H.261 Video Compression Standard

1
H.261 Video Compression Standard

• Presented by
• Parul Mathur
• Brad Gagné
• This report was prepared for Professor L.
  Orozco-Barbosa in partial fulfillment of the
  requirements for the course CEG 4183.

2
Presentation Overview

• Introduction
• Key Terms and Concepts
• H.261 Specifications
• How It Works
• Design Details
• Encoding Process Diagram
• Performance
• H.261 vs. MPEG
• Where It Is Used

3
Introduction

• H.261 is a video coding standard published by the
  ITU-T in 1990.
• It is the most widely used international video
  compression standard for video coding.
• H.261 is usually used in conjunction with other
  control and framing standards.

http//www.cs.ucl.ac.uk/staff/jon/mmbook/book/node
121.html
http//www-mobile.ecs.soton.ac.uk/peter/h261/h261.
html
4
Key Terms and Concepts (1)

• ITU-T International Telecommunication Union
• CIF - Common Intermediate Format
• A video format used in videoconferencing systems
  that easily supports both NTSC and PAL signals,
  and specifies a data rate of 30 fps, with 352x288
  pixel frames.
• DPCM Differential Pulse Code Modulation
• a form of PCM that produces a digital signal with
  a lower bit rate than standard PCM by recording
  only the difference between samples and adjusting
  the coding scale dynamically to accommodate large
  and small differences
• DCT Discrete Cosine Transform
• A technique for representing waveform data as a
  weighted sum of cosines commonly used for data
  compression, as in JPEG.

http//www.webopedia.com
5
Key Terms and Concepts (2)

• ISDN Integrated Services Digital Network
• An international communications standard for
  sending voice, video, and data over digital
  telephone lines or normal telephone wires
  supports data transfer rates of 64 kbps.
• Spatial Redundancy, Temporal Redundancy, Motion
  Compensation

http//www.webopedia.com
6
H.261 Specifications (1)

• The H.261 standard describes the video
  coding/decoding methods for the video portion of
  an audiovisual service
• Designed for data rates of p64 kbps, where p is
  in the range 1-30
• Targeted for circuit-switched networks (ISDN was
  the communication channel considered within the
  framework of the standard)

http//www-mobile.ecs.soton.ac.uk/peter/h261/h261.
html
7
H.261 Specifications (2)

• Defines two picture formats
• CIF (352x288) and QCIF(176x144)
• The H.621 encoding algorithm is a combination of
• inter-picture prediction (to remove temporal
  redundancy)
• transform coding (to remove spatial redundancy)
• motion vectors (for motion compensation)

http//www-mobile.ecs.soton.ac.uk/peter/h261/h261.
html
8
Design Details

• The three main elements are
• Prediction blocks are intra- or inter-coded
• Intra-coded blocks stand alone
• Inter-coded blocks are based on differences
  between the previous frame and the current one
• Block Transformation
• each block (inter and intra) is composed into 8x8
  blocks and processed by a 2-D DCT function
• Quantization Entropy Coding
• achieves further compression by representing DCT
  coefficients with only the necessary precision
• entropy encoding (non-lossy) using Huffman
  encoding

http//www.cs.ucl.ac.uk/staff/jon/mmbook/book/node
121.html
9
H.261 Encoding Process
demo
http//www.cogs.susx.ac.uk/users/ianw/teach/ms/img
7.gif
10
Performance

• Quality of picture depends on efficiency of the
  encoding and thoroughness of decoding
• Compromise between definition and fluency
• DCT takes advantage of the spatial and temporal
  redundancy found in picture data to remove
  redundant data bits, reducing data transmission
  rates
• Motion compensation moves only the section of
  video where motion has occurred, rather the
  entire video area for every frame, also reducing
  data transmission rates
• Even a minimum H.261 compliant codec can provide
  tremendous compression ratios (well beyond 1001)

http//web.nps.navy.mil/seanet/Distlearn/chp3.htm

http//hepwww.rl.ac.uk/sysman/may98/vcfayers/sld01
9.htm
11
Image Quality vs. Bitrate
http//www-mobile.ecs.soton.ac.uk/peter/h261/compa
re-averpsnr-v-bitrate.gif
12
H.261 vs. MPEG

