• H.261 is an algorithm that determines how to encode and compress the data electronically.

• It is a video coding standard published by the ITU (International Telecommunication Union) in 1990.

• It is the most widely used international compression technique for encoding videos.

• H.261 encoding technique can encode only video part of an audiovisual service.

• H.261 is a two-way communication over ISDN lines (Video conferencing and Video calling) and supports data rate in multiples of 64 KBPS.

• H.261 defines a video encoder that is intended to be used to compress video data that will be sent over Integrated Services Digital Network (ISDN) lines.

• The H.261 codec is intended primarily for use in video telephony and videoconferencing applications.

• H.261 was the first practical digital video coding standard. The H.261 design was a pioneering effort, and all subsequent international video coding standards (MPEG-1, MPEG-2/H.262, H.263, and even H.264 have been based closely on its design). Additionally, the methods used by the H.261 development committee to collaboratively develop the standard have remained the basic operating process for subsequent standardization work in the field.

• The images supplied as input to an H.261 compressor must meet both color space and size (width and height) requirements.

• In terms of color space, the images must be YCbCr images.

• In terms of size, the images must adhere to either the Common Interchange Format (CIF) or the Quarter-CIF (QCIF) format. Table below indicates the widths and heights defined by these formats.

How an H.261 Codec Works

The three main elements in an H.261 encoder are as follows:

• Prediction
• Block Transformation
• Quantization and Entropy Coding

Two types of image frames are defined:

• Intra-frames (I-frames) and Inter-frames (P-frames)
• I-frames are treated as independent images. Transform coding method similar to JPEG is applied within each I-frame, hence “Intra”.

• Macroblocks are of size 16 × 16 pixels for the Y frame, and 8 × 8 for Cb and Cr frames, since 4:2:0 chroma Subsampling is employed.

• A macroblock consists of four Y, one Cb, and one Cr 8 × 8 blocks.

• For each 8 × 8 block a DCT transform is applied, the DCT coefficients then go through quantization zigzag scan and entropy coding.

Inter-frame (P-frame) Predictive Coding:

• P-frame coding scheme is based on motion compensation. In motion compensation a search area is constructed/predicted in previous frame to determine the best reference macroblock.

• After the prediction, a difference macroblock is derived to measure the prediction error.

• Each of these 8×8 blocks go through DCT, quantization, zigzag scan and entropy coding procedures.

• The P-frame coding encodes the difference macroblock (not the Target macroblock itself).

• Sometimes, a good match cannot be found, i.e., the prediction error exceeds a certain acceptable level.

Intraframe Versus Interframe Encoding

• For each macroblock that it encodes, the H.261 encoder can perform Intraframe or Interframe compression. In intraframe mode, the compressor encodes the actual YCbCr values in the macroblock.

• In Interframe mode, the compressor:

• Looks at the YCbCr values in the macroblock it is encoding.

• Calculates the difference between the predicted values for the macroblock and the actual values in the macroblock. The predicted values are taken from the most recently compressed image, which is stored in a history buffer.

• Encodes the difference values if they are significant.

• Encoding YCbCr Values

• This encoding scheme is very similar to the one used in the JPEG still-image compression standard.