X6

MPEG glossary 371
Bit rate
MPEG is a format used for storage and transferring. With older formats (e.g. AVI) you
could predict that 20 seconds of movie would result in 20 MB of data. The file size is
this a direct measurement of quality.
This is different for MPEG: The amount of data available can be used differently for
different display modes. 20 MB can be 4 seconds of DVD Video or 5 minutes Internet
streaming in thumbnail format. The quality of an MPEG video is measured by the
width of the created data stream, the bit rate. This is the amount of the transmitted
data per time unit; it is stated in kBit/s or bit per second.
Bits, not bytes are used, since the data word width has to address the transmission
restrictions.
The file size can be calculated from the average bit rate, if its length is known:
F = (BRV + BRA) * t
F=File size BRV=
Video bit rate
BRA=
Audio bit rate
t=Length in s
Block
For almost all image file editing techniques the image is subdivided into 8 x 8 pixel
blocks (image points). This should be noted if you would like to used user-defined
image resolutions (width/height), and they should always be a multiple of 8.
Chroma format
The color value of each image point consists of the color values for the primary colors
red, green, and blue (RGB), and for traditional and technical reasons it is transformed
into one brightness value (Y = 0.299*R = 0.587*G + 0.114*B) and two color
difference values (U = R - Y, V = G -Y).
The Y value alone produces the black and white picture. These signal components
allow brightness and color information to be handled separately. The first data
reduction occurs when single rows comprising a picture are read. Because the human
eye has a lower color resolution than a brightness resolution, the color components
are recorded only for every other point of a row (4:2:2) for each four pixels grouped
(4:1:0), i.e. color signal under-reading.
4:2:2 This corresponds to the established TV standard. One piece of color information
is transmitted per row for two pixels which corresponds to a 2/3 compression of the
output data.