• based on JPEG
• encodes video only
• lossy algorithm with compression in space and
  time
• uses I and P-frames
• uses DCT on 8x8 blocks
• best for video with little motion (eg. video
  conferencing)
• optimized for bandwidth efficiency and low delay

• based on H.261 and JPEG
• encodes audio video
• lossy algorithm with compression in space and
  time
• uses I, P, and B-frames
• uses DCT on 8x8 blocks
• designed to handle moving picture components
• less bandwidth efficient

http//www.npac.syr.edu, http//web.nps.navy.mil/
seanet/Distlearn/chp3.htm
13
Where It Is Used (1)

• There are a number of compression techniques
  available for use in videoconferencing, and H.261
  is one of the most widely used in commercial
  videoconferencing products.
• It is currently used in such software as MS
  Windows NetMeeting, AudioVision, Intel Video
  Phone, Internet Phone

http//web.nps.navy.mil/seanet/Distlearn/chp3.htm
14
Where It Is Used (2)

• H.261 is currently used in teleconferencing
  standards H.320, H.323 and H.324.

http//www.ece.wpi.edu/courses/ee535/hwk97/hwk4cd9
7/bigles/sec01.html
15
Thank You

• Questions?

16
Appendix A I and P-Frames
17
Appendix B Standards
Video Teleconferencing Standards
http//www.it.kth.se/nv91-tob/table6.html
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References

• J.Anders, Inline H261 - player in JAVA
  (JITVERSION CIF Example), lthttp//rnvs.informati
  k.tu-chemnitz.de/jan/H261_JIT/H261_CIF_JIT.htmlgt,
  Accessed 31 January 2002
• Dr. Peter Cherriman, H.261 Video
  Coding,lthttp//www.cs.ucl.ac.uk/staff/jon/mmbook
  /book/node121.htmlgt, 14 September 1996
• Dr. Peter Cherriman, H.261 Video
  Coding,lthttp//www-mobile.ecs.soton.ac.uk/peter/
  h261/h261.htmlgt, 14 September 1996
• http//www.webopedia.com, Accessed 25 February
  2002
• Ian Wakeman, Multimedia Systems Techology
  Lecture Notes, lthttp//www.cogs.susx.ac.uk/users/
  ianw/teach/ms/img7.gifgt, 31 January 2002
• Mark V. Glover, MAJOR VIDEOCONFERENCING
  STANDARDS, lthttp//web.nps.navy.mil/seanet/Dist
  learn/chp3.htmgt, March 1998
• G.J.Fayers, Videoconferencing A Scenic Tour of
  Current Technologies, lthttp//hepwww.rl.ac.uk/sys
  man/may98/vcfayers/sld019.htmgt, 22 May 1998
• Michael Roberts, Overview of the H.320 Video
  Conferencing Protocol,
• lthttp//www.ece.wpi.edu/courses/ee535/hwk97/hwk4cd
  97/bigles/sec01.htmlgt, 21 January 1999
• Tobias Öbrink, Table VI. Video Teleconferencing
  Standards, lthttp//www.it.kth.se/nv91-tob/table
  6.htmlgt, 2 February 2002

19
Questions

• H.261 was designed specifically for
• LAN networks
• ISDN networks
• POTS networks
• none of the above
• True or False H.261 is based on MPEG 1. False.
• The three main components of H.261 encoding are
• temporal redundancy, spatial redundancy, motion
  compensation
• ITU-T, PCM, DCT
• Prediction, Block Transformation, Quantization
  Entropy Encoding
• I-frames, P-frames, B-frames
• True or False H.261 was originally designed for
  circuit-switching networks. True.
• H.261 is a compression standard for
• audio information
• video information
• data
• all of the above