1. H.261 Video Compression Standard Presented by Parul Mathur Brad Gagné This report was prepared for Professor L. Orozco-Barbosa in partial fulfillment of the requirements for the course CEG 4183.
17. Appendix B: Standards Video Teleconferencing Standards http://www.it.kth.se/~nv91-tob/table6.html
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Editor's Notes
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CIF – there is also QCIF (“Quarter CIF”), which is the same format except only half of the height and half of the width (so 176x144), and therefore one quarter size of CIF (hence the name). DPCM – in other words, basically PCM with an adaptive scale DCT is non-lossy compression, but algorithms that use it always round off the coefficients, which results in lossy compression
Spatial Redundancy : identical (or very similar) blocks within the same frame only one needs to be fully represented; the others can reference it Temporal Redundancy : identical (or very similar) blocks between adjacent frames blocks can refer back to a block in the previous frame I-frames and P-frames: I-frames contain a complete image P-frames record only the differences between the last frame and the current one Motion Compensation : using motion vectors, a P-frame can indicate that a portion of a block in the previous image has remained the same in appearance but has moved to a different location
H.261 is only for video ; there is no audio representation. P*64 kbps obtains data rates in the range of (approximately) 64kbps to 2Mbps, which is a perfect match for typical ISDN network data rates. Designed for circuit-switched networks; adaptations have been made to optimize it for packet-switching networks also, using packet-dropping when necessary to adapt to available data rates
CIF & QCIF – see slide 4 (Key Terms and Concepts) temporal redundancy, spatial redundancy, motion compensation – see slide 5 (Key Terms and Concepts)
Intra-blocks represent a portion of a frame’s image independently of other blocks Inter-blocks represent a portion of a frame’s image in terms of another similar intra-block DCT – see slide 4 – Key Terms and Concepts Huffman encoding : the block of text to encode in run through a frequency analyzer repetitive strings are encoded using short symbols the more common the string, the shorter the symbol that will be used to encode it
The H.261 Encoding Process full images as inputs, plus previous frame (used for removing temporal redundancy) images are divided into blocks (intra or inter) and a DCT function is applied (“DCT”) coefficients of the DCT function are rounded off (“QUANT”) Intra- and Inter- blocks are resized into 8x8 blocks and the 2-d DCT function is applied for further compression frame is put into frame store; used to calculate motion vectors between current and next frame frame is used as an input when encoding the next frame
Quality of picture depends on efficiency of the encoding and “thoroughness” of decoding: The encoded data may contain detailed information about the encoded images, but if the information is only done “roughly” then only a rough image will be produced Compromise between definition and fluency: With high image quality, frame rates must be lowered to compensate and keep a relatively small data rate (and vice versa). Motion Compensation – see slide 5 – Key Terms and Concepts
PSNR = (# bits in original image) / (# bits in decoded image) As can be seen on the graph, better image quality (PSNR) causes higher bitrates. Best performance is achieved when the encoding process uses motion vectors (“MV”) and loop filtering (“LF”). This is the “efficiency” and “thoroughness” referred to in the previous slide.
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Image at left shows a typical video conferencing device. Image at right shows where H.261 fits into the standards for video conferencing for ISDN, LAN, and POTS (“Plain Old Telephone System”) networks.
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Image shows a typical sequencing of I-Frames and P-Frames. For definitions, please refer to slide 5, under “Temporal Redundancy”.
This table shows where H.261 is used in the Video Teleconferencing Standards.
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