| Compression makes video data
capable of being stored, edited, played and streamed at a
reasonable rate, and acceptable picture and sound quality.
Video is basically a three-dimensional array of color pixels.
Two dimensions serve as spatial (horizontal and vertical)
directions of the moving pictures, and one dimension represents
the time domain.
Digital video sequencing results in extremely high data rates:
A television image with a resolution of 720 x 576 pixels
and a color depth of 16 bits produces a data stream of 1.35
MB per frame.
A rate of 25 frames per second is required to give smooth
video, at a data volume of 33.75 MB/s.
Most hard drives could not save this data stream. A CD could
only hold about 16 seconds of video. Video signals are therefore
compressed to remove or reorganize data; thereby reducing
the size of digital files.
Video compression typically operates on square-shaped groups
of neighboring pixels. These pixel groups or blocks of pixels
are compared from one frame to the next and the video compression
codec (encode-decode scheme) sends only the differences within
those blocks.
There are two methods of video compression:
- loss-free compression
- lossy compression.
Lossless Compression
Lossless compression retains the original data image sequences
after compression. Most lossless compression methods use run
length encoding to remove areas of the image that use the
same color.
Lossless compression rate is less than 3:1, depending on
the complexity of individual images, so are not widely adopted.
With lossless compression there is no readily discernible
difference between the original and the compressed data. Thus,
no loss in quality.
Lossless techniques are technically quite demanding and require
expensive equipment.
Lossly Compression
Lossy compression methods optimizes the data stream by removing
image information unlikely to be noticed by the viewer. The
original data and some image information is lost; the volume
depending on the degree of compression.
Most video compression is lossy. For example, DVDs use a
video coding standard called MPEG-2 that can compress ~2 hours
of video data by 15 to 30 times while still producing a high
quality standard-definition video.
Video compression, like data compression, is a tradeoff between
disk space, video quality and the cost of hardware required
to decompress the video in a reasonable time. Once excessive
video compression compromises image quality, it is difficult
or impossible to restore the original image quality.
Compression Standards
MPEG-2 and MPEG-4 Compression
Both MPEG-2 and MPEG-4 are popular and sophisticated compression
techniques.
Most consumer camcorders use a 4:1:1 format (such as DV,
DVCAM, DVCPRO, etc.), which depend on a greater level of quality-sacrificing
compression. Most people will not notice the loss in quality.
Two problems with MPEG2 and 4:
- Since MPEG compressed video relies on data in adjacent
frames, if the video is stopped on a particular frame —
as would be the case in editing — critical data will
be missing.
- MPEG data must be stored for brief moments in order for
the system to evaluate a sequence of frames, causing a slight
delay in processing the video. This can confuse video editing
equipment.
High compression ratios [30:1] are possible without degredation,
these editing limitations mean video editors prefer JPEG compression.
JPEG and JPEG 2000 Compression
Using JPEG Compression, each video frame is complete, meaning
frame accurate editing is possible.
JPEG and the newer JPEG 2000 compression compress in the
range of 15 to 20 to 1. The amount of compression, is determined
both by the user and the content of the images.
JPEG compression schemes use scalable [highly sophisticated
mathematical] approach to eliminate nonessential picture elements.
Editors can choose from a range of compression ratios. You
can find more information on JPEG and JPEG 2000 on the Joint
Photographic Experts Group, JPEG.org
website.
Compression Ratios
500 bits of data compressed to 50 bits, gives a 10:1 compression
ratio and no noticeable loss of quality.
At 20:1 ]depending on the subject matter], you still have
an excellent picture; only the trained eye will notice a slight
loss in quality.
At 50:1 and beyond, there is a noticeable and unacceptable
difference between the original picture and the compressed
version.
Top-of-the-line digital camcorders use a "no compromise"
digital 4:2:2 format.
- The first number refers to the luminance or black and
white part of the video and is an expression of the component-sampling
ratio (not the compression ratio).
- The second two numbers refer to the sampling ratio of
the color components of the video signal.
4:4:4 is a pure, uncompromised signal;
4:2:2 represents minimal and unnoticed compression; and
4:1:1, which is associated with DV camcorders, involves significant
signal compression
Data compression is necessary because today's equipment cannot
easily handle the high-speed data steams associated with digital
video.
As data storage becomes cheaper and more compact, and computer
chips become faster, uncompressed audio and video is predicted.
NEXT: Video Editing
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