Introducing digital video 3
Note that when the hard drive is used to time shift or record, it simply stores
the MPEG bitstream. On playback the bitstream is decoded and the picture
quality will be unimpaired. The generation loss due to using an analog VCR is
eliminated.
Ultimately digital technology will change the nature of television broadcasting
out of recognition. Once the viewer has non-linear storage technology and
electronic program guides, the traditional broadcaster’s transmitted schedule is
irrelevant. Increasingly viewers will be able to choose what is watched and when,
rather than the broadcaster deciding for them. The broadcasting of conventional
commercials will cease to be effective when viewers have the technology to skip
them. Anyone with a web site which can stream video can become a
broadcaster.
1.2 What is a video signal?
When a two-dimensional image changes with time the basic information is three-
dimensional. An analog electrical waveform is two-dimensional in that it carries
a voltage changing with respect to time. In order to convey three-dimensional
picture information down a two-dimensional channel it is necessary to resort to
scanning. Instead of attempting to convey the brightness of all parts of a picture
at once, scanning conveys the brightness of a single point which moves with
time.
The scanning process converts spatial resolution on the image into the
temporal frequency domain. The higher the resolution of the image, the more
lines are necessary to resolve the vertical detail. The line rate is increased along
with the number of cycles of modulation which need to be carried in each line.
If the frame rate remains constant, the bandwidth goes up as the square of the
resolution.
In an analog system, the video waveform is conveyed by some infinite
variation of a continuous parameter such as the voltage on a wire or the strength
or frequency of flux on a tape. In a recorder, distance along the medium is a
further, continuous, analog of time. It does not matter at what point a recording
is examined along its length, a value will be found for the recorded signal. That
value can itself change with infinite resolution within the physical limits of the
system.
Those characteristics are the main weakness of analog signals. Within the
allowable bandwidth, any waveform is valid. If the speed of the medium is not
constant, one valid waveform is changed into another valid waveform; a
timebase error cannot be detected in an analog system. In addition, a voltage
error simply changes one valid voltage into another; noise cannot be detected in
an analog system. Noise might be suspected, but how is one to know what
proportion of the received signal is noise and what is the original? If the transfer
function of a system is not linear, distortion results, but the distorted waveforms
are still valid; an analog system cannot detect distortion. Again distortion might
be suspected, but it is impossible to tell how much of the energy at a given
frequency is due to the distortion and how much was actually present in the
original signal.
It is a characteristic of analog systems that degradations cannot be separated
from the original signal, so nothing can be done about them. At the end of a
system a signal carries the sum of all degradations introduced at each stage