H.261 is a video compression standard and codec developed by the ITU-T for video conferencing and telephony services. It was one of the first practical video coding standards and has played a foundational role in the evolution of video communication. Here’s a brief overview of H.261:
Design Purpose: H.261 was specifically designed for the ISDN (Integrated Services Digital Network) in mind, which was popular during the late 1980s and early 1990s. The bit rates of H.261 were multiples of 64 kbps to align with ISDN’s B-channel rate.
Resolution: The codec supports two resolutions:
- CIF (Common Intermediate Format): 352 x 288 pixels
- QCIF (Quarter CIF): 176 x 144 pixels These resolutions were chosen considering the typical data rates and the need for real-time processing at that time.
Bit Rates: H.261 was optimized for two primary bit rates: p x 64 kbps, where “p” can be in the range of 1 to 30, essentially between 64 kbps to 1.92 Mbps.
Compression Techniques: H.261 uses both intra-frame (within the frame) and inter-frame (between frames) compression techniques. It employs Discrete Cosine Transform (DCT) for spatial redundancy reduction and motion compensation for temporal redundancy reduction.
Block-based Codec: H.261 divides frames into blocks, then further into 8×8 sub-blocks for the DCT.
Motion Compensation: H.261 introduced motion compensation to account for movement in video sequences. It’s a technique still used in modern video codecs to increase compression efficiency.
Legacy and Evolution: While H.261 was groundbreaking in its time, it has been surpassed by newer video codecs like H.263, H.264, and beyond, which offer better compression efficiency, error resilience, and adaptability to varying network conditions.
Usage: H.261 is rarely used in modern applications due to advancements in video codec technologies. However, it’s historically significant as it laid the groundwork for the video codecs that followed.
In summary, H.261 was pioneering during its era, setting foundational principles for video compression. Its concepts and techniques influenced subsequent video codecs, ensuring better video quality and more efficient bandwidth usage in video communication systems.