10
Compression Process
Compression of Video is accomplished via the following process.
1. Selective Sampling
2. Discrete Cosine Transform (DCT)
3. Predictive & Motion Encoding
4. Hoffman Encoding
Selective Sampling
The number of times per second that you sample a signal is called its
sampling frequency. In Audio, the sampling frequency is 44,000 samples/
sec which is approx 2 times the highest frequency in Audio (20,000 Hz).
In contrast, Video is sampled at 4 times the highest frequency (13.5Mhz).
Hence the term 4 in the sampling structure 4:4:4 representing the sam-
pling ratio of Y, R-Y and B-Y respectively.
Therefore, the same number of samples are taken of Y as they are of R-
Y and B-Y.
Time
Signal
4:2:2
Since the eye is less perceptive to color changes than to Luminance,
significant reduction of data can be accomplished in the Sampling
process if we sample the color components (R-Y and B-Y) half as
much as the Luminance component (Y). The result is a 1/3 reduction
or a bandwidth of 166Mbits/sec.
Video sampled using this technique is represented by a 4:2:2 sampling
structure. Y is sampled normally with R-Y and B-Y sampled half as much.
LINE 1
LINE 2
.
.
.
.
LINE 480
ONE
PIXEL
Samples taken in the Horizontal direction up to 720 per Line
Non Sampled components Illustrated as clear boxes
R-Y
B-Y
Y
Y
R-Y
B-Y
Y
Y
R-Y
B-Y
Y
Y
R-Y
B-Y
Y
Y
4:1:1
Consequently techniques such as the 4:1:1 further reduce color sam-
pling to 1/4
th
of the Y component and compress by ½ or reduce band-
width to 125Mbits/sec.
Содержание DTV-01
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