These notes describe some
concepts on the conversion of computer video to TV video signals,
generally performed by a device called ‘encoder’. We apologize to
the experts for some oversimplification here and there. But is is
important, we believe, to have some basic information on the process (of
conversion) and its ingredients to select and use the equipment
properly.
First of all some
agreement on terms. When we use the word "video" we tend to
think of a signal displayed on a "monitor". By far, the most
common monitor is the television monitor, hence by deduction, we
associate "video" with "television" and tend to
think of the two concept or words as equivalent. This is NOT so. There
are in fact two classes of video signals:
-
Computer Video signals
-
Television Video
signals
Converting any one
computer video signal to any one television video signal is the task of
a class of products referred to as scan converters or
"encoders".
For accuracy the term
"encoding" refers (and used to refer exclusively) to the
process of transferring (i.e. encoding) color information on to an
(otherwise) monochrome video signal (whether computer or tv). Usage
makes the law also with words and now ‘encoder’ refers loosely to
those devices that adapt or convert a computer video to a television
video signal. The accurate term for an ‘encoder’ is - and should be
- ‘scan converter’, because the key to conversion is to adapt the
‘scanning’ frequency of the computer video signal to the
‘scanning’ frequency of a television signal. Scan conversion can be
done through software, hardware or a combination of both. A true scan
converter does the conversion all in hardware, the other devices (and
very common) do it by combining hardware and software. We should
describe those other devices as "converters that modify somewhat
the computer video scanning frequency so as to make it compatible with
the television scanning frequency". For brevity we will refer to
these other encoders as "hybrid scan converters" (this is an
unofficial Computer Friends definition).
A true scan converter is
totally independent of the computer or of the computer operating system.
It can also ‘convert’ displays of higher resolutions than the
standard 640 x 480 pixels, for example 800 x 600, 1024 x 768 etc. And it
can work with computers whose vertical scanning frequency is different
from the standard 60 HZ,for example the Macintosh, which has a vertical
scanning frequency of 72 Hz - we will return later to the concepts of
horizontal and vertical scanning frequency. A hybrid scan converter can
only work on PC compatibles.
We just spoke of different
screen resolutions. To avoid misunderstandings, even a 25,000 $ scan
converter will give you, on output, a television signal which will yield
a screen image of (roughly) 640 x 480 screen resolution. What is the
point, you will ask, of converting a higher resolution computer video
signal if I cannot see an equivalent resolution in the output TV signal?
The point is that YOU CAN SEE IT. If you select a 1024 x 768 resolution
on a a hybrid encoder the tv output signal is a blank screen. Why bother
then? Because some programs, for example CAD/CAM and others make use of
the higher computer video screen resolution. For teaching or
demonstrating CAD/CAM on large TV screen or for recording related
training sessions to a VCR you need a true scan converter (if you use
the higher resolution display mode on your computer monitor).
The main trade-offs
between a true and a hybrid scan converters are:
-
price (hybrid scan
converters are really economical)
-
screen resolution (as
above, a hybrid scan converter is limited to converting 640 x 480
(in NTSC, American standard) and 800 x 600 (in PAL, the European
standard). This (640 x 480) is by far the most used and popular
resolution
-
occasional system
dependence (a problem now almost totally overcome). A hybrid encoder
relies for activation on TRS or terminal resident software, provided
with the unit.
-
Broadcast quality
video signal (for TV transmission, including closed circuit TV)
requires a signal with certain characteristics. Of the (4) encoders
we offer, the Hyperconverter has a broadcast quality video signal.
We will offer some
additional information on the characteristics of computer video versus
TV video signals, on flicker reduction etc.
Television
Standards
Here is a table showing
the characteristics of different Television Systems in the world.
|
TV System
|
total lines
|
horizontal freq.
|
vertical freq.
|
color freq.
|
|
NTSC(US)
|
525 lines
|
15.734 kHz
|
59.94 Hz
|
3.579 MHz
|
|
PAL (Europe)
|
625 lines
|
15.625 kHz
|
50.00 Hz
|
4.433 Mhz
|
|
SECAM (France)
|
625 lines
|
15.625 kHz
|
50.00 Hz
|
4.433 Mhz
|
-
NTSC stands for
(National Television Standards Committee)
-
PAL stands for (Phase
Alternate Line)
-
SECAM stands for
("Sequentiell Couleur A’ Memoire)
A bit of historical
levity. The first convention introduced in the world was the NTSC - each
system has trade offs, the trade off in the NTSC system resulted (at the
beginning) in color alterations under different conditions of
transmission. The French came up with their own system and the British
thought of reaching the best trade-off with theirs. In the internal
lingo of TV folks, NTSC stands for Never Twice The Same Color, SECAM
stands for Something Extremely Complicated and Anti-American and PAL
stands for Peace At Last.
Scanning
Rates
A video monitor displays
an image by scanning an electronic gun (3 guns in a color monitor) from
left to right and from top to bottom. This gradually builds an image on
the screen and must be done at least 50 times per second for the human
eye to perceive an unbroken image. TV transmission is now an ‘old’
technology. When it was introduced the standards were established as a
compromise between picture resolution (ability to represent small
detail) and flicker. Note that while it is easy to visualize the
vertical frequency (a beam going from left to right and from top to
bottom 50 times a second), the horizontal frequency is not so intuitive.
