SWE 423: Multimedia Systems

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SWE 423 Multimedia Systems

• Chapter 7 Data Compression (5)

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Outline

• Introduction to H.261
• H.261 Image Preparation
• H.261 Coding Algorithms
• H.263
• H.261 H.263 Properties
• H.261 vs. H.263
• H.264

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Introduction to H.261

• ISDN Integrated Services Digital Network is
  (and was) behind H.261
• A circuit-switched telephone network system,
  designed to allow digital transmission of voice
  and data over ordinary telephone copper wires.
• In ISDN, there are two types of channels, B (for
  "Bearer") and D (for "Delta"). B channels are
  used for data (which may include voice), and D
  channels are intended for signaling and control
  (but can also be used for data).
• In a narrow-band ISDN connection, exactly two
  B-channels and one D-channel is available
• One or both B channels can transfer video data,
  in addition to speech.
• This requires that both ends have to use the same
  video data coding schemes

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Introduction to H.261

• The primary applications of ISDN were video
  phones and video conferencing.
• Such dialogue applications require that coding
  and decoding being carried out in real-time.
• In 1984, Study group XV of CCITT formed a
  committee to draw up a video standard for
  compressing moving pictures
• The standard, H.261 Video CoDec for Audiovisual
  services at p?64Kbit/s was finalized after 5
  years and got accepted in December 1990.
• North America adopted it with slight
  modifications
• Since data rates of p?64 Kbit/s are considered,
  the recommendation was also known as p?64.
• Maximum combined signal delay is 150 ms

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H.261 Image Preparation

• Very precise format, unlike JPEG
• Refresh frequency at the input must be 30000/1001
  frames/s
• During encoding, lower frame rates are possible
  (10 or 15 frames/s)
• Images cannot be presented at the input to the
  coder using interlaced scanning
• The image is encoded as a luminance signal Y and
  chrominance difference signals Cb, Cr, according
  to the CCIR 601 sub-sampling scheme (211)
• This was later adopted by MPEG

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H.261 Image Preparation

• Two resolution formats are supported, both with
  an aspect ratio of 43
• Common Intermediate Format (CIF)
• Optional
• 352 lines, each with 288 pixels of luminance (Y)
  component
• As per the (211) requirement, the chrominance
  components are sub-sampled with ....... lines,
  each with ....... Pixels.
• Quarter CIF (QCIF)
• All H.261 CoDecs have to implement QCIF
• Has exactly half the resolution in all components.

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H.261 Image Preparation

• H.261 divides the Y, Cb and Cr components into
  blocks of 8?8 pixels
• A macro block results from combining 4 blocks of
  the Y matrix with 1 block each from the Cb and Cr
  components.
• A group of blocks consists of 3?11 macro blocks.
  Hence,
• CIF consists of ...... groups
• QCIF consists of ....... groups

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H.261 Coding Algorithms

• H.261 uses two different modes of coding
• Intra-frame coding
• Inter-frame coding
• H.261 does not specify any criteria for choosing
  one or the other.
• That decision is taken during encoding

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H.261 Coding Algorithms

• Intra-frame coding
• Considers data from image being coded
• Like JPEG, each block of 8?8 pixels is
  transformed into 64 coefficients using DCT.
• DC coefficients are quantized differently than AC
  coefficients
• Entropy encoding using variable-length code words
  is then performed.

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H.261 Coding Algorithms

• Inter-frame coding
• Considers data from other images
• A prediction method is used to find the most
  similar macro block in the preceding image.
• Motion vector is the relative position of the
  previous macro block w.r.t. the current macro
  block
• According to H.261, the encoder needs not
  determine a motion vector, thus may only consider
  differences between macro blocks located at the
  same position in successive images.
• The motion vector is processed and entropy
  encoded using variable-length code words
• The DPCM-coded macro block is processed and
  transformed using DCT if and only if its value
  exceeds a certain threshold value, linearly
  quantized, and entropy encoded using
  variable-length code words
• An optical low pass filter can be optionally
  inserted between the DCT transformation and
  entropy encoding to delete any remaining
  high-frequency noise.

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H.263

• Developed in 1996 to replace H.261 for many
  applications
• Designed for low bit rate transmission, but also
  suitable for higher bit rates applications
• Provides one of the most efficient video
  compression techniques available.

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H.263

• Inclusion of four negotiable options to improve
  performance (achieving same quality as H.261 with
  less than half as many bits)
• Syntax-based arithmetic coding
• Defines the use of arithmetic coding instead of
  variable length coding
• Forward and backward frame prediction
• Can increase frame rate without changing the bit
  rate by coding two images as one unit.
• Unrestricted Motion Vectors
• Makes it possible for motion vectors to point
  outside image boundaries.
• Useful for small images with motion in the
  direction of edges
• Advanced Prediction
• Uses the overlapped block motion compensation
  (OBMC) technique for P-frame luminance.
• An algorithm that obtains motion vectors from
  blocks next to the current macro block and uses
  them with the current macro block to achieve a
  more accurate predication and a smaller bit
  stream.
• Requires the use of unrestricted motion vectors.

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H.261 H.263 Properties

• The data stream contains information for error
  correction, although the use of external error
  correction standards (e.g. H.223) is recommended.
• Each image in H.261 includes a 5-bit image number
  that can be used as a temporal reference. H.263
  uses 8-bit image numbers
• During decoding, a command can be sent to the
  decoder to freeze the last video frame.
• It is possible to switch between still images and
  moving images using an additional command sent by
  the coder.

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H.261 vs. H.263

• Use similar coding algorithms
• With some enhancements and error correction in
  H.263
• H.263 uses half pixel precision for motion
  compensation, while H.261 uses full pixel
  precision with loop filter.
• Some parts of the hierarchical structure of the
  data stream are now optional, so the codec can be
  configured for a lower data rate or better error
  recovery.
• Inclusion of four negotiable options to improve
  performance (achieving same quality as H.261 with
  less than half as many bits)
• Unrestricted Motion Vectors,
• Syntax-based arithmetic coding,
• Advance prediction, and
• Forward and backward frame prediction
• similar to MPEGs P and B frames.
• H.263 supports three more resolutions (SQCIF,
  4CIF, and 16CIF) in addition to the two supported
  by H.261 (QCIF and CIF)
• SQCIF is approximately half the resolution of
  QCIF.
• 4CIF and 16CIF are 4 and 16 times the resolution
  of CIF respectively.
• The support of 4CIF and 16CIF means the codec
  could then compete with other higher bitrate
  video coding standards such as the MPEG
  standards.
• Check http//www-mobile.ecs.soton.ac.uk/peter/h263
  /h263.html for samples

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H.264

• H.264 was finalized and published in March 2005
  and represents an evolution of the existing video
  coding standards (H.261,H.262, and H.263)
• It was developed in response to the growing need
  for higher compression of moving pictures for
• various applications such as videoconferencing,
  digital storage media, television broadcasting,
  Internet streaming, and communication and enable
  the use of the coded video representation in a
  flexible manner for a wide variety of network
  environments.
• It also allows motion video to be manipulated as
  a form of computer data and to be stored on
  various storage media, transmitted and received
  over existing and future networks and distributed
  on existing and future broadcasting channels.

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H.264

• The revision contains modifications of the video
  coding standard to
• add four new profiles, High, High 10, High 422,
  and High 444 profiles
• improve video quality capability
• extend the range of applications addressed by the
  standard (for example, by including support for a
  greater range of picture sample precision and
  higher-resolution chroma formats).
• define new types of supplemental data has been
  specified to further broaden the applicability of
  the video coding standard.