Terminology » Other Wireless » H.261 vs H.263: Video Coding Standards Compared

H.261 vs H.263: Video Coding Standards Compared

video coding h.261 h.263 video conferencing coding standard

This article compares the H.261 and H.263 video coding standards, outlining the key differences between them. Both standards were adopted by the ITU-T and are utilized for real-time encoding and decoding, often on Power-PC 604 processors. They are commonly found in international standard teleconferencing codecs.

H.261

H.261 was specifically designed and developed for video conferencing applications intended for use over ISDN lines. ISDN provides bit rates of 64 Kbps and higher, in multiples of 64 Kbps.

  • Key Features:
    • Maximum coding delay of 150 msec.
    • Supports I-frames and P-frames.
      • I-frames: Do not reference any previously coded frames.
      • P-frames: Use the previous frame as a reference.
    • The MPEG-1 coding standard was derived from the H.261 standard.

H.263

H.263 builds upon the foundations of MPEG-1 and H.261. It’s an improved standard designed for low bit-rate applications.

  • Key Features:
    • Similar to H.261, it uses transform coding for intra-frames and predictive coding for inter-frames.
    • Supports POTS (Plain Old Telephone Service) at rates of 64 Kbps or less.
    • Utilizes bi-directionally predicted (B) frames.
      • B-frames: Represent an average of two P-frames. They use both the previous and future frames to predict the current input frame.
      • B-frame size is approximately 1/3 of the P-frame size.
    • Employs an advanced prediction mode.
      • 4 motion vectors per macroblock: Each 8x8 block is associated with one motion vector, instead of a single motion vector for the entire macroblock. This allows for better motion representation but increases the number of bits needed to encode the motion vector.
      • OBMC (Overlapped Block Motion Compensation): This technique smooths the image.

H.261 vs H.263: Specifications Compared

SpecificationsH.261H.263
Picture formatsCIF (352 x 288 pixels for luminance channel, 176 x 144 pixels for chrominance channels U & V)Supports five picture formats: Sub-QCIF (128 x 96 pixels Y, 64 x 48 pixels U,V), QCIF (176 x 144 pixels Y, 88 x 72 pixels U,V), CIF (352 x 288 pixels Y, 176 x 144 pixels U,V), 4CIF (704 x 576 pixels Y, 352 x 288 pixels U,V), 16 CIF (1408 x 1152 pixels Y, 704 x 576 pixel U,V)
Temporal rates(7.5, 10, 15, 30 frames/sec) are supported
Bit rate40 kbps to 2 MbpsSupports lower bit rates compared to H.261
Motion estimationMotion vectors are expressed in integer pixel units.Uses half pixel motion estimation.
Frame modesI frame, P frameIt uses PB frame mode. It increases frame rate without increasing bitrate.

Note: In the table, CIF stands for Common Intermediate Format, and QCIF stands for Quarter of Common Intermediate Format.