The horizontal signal is a modulated wave where the modulation carries
information about the picture and its color. By increasing the vertical
frequency we increase resolution - however, at the same time the
horizontal frequency must also be increased otherwise there would be
flicker. In fact an SVGA signal has a vertical frequency varying from 45
to 72 Hz and a corresponding horizontal frequency of 31.5 KHz. Why
should not the TV signal have the same characteristics of a computer
video signal? Because by increasing these frequencies, we increase the
bandwidth (i.e the portion of the wave spectrum) of the TV signal. There
is only a limited number of frequencies available for TV transmission,
if we wish to transmit more than (1) channel - hence the compromise and
the limitations and the need of converters (encoders). With the advent
of Digital TV in a few years time these limitations will be gone, but
technology, like nature, advances gradually (see the Latin proverb,
Natura non facit saltus", "Nature does not advance by
jumps").
Interlaced
and Non Interlaced Displays
An image can be displayed
in two modes, interlaced and non-interlaced. This refers to the way an
electron beam that creates the image is moved horizontally and
vertically across the screen. A non interlaced image is completely
displayed during one vertical scan of the video monitor. An interlaced
image requires (2) vertical scans. During the first scan, the electron
beam writes the first line, jumps a line and write the next etc. At the
end of the first scan the beam retraces back to the top and writes the
lines that were ‘skipped’ during the first vertical scan. This is a
way to double the resolution without increasing the scanning rate.
TELEVISION IMAGES ARE ALWAYS INTERLACED. Interlacing works with images
of soft edges, without fine detail, that would cause flicker. COMPUTER
VIDEO IMAGES are rarely interlaced.
The unusual shape of color
video waveforms
As indicated earlier,
‘encoding’ refers more properly to the process that makes a waveform
carry color information. Color is reproduced by combining in different
amounts (for each represented point) the individual components of RED,
GREEN and BLUE (RGB). Let’s first consider a TV signal. In a
monochrome TV the screen needs only one signal, that is, brightness or
LUMINANCE to convey the black and white information. In color the (3)
components, red, green and blue are needed. But TV signals are not
transmitted using (3) separate RED, GREEN and BLUE signals. This is
because:
-
The TV transmission
color system has to be compatible with existing black/white set -
that is you should be able to view a TV signal in black/white on a
monochrome monitor and in color on a color TV monitor
-
The need to reduce the
frequency band used by each channel so as to have more channels
available
-
The human eye is less
sensitive to color clarity (chrominance) than to intensity
(LUMINANCE). Hence the color differences can be transmitted in less
detail
To transmit color over the
air waves, TV uses a single signal, formed by adding together:
-
the luminance value
(Y)
-
the color differences,
referred to as (U) and (V)
-
a synchronizing pulse
indicating the beginning of each line and field.
We refer to this TV signal
as a COMPOSITE signal, because it is ‘composed’ of individual,
superimposed signals. As color information is conveyed with less detail,
there is some color blending on smudging. Also the superimposed signals
must be filtered at the receiver. These are some of the reasons why a TV
video signal looks inferior to a computer video signal.
In S-video two signal are
used, one to carry the Luminance value (Y) and the other (C) to carry
the (2) color difference signals (chrominance). S-VHS does not require
filtering and picture quality is significantly improved.
YUV professional video,
uses (3) signals, Y (luminance) and U and V, the color difference
signals. No color decoding or filtering is required and this results in
excellent picture quality.
In RGB (the signal you
typically see on a computer video monitor) (4) signals, Red, Green, Blue
and a sync signal are used to display the image. This is an almost
direct connection to the TV picture tube and picture quality is
excellent.
More
on Converting a Computer Video to a TV Video Signal
We said we offer two types
of converters, one type (hybrid) requiring TRS terminal resident
software and one where the conversion is all in hardware and requires no
software.
The hybrid converter type
uses a memory buffer (FIFO first in first out) to convert one scanning
rate to another. Data is sampled at the input and read back at exactly
half the sampling rate. This works quite well because the VGA horizontal
scanning frequency is approximately double the TV horizontal scanning
rate or frequency, 31.5 kHz versus 15.734 kHz. With 800 x 600 displays
the scan rate is higher and this hybrid system does not work.
The full Scan Converters
(TV SuperScan and Hyperconverter) convert both the scan rates,
horizontal and vertical in hardware. They include a large memory buffer
(more memory, more money). The buffer stores a complete computer image
received at one scanning rate and outputs it at another TV-compatible
scanning rate. Both operations occur simultaneously.
Overscan
and Underscan
A computer image is
underscanned - there is always a black b order around the edge. A TV
picture is always overscanned, i.e. no black border around the edge.
Why? Because in a computer image the borders are used to display menus,
scroll bars etc. which are critical functions or commands for running
the computer. Underscanning ensures that they are not off the edge of
the screen. TV images are made as big as possible so as to fill the
entire screen. When converting computer video images to computer TV
images there will be occasions when the image must be underscanned (for
example a Windows program) and when it should be overscanned (picture of
a person, landscape etc.). With the hybrid converters under/over
scanning is achieved by a software command. In hardware scan converters
the function is performed by actual switches.
Flicker
Reduction
Flicker is inherent to a
TV display due to the limitations in the signals involved and the
effects due to interlacing. If you have one white, pixel-wide,
horizontal line, it will be displayed 25 times per second (refer to the
interlacing diagram). The white line will appear to pulse on and off.
Remember that any interlaced video image (frame) is composed of (2)
fields. One field refers to the odd scan lines that make up the first
half of the frame, the other refers to the even scan lines that make up
the other half of the frame. To reduce flicker the software (and the
hardware) slightly soften the image vertically, thus avoiding a bright
horizontal line to be next to a dark horizontal line, which is the
original cause of the flicker. Flicker reduction works, but we advise
some practice to optimize the use of the flicker reduction function.
Why
not use a video card that has built in a TV signal output?
The reason is simple -
computer video cards evolve more rapidly than does TV broadcast
television. If you lock yourself into a VGA card because of its TV
signal output capabilities, you will probably end up not being able to
use it because you need an upgraded VGA card thus also losing the
conversion to TV function.
