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Multimedia Data Basics
Multimedia systems/applications have to deal with the
- Generation of data,
- Manipulation of data,
- Storage of data,
- Presentation of data, and
- Communication of information/data Lets consider some broad implications of the above
Discrete v Continuous Media RECALL: Our Definition of Multimedia
- All data must be in the form of digital information.
- The data may be in a variety of formats:
- text,
- graphics,
- images,
- audio,
- video.
Static and Continuous Media
- Static or Discrete Media — Some media is time independent: Normal data, text, single images, graphics are examples.
- Continuous media — Time dependent Media: Video, animation and audio are examples.
Analog and Digital Signal Conversion
The world we sense is full of analog signals:
- Electrical sensors convert the medium they sense into electrical signals
- E.g. transducers, thermocouples, microphones.
- (usually) continuous signals
- Analog: continuous signals must be converted or digitised for computer processing.
- Digital: discrete digital signals that computer can readily deal with.
Digital-to-Analog Converter (DAC)
- Playback – a converse operation to Analog-to-Digital
- Takes digital signal, possible after modification by computer (e.g. volume change, equalisation)
- Outputs an analog signal that may be played by analog output device (e.g. loudspeaker, CRT display)
Analog-to-Digital-to-Analog Pipeline (1)
Begins at the conversion from the analog input and ends at the conversion from the output of the processing system to the analog output as shown:
(Strictly Analog-to-Digital-to-Analog is within the dotted box)
Multimedia Data: Input and format How to capture and store each Media format? Note that text and graphics (and some images) are mainly generated directly by computer/device (e.g. drawing/painting programs) and do not require digitising: They are generated directly in some (usually binary) format.
- Printed text and some handwritten text can be scanned via Optical Character Recognition
- Handwritten text could also be digitised by electronic pen sensing
- Printed imagery/graphics can be flatbed scanned directly to image formats.
Text and Static Data
- Source: keyboard, speech input, optical character recognition, data stored on disk.
- Stored and input character by character:
- Storage: 1 byte per character (text or format character), e.g. ASCII.
- For other forms of data (e.g. Spreadsheet files). May store as text (with formatting) or may use binary encoding.
Graphics
• Format: constructed by the composition
of primitive objects such as lines, polygons,
circles, curves and arcs.
• Input: Graphics are usually generated by
a graphics editor program (e.g. Illustrator,
Freehand) or automatically by a program
(e.g. Postscript).
Graphics (cont.)
- Graphics input devices: keyboard (for text and cursor control), mouse, trackball or graphics tablet.
- Graphics are usually selectable and editable or revisable (unlike images).
- Graphics standards : OpenGL.
- Graphics files usually store the primitive assembly
- Do not take up a very high storage overhead.
Images (cont.)
- Input: scanned for photographs or pictures using a digital scanner or from a digital camera.
- Input: May also be generated by programs similar to graphics or animation programs.
- Analog sources will require digitising.
- Stored at 1 bit per pixel (Black and White), 8 Bits per pixel (Grey Scale, Colour Map) or 24 Bits per pixel (True Colour)
- Size: a 512x512 Grey scale image takes up 1/4 Mb, a 512x512 24 bit image takes 3/4 Mb with no compression.
- This overhead soon increases with image size — modern high digital camera 10+ Megapixels 29Mb uncompressed! B. Okuku^16
Images (cont.)
- Can usually only edit individual or groups of
pixels in an image editing application, e.g.
photoshop.
Video
- Input: Analog Video is usually captured by a
video camera and then digitised, although
digital video cameras now essentially perform
both tasks.
- There are a variety of video (analog and digital)
formats
- Raw video can be regarded as being a series of
single images.
- There are typically 25, 30 or 50 frames per
second.
Video (cont)
Video Size:
- A 512x512 size monochrome video images take 25*0.25 = 6.25Mb for a minute to store uncompressed.
- Typical PAL digital video (720 576 pixels per colour frame) 1:2 25 = 30Mb for a minute to store uncompressed.
- High Definition DVD (14401080 = 1.5 Megapixels per frame) 4:5 25 = 112.5Mb for a minute to store uncompressed. (There are higher possible frame rates!)
- Digital video clearly needs to be compressed.
