Video

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Video

• To begin looking at video we need to first go
  over some terms used in its description. Here
  comes the jargon!

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Describing the video stream

• Frame rate
• The number of still pictures per unit of time of
  video,
  
• old mechanical cameras from six or eight (fps
• new professional cameras 120 or more frames per
  second
  
• PAL and SECAM standards specify 25 fps,
• NTSC specifies 29.97 fps.
• To achieve the illusion of a moving image, the
  minimum frame rate is about ten frames per
  second.
  
• Film is shot at the slower frame rate of 24fps.

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• Interlacing vs progressive scan
• Interlaced
• Interlacing was invented as a way to achieve good
  visual quality within the limitations of a narrow
  bandwidth.
  
• The horizontal scan lines of each interlaced
  frame are numbered consecutively and partitioned
  into two fields the odd field consisting of the
  odd-numbered lines and the even field consisting
  of the even-numbered lines. NTSC, PAL and SECAM
  are interlaced formats.

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• Progressive scan
• Progressive or non-interlaced scanning is any
  method for displaying, storing or transmitting
  moving images in which the lines of each frame
  are drawn in sequence. This is in contrast to the
  interlacing used in traditional television
  systems.
• Progressive scan is used in most CRTs used as
  computer monitors. It is also becoming
  increasingly common in high-end television
  equipment, which is often capable of performing
  deinterlacing so that interlaced video can still
  be viewed.

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• Advantages of progressive scan include
• Subjectively increased vertical resolution. The
  perceived vertical resolution of an interlaced
  image is usually equivalent to multiplying the
  active lines by about 0.6. This explains, for
  example, why HDTV standards such as 1080i
  (1920x1080, interlaced) in most cases deliver a
  quality equal to or slightly poorer than that of
  720p (1280x720, progressive), despite containing
  far more lines of resolution.
• No flickering of narrow horizontal patterns
• Simpler video processing equipment
• Easier compression

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• Video resolution
• The size of a video image is measured in pixels
  for digital video or horizontal scan lines for
  analog video.
  
• Standard-definition television (SDTV) is
  specified as 640_80i60 for NTSC and 720_76i50 for
  PAL or SÉCAM resolution. New high-definition
  televisions (HDTV) are capable of resolutions up
  to 1920_080p60, i.e. 1920 pixels per scan line by
  1080 scan lines, progressive, at 60 frames per
  second.

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• Aspect ratio
• Aspect ratio describes the dimensions of video
  screens and video picture elements.
  
• traditional television screen is 43, or 1.331.
• High definition televisions 169, or about
  1.781.
  
• full 35 mm film (also known as "Academy
  standard") is around 1.371.
  

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• Some common aspect ratios

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• Digital Video
• Digital video is a type of video recording system
  that works by using a digital, rather than
  analog, representation of the video signal.
  
• This generic term is not to be confused with the
  name DV, which is a specific type of digital
  video.
  
• Most often recorded on tape, then distributed on
  optical discs, usually DVDs.
  
• Some exceptions, camcorders that record directly
  to DVDs, Digital8 camcorders which encode digital
  video on conventional analog tapes.
  

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• Main issue with digital video
• Digital Video in its raw form can be huge.
• e.g. 25 fps of 720 576 3bytes.
• Leeds to two interrelated problems
• Large storage requirements
• High Bandwidth requirement during playback and
  recording.

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• The solution compression
• In fact this problem of large storage and high
  bandwith requirement is a defining feature of
  much multimedia not just video.
  
• We also must deal with compression in the context
  of sound.
  
• For now we focus on digital video

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Video Compression

• Compression is a conversion of data to a format
  that requires fewer bits, usually performed so
  that the data can be stored or transmitted more
  efficiently.
• The inverse process is known as decompression.
  Depending on the compression method used to
  encode the original, decompression may or may not
  yield an exact copy of the original unencoded
  data.

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• Two broad types of compression
• Temporal and Spatial ( a slightly false
  dichotomy, many codecs use a combination of the
  two.)
  
• Before we look at these remember what we mean by
  a frame.
  
• Frame
• A frame is a set of all pixels that
  (approximately) correspond to a single point in
  time. However, in interlaced video, the set of
  horizontal lines with even numbers and the set
  with odd numbers are grouped together in fields.
  The term "picture" can refer to a frame or a
  field.

