Multimedia Representations

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Multimedia Representations
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Goal

• Multimedia hypertext system
• The world-wide web
• Media
• Images, sound, video

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Image Formats

• BMP
• Bitmap
• GIF
• Graphics Interchange Format
• JPEG (JPEG, JPG)
• Joint Photographic Experts Group
• PNG
• Portable Network Graphics

http//www.yourhtmlsource.com/images/fileformats.h
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BMP

• The simplest data to read is 24 bit true color
  images. In this case the image data follows
  immediately after the information header, that
  is, there is no color palette. It consists of
  three bytes per pixel in b,g,r order.

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Problem

• Image files too large for use in timely manner.
• Performance of network technology inadequate.

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Image Compression

• Need ways to capture images in smaller files
  (based on size).
• Issue Can the image be saved then restored
  without loosing information?
• Lossless
• Lossy

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GIF

• Compuserves 8-bit GIF format has long been the
  most popular on the Internet, mainly because of
  its small size. It is ideal for small
  navigational icons and simple diagrams and
  illustrations where accuracy is required, or
  graphics with large blocks of a single color. The
  format is loss-less, meaning it does not get
  blurry or messy.
• GIFs use a simple technique called LZW
  compression to reduce the filesizes of images by
  finding repeated patterns of pixels, but this
  compression never degrades the image quality.

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JPEG

• The 16-bit JPEG format, was designed with
  photographs in mind. It is capable of displaying
  millions of colors at once. JPEGs use a complex
  compression algorithm, which can be applied on a
  sliding scale. Compression is achieved by
  forgetting certain details about the image, which
  the JPEG will then try to fill in later when it
  is being displayed.
• A compression setting of about 60 will result
  in the optimum balance of quality and filesize,
  without letting the compression do too much
  damage.

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PNG

• One version of the format, PNG-8, is similar to
  the GIF format. It can be saved with a maximum of
  256 colors and supports 1-bit transparency.
  Filesizes will be noticeably smaller than the GIF
  counterpart, as PNGs save their color data more
  efficiently.
• PNG-24 is another flavor of PNG, with 24-bit
  color support, allowing ranges of color akin to a
  high color JPEG.

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Intellectual Property
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Sound

• Sound is made when objects vibrate producing
  pressure waves that can be picked up by our ears.
  These waves can be captured when they vibrate the
  membrane of a microphone and can be re-created by
  the amplified vibrations of the membrane in a
  speaker. 

http//www.cortland.edu/flteach/mm-course/sound.ht
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• If we graph the intensity of this wave or of the
  motion of the microphone membrane over time, we
  will get a waveform curve in which the frequency
  of a sound is the number of peaks per second
  (Hertz cycles per second). 
• The distance between two peaks is the
  wavelength. 
• This is an analog signal.

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• To store the signal in a computer, we need to
  represent the analog signal by a digital signal
  which is an approximation.
• A series of numbers measuring samples of the
  height of the analog curve.  The more numbers you
  have (sampling rate in kHz) and the more precise
  those number are (resolution of each sample in
  bits), the more the digital sound will resemble
  the original, but the larger your sound files
  will be.
• A digital waveform can never be completely smooth.

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Bits

• Digital representation of audio CDs uses 16-bits.
• 16-bits allows us to capture 65536 (216) levels.

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MP3

• A CD stores music using 44,100 samples per
  second, 16 bits per sample and two channels (for
  stereo sound). This means that a CD stores about
  10 million bytes (megabytes) of data per minute
  of music on the CD. A three-minute song therefore
  requires 30 megabytes of data.

http//entertainment.howstuffworks.com/question118
.htm
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• MPEG (The Moving Picture Experts Group) has
  developed compression systems used for video
  data. For example, DVD movies, HDTV broadcasts
  and DSS satellite systems use MPEG compression to
  fit video and movie data into smaller spaces. The
  MPEG compression system includes a subsystem to
  compress sound, called MPEG Audio Layer-3. We
  know it by its abbreviation, MP3.
• MP3 can compress a song by a factor of 10 or 12
  and still retain something close to CD quality.

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