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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