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• Spatial Compression (intraframe)
• Compresses each frame individually. It treats the
  video stream as a series of individual images.
  
• takes advantage of the fact that the human eye is
  unable to distinguish small differences in colour
  as easily as it can changes in brightness and so
  very similar areas of colour can be "averaged
  out" in a similar way to jpeg images.

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• Temporal compression (interframe)
• With temporal compression only the changes from
  one frame to the next are encoded.
  
• Often a large number of the pixels will be the
  same on a series of frames. (blue sky)
  
• By identifying differences between consecutive
  frames of video, and storing just those frame
  differences in the compressed file, temporal
  compression can dramatically decrease video data
  size.
• Interframes Frames compressed with temporal
  compression are called interframes

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Codecs

• A Codec is a device or program capable of
  performing Encoding and Decoding on a data stream
  or signal.
  
• The word "codec" is a portmanteau of any of the
  following 'Compressor-Decompressor',
  'Coder-Decoder', or 'Compression/Decompression
  algorithm'.

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• Cinepak
• developed by Radius Inc to accommodate 1x (150
  kbyte/s) CD-ROM transfer rates.
  
• primary video codec of early versions of
  QuickTime and Microsoft Video for Windows,
  
• superseded by Sorenson Video, Intel Indeo, and
  most recently MPEG-4 and h.264.
  
• based on vector quantization, which is a
  significantly different algorithm from the
  discrete cosine transform (DCT) algorithm used by
  most current codecs (in particular the MPEG
  family, as well as JPEG). This permitted
  implementation on relatively slow CPUs, but
  tended to result in blocky artifacting at low
  bitrates.

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• Sorenson codec (Sorenson Video Codec, Sorenson
  Video Quantizer or SVQ)
  
• devised by the company Sorenson Media
• The Sorenson codec first appeared in QuickTime 3.
  
  
• The specifications of the codec were not public,
  and for a long time the only way to play back
  Sorenson video was to use Apple's QuickTime
  player, or the MPlayer for Unix/Linux, which in
  turn piggy-backed Microsoft Windows DLL-files
  extracted from Apple's player.

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• According to an anonymous developer of FFmpeg,
  reverse engineering of the SVQ3 codec revealed it
  as a tweaked version of H.264. The same developer
  also added support for this codec to FFmpeg,
  making native playback on all platforms supported
  by FFmpeg possible.

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• MPEG-1 video
• Originally designed for 1.5M bit/second data
  rates and 352x240 resolution.
  
• Later improvements allow for up to 4M bit/second
  for better quality.
  
• MPEG-1 is the most compatible format in the MPEG
  family it is playable in almost all computers
  and VCD/DVD players.
  
• One big disadvantage of the MPEG-1 is that it
  supports only progressive scan video, which is
  part of the reasons which prompted the more
  advanced MPEG-2.

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• MPEG-2 Video
• Standard for DVD encoding
• similar to MPEG-1, but also provides support for
  interlaced video (the format used by broadcast TV
  systems)
  
• not optimized for low bit-rates (less than
  1 Mbit/s), but outperforms MPEG-1 at 3 Mbit/s and
  above.
  
• All standards-conforming MPEG-2 Video decoders
  are fully capable of playing back MPEG-1 Video
  streams.

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• Mpeg 2 uses a combination of temporal and spatial
  encoding
  
• An MPEG-2 video bitstream is made up of a series
  of data frames encoding pictures.
  
• The three ways of encoding a picture are
  intra-coded (I picture), forward predictive (P
  picture) and bidirectional predictive (B
  picture).
• I pictures encode for spatial redundancy, P and B
  pictures for temporal redundancy. Because
  adjacent frames in a video stream are often
  well-correlated, P pictures may be 10 of the
  size of I pictures, and B pictures 2 of their
  size.

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• The sequence of different frame types is called
  the Group of Pictures (GOP) structure.
  
• There are many possible structures but a common
  one is 15 frames long, and has the sequence
  I_BB_P_BB_P_BB_P_BB_P_BB_.
  
• The ratio of I, P and B pictures in the GOP
  structure is determined by the nature of the
  video stream and the bandwidth constraints on the
  output stream, although encoding time may also be
  an issue.

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• MPEG 4
• introduced in late 1998,
• the designation for a group of audio and video
  coding standards and related technology agreed
  upon by the ISO/IEC Moving Picture Experts Group
  (MPEG)
• The primary uses for the MPEG-4 standard are web
  (streaming media) and CD distribution,
  conversational (videophone), and broadcast
  television.

