Compression makes data "smaller" so more information can be transmitted over
telephone lines. It is a technique to increase the capacity of telephone lines. With
compression, data to be transmitted is made smaller by removing white spaces and redundant
images, as well as by abbreviating the most frequently appearing letters. For example,
with a facsimile, compression removes white spaces from pictures, and only transmits the
images.
Modems use compression to achieve greater throughput, or rates of transmitted information.
When a modem equipped with compression transmits text, repeated words are abbreviated into
smaller codes. For example, since E, T, O and I appear frequently in a text, compression
sends shortened versions of 3 bits rather than 7 or 8 bits. Therefore, a page would
consist of about 1,600 bits instead of 2,200 bits.
If a word processing file was 10 pages long, compression that removed white spaces,
redundant characters and abbreviated characters might compress the document to 7 pages,
which takes less time to transmit. Thus, a modem using compression would be able to send
greater amounts of computer data in a smaller amount of time over analog lines. This
increases throughput. In order for compression to work, the two sites transmitting and
receiving must have matching compression.
Video Compression
Video compression works by transmitting only the changed image and not the same image
repeatedly. In video conferencing, nothing is transmitted until the person being
photographed moves or speaks. Fixed objects such as walls, and background are not
repeatedly transmitted.Another method of video compression is to only transmit part of the
image. The coder, or device doing the compression, knows that discarding minor changes in
images would not distort the viewed image noticeably. Improvements in video compression in
the mid-1980s gave birth to the commercial viability of video conferencing rooms. These
systems made it economical to use video since they required less bandwidth, which means
cheaper telephone lines. Video conferencing became affordable to a wider range of
organizations since they could lease cheaper phone lines as low as $14 per hour instead of
using the traditional T-1 lines at hundreds of dollars per hour.
In most cases, the cheaper lines still had acceptable video quality. New compression
algorithms meant that slower speed digital lines were an acceptable choice for video
meetings. As such, a new industry was created.
Compression Methods
Compression & Multiplexing
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There are various types of
compression methods. Companies such as AT&T, Motorola, PictureTel and Compression Labs
have all designed unique compression schemes using math algorithms.
A Codec (which is an abbreviation for a coder-decoder) encodes text, audio, video or image
using a compression algorithm. For this compression to work, both the sending and
receiving ends must have the same compression method. The sending end looks at the data,
voice or image, then codes it, while the receiving end of the transmitter decodes it.
For devices from multiple manufacturers to operate together, compression standards have
been agreed upon for modems, digital television, video conferencing and other devices.
Digital television compression works much like video compression. It allows pictures to be
transmitted in a highly abbreviated form.
With most video pictures, the image in one frame is similar to that in the previous frame
since the background remains the same while the actors move only slightly from one frame
to the next. Therefore, instead of transmitting the entire image again, a compression
system sends only the parts of the picture that change. Digital compression makes it
possible to represent continuous color-TV signal. This compression squeezes video and
analog signals into small enough units so that studio-quality television can be sent on
standard digital Television channels. The analog standard for television is 525 scan
lines, or lines of images. High definition television (HDTV) will enable a television
screen to display 1125 scanned lines.
A greater number of scan lines results in clearer, studio-quality pictures. Additional
lines of image are seen as a denser higher resolution of detailed images on the screen.
This is done through computer manipulation of the video and audio portions of the
television signals.
Because of the powerful compression and decompression tool used by computers, very little
of the images are lost to the viewer. The quality of digital television is high enough so
that people watching television will perceive the quality to be similar to movies in
theaters.
