Video Compression Basics

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Video CompressionBasics

• Nimrod Peleg
• Update Nov. 1999

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Video Compression list of topics

• Analog and Digital Video Concepts
• Block-Based Motion Estimation
• Resolution Conversion
• H.261 A Standard for VideoConferencing
• MPEG-1 A Standard for CD-ROM Based App.
• MPEG-2 and HDTV All Digital TV
• H.263 A Standard for VideoPhone
• MPEG-4 Content-Based Description

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Analog Video Signal Raster Scan
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Analog Video Signal Image line (NTSC)
Black
White
53.5microSec
10uSec
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Analog Video Standards

• All video standards are in color
• Almost any color can be reproduced by mixing the
  3 additive primaries R (red), G (green) and B
  (blue)
• 3 main different representations
• Component
• Composite
• S-Video (Y/C)

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Component Analog Video

• Each primary is considered as a separate
  monochromatic video signal
• Basic presentation RGB
• Other RGB based
• YIQ
• YCrCb
• YUV
• HSI

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Composite Video Signal

• Encoding the Chrominance over Luminance into one
  signal (saving bandwidth)
• NTSC (National TV System Committee)
• North America, Japan
• PAL (Phased Alternation Line)
• Europe (Including Israel)
• SECAM (Systeme Electronique Color Avec Memoire)
• France

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Analog Standards Comparison

• NTSC PAL/SECAM
• Defined 1952 1960
• Scan Lines/Field 525/262.5 625/312.5
• Active lines 485 525
• Subcarrier Freq. 3.58MHz 4.43MHz
• Interlacing 21 21
• Aspect ratio 43 43
• Horiz. Resol.(el/lin) 452
• Frames/Sec 29.97 25
• Component Color TUV YCbCr

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NTSC/525 Advantages

• Higher Frame Rate - Use of 30 frames per second
  (really 29.97) reduces visible flicker.
• Atomic Color Edits - With NTSC it is possible to
  edit at any field boundary point without
  disturbing the color signal.
• Less inherent picture noise - Almost all pieces
  of video equipment achieve better signal to noise
  characteristics in their NTSC/525 form than in
  their PAL/625.

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NTSC/525 Disadvantages

• Lower Number of Scan Lines - Reduced clarity on
  large screen TVs, line structure more visible.
• Smaller Luminance Signal Bandwidth - Due to the
  placing of the color sub-carrier at 3.58MHz,
  picture defects such as moire, cross-color, and
  dot interference become more pronounced. This is
  because of the greater likelihood of interaction
  with the monochrome picture signal at the lower
  sub-carrier frequency.

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NTSC/525 Disadvantages (contd)

• Susceptablity to Hue Fluctuation - Variations in
  the color sub-carrier phase cause shifts in the
  displayed color, requiring that the TV receivers
  be equipped with a Hue adjustment to compensate.
• Lower Gamma Ratio - The gamma value for NTSC/525
  is set at 2.2 as opposed to the slightly higher
  2.8 defined for PAL/625. This means that PAL/625
  can produce pictures of greater contrast.

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NTSC/525 Disadvantages (contd)

• Undesirable Automatic Features - Many NTSC TV
  receivers feature an Auto-Tint circuit to make
  hue fluctuations less visible to uncritical
  viewers. This circuit changes all colors
  approximating to flesh tone into a "standard"
  flesh-tone, thus hiding the effects of hue
  fluctuation. This does mean however that a
  certain range of color shades cannot be displayed
  correctly by these sets. Up-market models often
  have this (mis)feature switchable, cheaper sets
  do not.

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PAL/625 Advantages

• Greater Number of Scan Lines - more picture
  detail.
• Wider Luminance Signal Bandwidth - The placing of
  the color Sub-Carrier at 4.43MHz allows a larger
  bandwidth of monochrome information to be
  reproduced than with NTSC/525.
• Stable Hues - Due to reversal of sub-carrier
  phase on alternate lines, any phase error will be
  corrected by an equal and opposite error on the
  next line, correcting the original error.

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PAL/625 Advantages (contd)

• Higher Gamma Ratio - The gamma value for PAL/625
  is set at 2.8 as opposed to the lower 2.2 figure
  of NTSC/525. This permits a higher level of
  contrast than on NTSC/525 signals. This is
  particularly noticeable when using multi-standard
  equipment as the contrast and brightness settings
  need to be changed to give a similar look to
  signals of the two formats.

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PAL/625 Disadvantages

• More Flicker - Due to the lower frame rate,
  flicker is more noticable on PAL/625
  transmissions particularly so for people used to
  viewing NTSC/525 signals.
• Lower Signal to Noise Ratio - The higher
  bandwidth requirements cause PAL/625 equipment to
  have slightly worse signal to noise performance
  than it's equivalent NTSC/525 version.
• Loss of Color Editing Accuracy - Due to the
  alternation of the phase of the color signal, the
  phase and the color signal only reach a common
  point once every 8 fields/4 frames. This means
  that edits can only be performed to an accuracy
  of /- 4 frames (8 fields).

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PAL/625 Disadvantages (contd)

• Variable Color Saturation - Since PAL achieves
  accurate color through canceling out phase
  differences between the two signals, the act of
  canceling out errors can reduce the color
  saturation while holding the hue stable.
  Fortunately, the human eye is far less sensitive
  to saturation variations than to hue variations,
  so this is very much the lesser of two evils.

