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H.261 vs H.263 | Difference between H.261 and H.263

This page compares H.261 vs H.263 standards and mentions difference between H.261 and H.263. Both are video coding standards adopted by ITU-T. Both are used for real-time encoding and decoding on Power-PC 604. They are employed in International standard tele-conferencing codec.

H.261

It is designed and developed for video conferencing applications to be used over ISDN lines. The ISDN has bit-rate of 64 Kbps and higher in the multiple of 64 Kbps. It offers maximum coding delay of 150 msec.

H.261 supports I-frames and P-frames. I frames do not take any reference to previously coded frames. P frames take previous frame as reference. MPEG-1 coding standard was derived from H.261 standard.

H.263

H.263 is developed based on MPEG-1 and H.261. H.263 is improved standard for low bit-rate application. Similar to H.261, it uses transform coding for intra-frames and predictive coding for inter-frames. It supports POTS at very old rates i.e. 64 Kbps or less.

H.263 uses bi-directionally predicted (B) frames. B-frame is basically average of two P-frames. It uses previous frame and future frame to predict current input frame. B frame size is about 1/3 of the P-frame size.

H.263 uses advanced prediction mode.
• It uses 4 motion vectors per macroblock. Each 8 x 8 block is associated with 1 motion vector instead of 1 motion vector for entire macroblock. Hence better motion representation can be achieved at the cost of increase in number of bits to encode motion vector.
• It uses OBMC (Overlapped Block Motion Compensation). Hence image gets smoothened.


Specifications H.261 H.263
Picture formats CIF 352 x 288 pixels for luminance channel
CIF 176 x 144 pixels for chrominance channels U & V
Temporal rates (7.5, 10, 15, 30 frames/sec) are supported
QCIF with 176 x 144 pixels for Y and 88 x 72 pixels for U & V
Supports five picture formats as follows:
Sub-QCIF 128 x 96 pixels(Y), 64 x 48 pixels(U,V)
QCIF 176 x 144 pixels(Y), 88 x 72 pixels(U,V)
CIF 352 x 288 pixels(Y), 176 x 144 pixels(U,V)
4CIF 704 x 576 pixels(Y), 352 x 288 pixels(U,V)
16 CIF 1408 x 1152 pixels(Y), 704 x 576 pixel (U,V)
Bit rate 40 kbps to 2 Mbps Supports lower bit rates compare to H.261
Motion estimation for compensation Here motion vectors are expressed in integer pixel units. Uses half pixel motion estimation.
Frame modes I frame, P frame It uses PB frame mode. It increases frame rate without increasing bitrate.

In the table CIF stands for Common Intermediate Format and QCIF stands for Quarter of Common Intermediate Format.

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