H.263 is a video compression standard and codec developed by the ITU-T as an improvement over the earlier H.261 standard. It was designed to address the shortcomings of H.261 and provide better video quality at lower bit rates, making it suitable for a wider range of applications including video conferencing, telephony, and streaming. Here’s a closer look at H.263:
Design Purpose: H.263 was primarily developed for video communication over low bit-rate channels. It was optimized for both the PSTN (Public Switched Telephone Network) and wireless communication.
Resolutions: The codec initially supported resolutions such as:
- QCIF (Quarter CIF): 176 x 144 pixels
- CIF (Common Intermediate Format): 352 x 288 pixels
However, later versions added support for higher resolutions.
Bit Rates: It was designed to be efficient at lower bit rates, particularly between 5 kbps to 64 kbps, but it’s capable of handling higher rates as well.
Compression Techniques: Like H.261, H.263 uses Discrete Cosine Transform (DCT) for spatial compression and motion compensation for temporal compression. However, it introduced several enhancements:
- Improved motion estimation.
- Half-pixel and quarter-pixel motion compensation for better accuracy.
- Loop filter to reduce blocking artifacts.
- PB-frames which combine the properties of both P-frames (predictive coded) and B-frames (bidirectionally predictive coded).
Annexes: H.263 introduced multiple annexes (extensions) that added optional features to improve compression efficiency and video quality. Some annexes catered to error resilience, an essential feature for unreliable networks.
Legacy and Evolution: H.263 served as a basis for subsequent video codecs. Its principles were used in the development of the MPEG-4 Part 2 standard. Later, H.264 (or AVC) was developed, offering significant improvements over both H.263 and MPEG-4 Part 2.
Usage: While H.263 was a popular choice for video conferencing and video streaming in the late 1990s and early 2000s, it has been largely superseded by more efficient codecs like H.264. However, its influence on the design of modern video codecs is undeniable.
In summary, H.263 was a significant step forward in video codec design, offering better compression, video quality, and flexibility compared to its predecessor. Its techniques and ideas paved the way for the advanced video codecs in use today.