabled in less complex profiles A lapped transform (input spans to pixels from other blocks as well) is used at the edges Used in spatial domain as pre and post processing Used only at low bit rates where blocking artifacts are higher Signaled at macroblock level so can be turned off in smooth regions

Low rate tools (<100 Kbps) Code frames at multiple resolutions (both in X and Y direction) A frame can be downscaled at the encoder and then upscaled at the decoder for LBR transmission The downscaling factor needs to remain same from the start of 1 I frame to the start of next I frame. The frame must be upscaled prior to display (upscaling out of scope of the standard).

Fading compensation Large amount of bits required for scenes having effects like fade-to-black,fade-from-black Not possible to predict motion using normally used techniques. Effective fading detection (original refrence image- current video image>threshold=fading) If detected then encoder computes fading parameters which specify a pixel-wise first order linear transform for the reference image. Also signaled to the decoder

References Windows Media Video 9: overview and applications Sridhar Srinivasan, Pohsiang (John) Hsu, TomHolcom b, Kunal Mukerjee, Shankar L. Regunathan, Bruce Lin, Jie Liang, Ming-Chieh Lee, Jordi Ribas-Corbera Windows Digital Media Division, Microsoft Corporation, Redmond, WA 98052, USA available online at

AVS China’s next generation video coding standard

Introduction Streamlined video coder dedicated to coding HDTV content (1920x1080 in 4:2:0 and 4:2:2) 4 levels are defined Level 4.0 : Standard Definition 4:2:0 Level 4.2 : Standard Definition 4:2:2 Level 6.0 : HD 4:2:0 Level 6.2 : HD 4:2:2 (HD: High Definition) Designed to provide near optimum performance and considerable reduction in complexity (low cost implementation) Applications include broadcast TV,HD-DVD and broadband video networking

Data Formats Progressive scan results in significantly less coding complexity Interlaced scan

Structure Have start codes

Structure Slice

Structure 4:2:0 4:2:2

Block Diagram (Encoder)

Block Diagram (Decoder)

Buffers The rate buffer at the encoder side helps in converting variable data rate produced by encoder to fixed data rate by controlling quantization using feedback The rate buffer at the decoder side gets the fixed rate data and stores it and then passes on to the decoder at a rate demanded by decoding of each macroblock and frame.