262a
The data to create an HD signal runs about 1.5 Gbps (gigabits
per second), too much for affordable VCRs to handle. Some method
of compression is necessary to reduce the amount of data in a
picture. As more compression is used, the VCR expense and tape
expense both go down, but also image quality goes down. Since
the whole point of high definition is sharp pictures, it behooves
us to compress the data the least amount possible. The method
of data compression commonly used by VCRs is DCT (Discrete Cosine
Transform), a system that attempts to throw away picture detail
that your eye wouldn't see anyway. Compression ratios below 5:1
show very little loss. Compression systems break down a bit when
there is a high degree of motion or detail in the image; there
is so much data that the system must throw away even more picture
detail to stay within its design limitations. So already you can
see that one can't describe picture sharpness as a static thing;
it changes, depending upon the content of the picture.
DCT can be used for intraframe compression (reducing the data
for each video frame independent of the others) or can be applied
to interframe compression, where a group of video frames are compared
to each other and redundant data tossed out. MPEG-2 is an example
of this. MPEG can work at different levels. At its highest compression
level, it could record one "real" frame (called an I
frame) and manufacture seven other frames from it (called P and
B frames) which contain only data about what changes occurred
from frame to frame, not the data that creates the whole image
itself. This high compression MPEG-2 would be used on home and
office VCRs designed to record DTV broadcasts. For true production
work, one must use the low compression MPEG-2 compression scheme
that uses only I-frames (only "real" frames). Most VCRs
use only DCT compression while others use MPEG-2, a subset of
DCT. (Sony uses MPEG-2 on its Betacam-SX for HD).
It is hard to discuss picture resolution when compression is involved.
One could sample an image many times, creating a lot of data to
describe the image accurately, then compress the heck out of it
to end up with a fuzzy, artifact-laden image. One the other hand,
one could sample the image less, making an image with less data
(which is theoretically fuzzier) then compress the image less,
preserving the data, leaving it technically sharper than the case
mentioned above. Thus, picture sharpness (and indeed cost) is
a tradeoff between the number of pixels in an image (i.e. a camers's
CCD chip) that determines the initial image sharpness, and the
amount of compression necessary to squeeze the data onto the video
tape.
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