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H.261 is an early video compression standard developed by the ITU-T for the purpose of video telephony and conferencing. It was introduced in 1990 and was one of the first video compression formats to gain significant traction in commercial applications. The standard is designed to compress video at bit rates ranging from 64 kbps to 2 Mbps, making it suitable for use over ISDN (Integrated Services Digital Network) and other low-bandwidth connections.
The core of H.261's compression technique relies on a combination of various coding methods, including motion compensation and transform coding. By predicting motion between frames and encoding only the differences, H.261 achieves efficient compression rates while maintaining acceptable video quality. The format supports various resolutions, with the most common being CIF (Common Intermediate Format) at 352x288 pixels and QCIF (Quarter CIF) at 176x144 pixels.
Despite its age, H.261 laid the groundwork for subsequent video coding standards, such as H.263 and H.264, which built upon its principles to offer higher compression efficiency and better quality. H.261 is still found in some legacy systems and applications, particularly where bandwidth constraints are a critical factor.
H.261 operates on a frame-by-frame basis, allowing for both intra-frame and inter-frame coding. The standard specifies various profiles that dictate the coding parameters, enabling flexibility in implementation depending on the application requirements. This adaptability has contributed to its longevity in certain niche markets.
Overall, H.261 remains an important part of the evolution of video compression technologies, illustrating the challenges and solutions associated with transmitting video over limited bandwidth. Its influence can still be seen in modern video codecs that prioritize efficient data transmission while aiming to preserve visual fidelity.