Video - A Practical Guide to Video and Audio Compression - page 99

Figure 5-21 Analog waveform for one line of video.

5.10.2 Conversion to Digital Form

Video is converted from analog to digital form by measuring the height of the waveform at
an instant in time and storing that number. The conversion takes place either in the red-
green-blue domain, or the component signals are digitized and the conversion to separated
color is accomplished in the digital domain. Information straight from a camera imaging
charge coupled device ( CCD ) will already be in RGB but will be converted to other formats
within the camera. Computer imaging is normally in RGB, but there are many alternative
color spaces available.

5.10.3 Bandwidth and Sample Rates

Analog bandwidth determines the maximum resolution of the picture in the horizontal
direction. Recording your content onto VHS videotape drastically reduces the resolu-
tion available to you in the horizontal axis. That is because the tape cannot record at
the broadcast bandwidth and the higher-frequency information (the detail) must be
discarded.
The basis for the relationship between bandwidth and horizontal resolution is this:

Bandwidth = scan rate * lines per raster * sample rate

The Nyquist theory indicates that the sample rate must be high enough to take two sam-
ples per cycle of the highest frequency of visible detail.
Some experts argue that significantly more than two samples are necessary to avoid
aliasing artifacts. Refer to Chapter 7 for details of digital audio because it is more relevant
in that area. The same theoretical arguments would apply to video, however. We would
see aliasing artifacts in video as a moving pattern, for example, if a presenter is wearing a
jacket with a particularly bold checkered pattern.