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Analog Video Equipment

• Cameras (Vidicon, Film, CCD)
• Video Tapes (magnetic) Betacam, VHS, SVHS,
  U-matic, 8mm ....
• Optical Video Disk
• Displays
• All for all video standards !

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Digital Video

• Developed with CD-ROM Technology
• Brings computers and communication together in a
  truly revolutionary matter
• Personal computer
• VideoPhone, FAX, HDTV, Live video (with
  processing), Local image printer,
• Video Conferencing, ......

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Digital Video Formats

• For a number of years professional TV studios
  used D1 (component) and D2 (composite) digital
  formats.
• CCIR digitization (CCIR601) and interfacing
  (CCIR656) for digital video is base line for all
  formats, using component form known as 422 (Y,
  Cb, Cr).

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CCIR Digitization

• For component video signals (studio source) with
  BW6MHz, CCIR sampling rate is 13.5MHz,
  independent of scanning standard
• This rate represents 864 x Fh for 625 systems and
  858 x Fh for 525 systems.
• Active lines per frame is 720 for both.
• For 8 bits/sample 13.5M x 8 108 bit/sec.

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Orthogonal Sampling

• In the CCIR method, samples are at the same fixed
  place on all lines in a frame, and also from
  frame to frame, such that they are situated on a
  rectangular grid.
• Other sampling schemes are not line-locked and
  their sampling grid is not rectangular.

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Digital Video (CCIR 601)

• Coded Signals Y, Cb, Cr
• Samples/line 858 (NTSC) / 864 (PAL)
• for color components 429 / 432
• Active samples 720
• for color components 360
• Quantizer Uniform PCM , 8 bit/sample (Y,Cb,Cr)
• Gray levels Scale 0 - 255
• Y 220 Q levels (black16, gray-white235)
• Cb,Cr 225 Q levels (zero 128)

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Digital Video Signal

• TrueColor RGB , 24 bit (16M colors)
• Resolution lack of sufficient resolution causes
  pixellation (blockization).
• Synchronization No need (computer made).
• Conversion A/D D/A after cameras and before
  display (still too expensive).
• The bottleneck BITRATE

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BitRates

• CD Quality digital audio
• 44.1KHz 16bps 2 (stereo) 1.4Mbps
• HDTV about 1Gbps
• One picture worth (almost) a 1000 words...

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Digital Video Resolution (CCIR601, 422)

• 525/60(NTSC) 625/50(PAL) CIF
• Active pel/line
• Lum (Y) 720 720 360
• Chroma(U,V) 360 360 180
• Active lines/pic
• Lum (Y) 480 576 288
• Chroma(U,V) 480 576 144
• Interlacing 21 21 11
• Rate/Aspect Ratio 60/43 50/43 30/43

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Network protocols and Bitrates

• Conventional telephone 0.3-56Kbps
• ISDN 64-144Kbps
• T-1 1.5Mbps
• EtherNet (Packet based LAN) 10Mbps
• Broadband ISDN 100-200Mbps
• ATM (Cell based LAN/WAN) 155-620Mbps

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Image Compression Standards

• CCITT G3/G4 Binary Images (FAX)
• JBIG FAX and Documents
• JPEG Still Images (b/w, color)
• H.263 VideoPhone (lt16Kbps) (upto h.261)
• H.261 VideoConferencing (Px64Kbps)
• MPEG-1 CD-ROM (1.5Mbps)
• MPEG-2 Broadcasting (10-20Mbps)
• MPEG-4 ? 4-8Kbps

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Proprietary Video Formats

• DVI, Indeo Intel
• QuickTime Apple
• CD-I Philips
• PhotoCD Eastman Kodak
• CDTV Commodore

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Why Digital Video?

• Quality
• Error correction
• Interactivity
• Computer control (s/w)
• Real-time playback and all kinds of editing
• Integration of many multimedia platforms
• .......

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A Summary of Video Formats

• CCIR Size (D1) Progressive Pictures
• NTSC - 720x480 (29.97 Pictures/Sec)
• PAL - 720x576 (25 Pictures/Sec)
• CCIR Size (D1) Interlaced Pictures
• NTSC - 720x(240x2) (29.97 Pictures/Sec)
• PAL - 720x(288x2) (25 Pictures/Sec)
• HD1 (Half D1) Progressive Pictures
• NTSC 352x480 (29.97 Pictures/Sec)
• PAL 352x576 (25 Pictures/Sec)
• HD1 (Half D1) Interlaced Pictures
• NTSC 352x(240x2) (29.97 Pictures/Sec)
• PAL 352x(288x2) (25 Pictures/Sec)

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Video Formats (Contd)

• SIF (Source Input Format)
• NTSC 352x240 (29.97 Pictures/Sec)
• PAL 352x288 (25 Pictures/Sec)
• QSIF
• NTSC 176x112 (29.97 Pictures/Sec)
• PAL 176x144 (25 Pictures/Sec)
• CIF (Common Intermediate Format)
• 352x288 (30 Pictures/Sec)
• QCIF Size Pictures
• 176x144 (30 Pictures/Sec

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YCbCr 4xy Chroma Formats
YCbCr 444
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YCbCr 422
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YCbCr 411
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YCbCr 420 Centered (MPEG-1 Style)
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YCbCr 420 Co-Sited (MPEG-2 Style)
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