TIMEnet Multimedia

Digital Multimedia

The digital multimedia format has become the default platform for delivering multimedia to large audiences. So, what is digital multimedia?

Multimedia is the integration of various medium used to present data, video/audio recordings, graphs or any visual information to an audience. In the past, the medium to deliver multimedia has been through cables as analogue signals. Simplistically, an analogue signal can be defined as a variation of electrical current. In analogue transmissions the image is scanned electronically and transmitted through cables as variations of electrical intensity. At the receiving end these variations are converted back into an image.

Bits and Bytes

Digital multimedia is the computerized platform for transmitting and displaying the various forms of visual and audio media. Computers break down the media into smaller and smaller representations until each part of it is represented by a series of 0’s and 1’s.

For instance, the character ‘A’ is represented by a string of eight digits (bits) namely ‘01000001’. ‘B’ is represented by the string ‘01000010’ and so forth. The computer allocates a string of eight 0’s and 1’s to represent every character of the alphabet. The string of eight 0’s and 1’s is called a byte.

eight bits = one Byte

So, every character in the alphabet can be represented on the computer by one byte.

… 01000001 … 01000010 …
… A … B …
… byte … byte …

Video Image

In a similar fashion, when the computer scans an image it breaks it down into a series of 0’s and 1’s. This is the only language the computer can process. It processes the image into finite points similar to the dots that make up photographs and words in a newspaper. These finite points are called pixels.

The computer represents these pixels by a series of 0’s and 1’s. The resolution of a computer screen or a hardcopy of a photograph is the number of these pixels or dots in a given space. So if there are 1000 pixels/dots in a space of one inch, the resolution is 1000 dots per inch abbreviated as dpi.

When the computer wants to display a picture on a screen it breaks it down into pixels. It starts at the top corner of the screen, displaying each pixel across the screen and then works its way down in rows until it reaches the bottom of the screen. When it reaches the bottom of the screen it starts the process all over again and again.

The speed at which it refreshes the screen from top to bottom is called the Refresh Rate. This is generally 60 times every second known as 60 Hertz.

In order for the computer to record the changing events on the screen it needs to store this information on a storage device generally this is its hard drive. Information about each pixel has to be recorded so that it can recall the information when requested. Recording every single pixel on a moving picture (frame) would require enormous amounts of storage space. Not only that, but the computer would have to record them at an enormous speed.

Video Compression

To reduce the amount of information that has to be saved various storage methods and algorithms have been devised. One such method is to record only the changes between successive frames. This reduces the amount of information that needs to be saved. On a fast moving video this information is very large and takes up a lot of storage space whilst a scene that has a lot of blue sky with very little movement, the storage requirements are small. On the average, the amount of storage space required dramatically reduced.

The process of reducing the amount of video information is called compression. The reverse process is called decompression. There are many variations to the methods used for compression / decompression currently used for storing digital multimedia. These include AVI, MPEG and WMV.

MPEG Compression

MPEG derives its name from the group that established its structure and configuration. The Moving Picture Experts Group is charged with the development of video and audio encoding standards. Its first meeting was in May of 1988 in Ottawa, Canada.

MPEG is perhaps the current international standard for compression, decompression, processing, and coded representation of video and audio signals. It has the following compression formats:

MPEG-1: This encoding format was the original video and audio compression standard used for Video CD and MP3 audio formatting.
MPEG-2: A video and audio standard used for broadcast-quality television. It is used for over-the-air digital television, digital satellite TV services and digital cable television signals. Today, videos are recorded on DVD utilize this format.
MPEG-3: The original format designed for HDTV, but was abandoned when it was realized that MPEG-2 had sufficient capacity for HDTV.
MPEG-4: This compression standard expands upon the MPEG-1 format to support video and audio content for low bitrate encoding. There are several new higher efficiency video standards included in MPEG4 that are used on HD DVD and Blu-ray discs.

• MPEG-1 is used for video CDs.
• MPEG-2 used for DVDs and Super-VCDs.
• MPEG-4 good for online distribution of large videos and video recorded to flash memory.

Resolution and Bitrate

Resolution: This is the term used to define clarity and sharpness of a display screen. A screen with a resolution of 1024 x 768 has 1024 pixels (dots) across the screen and 768 rows down the screen.

Bitrate: A measure of the speed at which the 0’s and 1’s are transmitted between two points. A bitrate of 2,000 kbps indicates that for every second 2,000,000 (2Mbps) zeroes and ones pass a particular point.
As mentioned earlier, digital video is achieved by streaming a series of ON and OFF electrical pulses (0’s and 1’s) on a cable or by streaming pulses of light through an optical fibre. Digital video distribution is the transmission of one or more media files to one or more viewers.

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