MPEG
A great deal of information about MPEG can be learned by studing Frequently Asked Questions.
You also may wish to refer to a PDF version of the standard.
Overview of the ISO/IEC standard MPEG (Motion Pictures Expert Group):
- Approval: video standard specified in 1991, audio and then systems later
- Adoption:
- CD-I add-on
- 3DO add-on
- Sigma Designs Reel Magic CD-ROM Upgrade Kit
- MPEG II:
- Higher resolution (to HDTV) and hence bitrate 704 x 480 at 30 fps
- Direct satellite broadcasts, cable TV
- MPEG details:
- Interframe coding has (I) reference frames, and predicted frames going forward (P) as well as bi-directional (B)
- Decoding includes handling of motion vectors for inter-frame coding and inverse DCT and Huffman for intra-frame coding
- Interframe decoding also involves interpolation
- Net effect: single-speed CD-ROM, 320x240, 30 fps
The main steps of MPEG compression, along with the compression ratio of each, include:
- Filtering: Simple approach is decimation; Better approach is averaging, using multiple taps (pixels/lines)
- Color-space conversion (MPEG compression 1.5:1): RGB to/from YUV
- Digitizing (2:1)
- Scaling or subsampling (4:1):
- Pick resolution and sample rate / sampling; YUV studio quality is 4:2:2 while MPEG and DVI use 4:1:1
- QuickTime originally to 160x120, MPEG to 320x240
- Also, pixel depth reduction in number of colors (CLUT)
- Playback to double or quadruple: smart scaling interpolation
- Transforms: DCT, Wavelet
- Quantization (3:1):
- Different accuracy of representation for different parts
- Less detail for high frequency due to Human Visual System
- Entropy coding (1.5:1):
- Huffman or arithmetic
- Runs of zeroes
- Interframe (5:1):
- Predictive: Store difference values instead of actuals
- Motion estimation and compensation: Find motion vector indicating pixel offset of 16x16 macroblock
- Picture interpolation:
- Average prior and following to predict current
- Selectively use prior or following to predict each region
- Save difference from predicted value
Hierarchy