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• And thats not even the beginning of codecs
• http//www.fourcc.org/codecs.php

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Container formats

• A container format is a computer file format that
  can contain various types of data, compressed in
  a manner of standardized codecs.
  
• The container file is used to be able to identify
  and interleave the different data types. Simpler
  container formats can contain different types of
  audio codecs, while more advanced container
  formats can support audio, video, subtitles,
  chapters, and meta-data (tags) - along with the
  synchronization information needed to play back
  the various streams together.

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Audio containers

• Some containers are exclusive to audio such as
• WAV (RIFF file format, widely used on Windows
  platform)
  
• AIFF (AIFF file format, widely used on Mac OS
  platform)

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More flexible containers

• Other flexible containers can hold many types of
  audio and video, as well as other media.
  
• The most popular multi-media containers are
• AVI (the obsolete standard Microsoft Windows
  container, also based on RIFF)
  
• MOV (standard QuickTime container)
• MPEG-2 TS (acronym of Transport Stream, standard
  container for digital broadcasting

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• MP4 (standard container for the MPEG-4 multimedia
  portfolio)
  
• Ogg (standard container for Xiph.org codecs)
• ASF (standard container for Microsoft WMA and
  WMV)
  
• RealMedia (standard container for RealVideo and
  RealAudio)
  
• Matroska (not standard for any codec or system,
  but it is an open standard).
  
• 3gp (used by many mobile phones)

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Lesser known containers

• There are many other container formats, such as
  NUT, MPEG, MXF, ratDVD, SVI,and
  
• DivX Media Format. (becoming very well known)

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Enough definitions! One more

• DV_ digital video. Yet another standard.
• a video format launched in 1996, and, in its
  smaller tape form factor MiniDV.
  
• has since become one of the standards for
  consumer and semiprofessional video production.
  
• The DV specification (originally known as the
  Blue Book, current official name IEC 61834)
  defines both the codec and the tape format.
  

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• Features include intraframe compression for
  uncomplicated editing,
  
• a standard interface for transfer to non-linear
  editing systems (FireWire also known as IEEE
  1394)
  
• good video quality, especially compared to
  earlier consumer analog formats such as 8 mm,
  Hi-8 and VHS-C.
  
• DV now enables filmmakers to produce movies
  inexpensively, associated with no-budget cinema.
  

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• DVs Video compression
• DV uses DCT intraframe compression at a fixed
  bitrate of 25 megabits per second (25.146
  Mbit/s), which, when added to the sound data
  (1.536 Mbit/s), the subcode data, error
  detection, and error correction (approx 8.7
  Mbit/s) amounts in all to roughly 36 megabits per
  second (approx 35.382 Mbit/s) or one Gigabyte
  every four minutes.
• At equal bitrates, DV performs somewhat better
  than the older MJPEG codec, and is comparable to
  intraframe MPEG-2. (Note that many MPEG-2
  encoders for real-time acquisition applications
  do not use intraframe compression.)

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• DVs audio compression
• DV allows either 2 digital audio channels
  (usually stereo) at 16 bit resolution and 48 kHz
  sampling rate,
  
• or 4 digital audio channels at 12 bit resolution
  and 32 kHz sampling rate.
  
• For professional or broadcast applications, 48
  kHz is used almost exclusively.
  
• In addition, the DV spec includes the ability to
  record audio at 44.1 kHz (the same sampling rate
  used for CD audio), although in practice this
  option is rarely used.

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• Variations on DV
• Sony's DVCAM is a semiprofessional variant of the
  DV standard that uses the same cassettes as DV
  and MiniDV, but transports the tape 50 faster,
  leading to a higher track width of 15
  micrometres.
• The codec used is the same as DV, but because of
  the greater track width available to the recorder
  the data are much more robust, producing 50 less
  errors known as dropouts.

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• Other Variants include DVCPRo and HDV
• Further reading
• http//en.wikipedia.org/

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TO DO

• Start up Adobe premier pro.
• Begin a new project that is PAL and 48kHz.
• Acquaint yourself with the different areas of the
  interface.
  
• The timeline
• The bin
• Import some video from the samples folder in the
  premier folder into your bin and drop them onto
  the timeline on video 1 track and video 2 track.