The ITU-T H.261 recommendation embraces video codecs for audio visual services at p x 64 kbit/s data rate, where p is between 1 and 30. Thus, the standard is informally called “p x 64.” It is aimed at low bit rate media, and is used in the H.320 videoconferencing recommendation.

H.261 provides a resolution of 352 x 288 pixels (CIF) or 176 x 144 pixels (QCIF), independent of bit rate. The H.261 recommendation defines both encoding and decoding. However, it defines, more strictly, how to decode than to encode the bit stream, and has room for options in the encoder.

The coding is based on DCT with word-length encoding. H.261 defines both independently coded frames (key frames), and frames that frame by using block-based motion compensation (non-key frames). H.261 also defines error-correction codes, and it allows rate control by varying quantization and by dropping frames and jumping blocks.