H.261 and H.263

H.261 was published in 1990, in order to enable video conferencing using 1~30 ISDN channels.
* Picture Formats Supported
QCIF(7.5Hz) and CIF(15Hz), progressive scanning

* Video Coding Tools
- I and P picture only; due to realtime requirement, no B picture.
- Intra and inter MB modes
- Forward motion compensation mode only with precision of 1 pel
- Motion vector range is +/-16 pels
- Two quantizers for DCT coeffs: Uniform quantizer of stepsize eight for DC in intra mode and a midtread quantizer with variable stepsize for other cases.
- run-length and variable length coding
- One loop filter is used to reduce the predication noise

H.263/H.263+/H.263++ is based on the framework of H.261. This standard was completed from 1995 to 2000.
* Picture Formats Supported
Sub-QCIF(128X96; 4:2:0), QCIF and CIF, progressive scanning

* Video Coding Tools
- I and P picture only
- Intra and inter MB modes
- Forward motion compensation mode only with precision of 0.5 pel
- MV could be the median of three ones of neighboring MBs
- Improved VLC: 3D VLC, i.e. (last, run, level) run-length coding
- Reduced GOB structure

In addition to these, H.263 offers a list of optional features that are defined in annexes to the standard.
- Unrestricted motion vectors. The MV range is [-31.5,31].
- Syntax-based arithmetic coding
- Advanced prediction mode: OBMC, and four block MVs for one MB
- Advanced Intra Coding
- PB picture

The system/signaling part for audiovisual comm. associated with H.261/263 is H.323 for Internet comm. and H.324 for PSTN comm.