|
||||||
|
Video Transmission Standards
Video is simply a sequence of images with the possible inclusion of the corresponding audio. We will at a later time talk about digital video formats and compression standards. Here we concentrate on the video transmission formats. This is what your TV set uses.How video is displayed on your TV
Your conventional TV is basically an analog device. To form an image some sort of gun is swept back and forth and up and down across the screen. For color there is usually three guns, one for Red, one for Green and one for Blue.What is the pattern of tracing that takes place?
NTSC(National Television Standards Committee)
This is one of the commonly used, currently in use in the United States and Japan, video standards that addresses the pattern of scanning, the number of rows, columns, and the timing involved. NTSC was devised in 1953. Besides transmitting this analog signal, it can be recorded on tapes such as VHS and Betacam.NTSC countries are: USA, Antigua, Bahamas, Barbados, Belize, Bermuda,
Bolivia, Burma, Canada, Chile, Colombia, Costa Rica, Cuba, Dominican Republic,
Ecuador, El Salvador, Greenland, Guam, Guatemala, Guyana, Honduras, Jamaica,
Japan, South Korea, Mexico, Netherlands Antilles, Nicaragua, Panama, Peru,
Philippines, Puerto Rico, St. Vincent & the Grenadines, St. Kitts, Saipan,
Samoa, Surinam, Taiwan, Tobago, Trinidad, Venezuela, Virgin Islands.
The NTSC standard is incompatible with most computer video standards, which
generally use RGB video signals. However, you can insert special video adapters
into your computer that convert NTSC signals into computer video signals and
vice versa.
Features:
- Interlaced versus Non-Interlaced: Non-Interlaced means that one row of data after another is traced out. Interlaced video means first the even rows are completely drawn and then the odd rows are drawn. The odd or even set of rows are refered to as fields of the image. An advantage to Interlaced video is that you can start to see what the image will look like before it is completely drawn quicker than in a Non-Interlaced version.
- Number of rows/lines of video 525.
- Number of Frames per second. 30. (hence 60 fields per second).
- Number of visible rows/lines of video 488. The remaining lines are used for vertical blanking which is the return of the gun back to the top of the image. Some of these lines can be used to store cueing, testing and other information.
PAL and SECAM
PAL is the video standard used by many countries in Europe. SECAM is the standard in France.PAL stands for Phase Alternation by Line, and was first used in 1967. An
advantage of this system is a more stable and consistent hue (tint). (PAL-M
is used only in Brazil. It has 525 lines, at 30 frames per second. ) PAL
countries include: Afghanistan, Algeria, Argentina (PAL-N), Australia, Austria,
Bahrain, Bangladesh, Belgium, Brunei, Cameroon, Canary Islands, China, Cyprus,
Denmark, Finland, Germany, Ghana, Gibralter, Greece (also SECAM), Hong Kong,
Iceland, India, Indonesia, Ireland, Israel, Italy, Jordan, Kenya, North Korea,
Kuwait, Liberia, Luxembourg (also SECAM), Madeira, New Zealand, Nigeria, Norway,
Oman, Pakistan, Paraguay (PAL-N), Portugal, Qatar, Saudi Arabia (also SECAM),
Siera Leone, Singapore, South Africa, Spain, Sri Lanka, Sudan, Swaziland,
Tanzania, Thailand, Turkey, Uganda, United Arab Emirates, United Kingdom,
Uruguay (PAL-N), Yeman (the former Yeman Arab Republic was PAL, and the former
People's Democratic Republic of Yeman was NTSC ), Yugoslavia, Zambia, Zimbabwe.
The above was gathered from Web searches and may not reflect the current
situation.
SECAM stands for Systeme Electronique Couleur Avec Memoire, which was first
used in 1967 and developed by the French. Countries include: Albania, Benin,
Bulgaria, Congo, former Czechosolvakia, Djibouti, Egypt, France, French Guiana,
Gabon, Greece (also PAL), Guadeloupe, Haiti, Hungary, Iran, Iraq, Ivory Coast,
Lebanon, Libya, Luxembourg (also PAL), Madagascar, Martinique, Mauritius, Monaco
(also PAL), Mongolia, Morocco, New Caledonia, Niger, Poland, Reunion, Romania,
Saudi Arabia (also PAL), Senegal, Syria, Tahiti, Togo, Tunisia, former USSR,
Viet Nam, Zaire. The above was gathered from Web searches and may not reflect
the current situation.
PAL and SECAM Features
- Number of rows/lines of video 625.
- Number of Frames per second. 25. (hence 50 fields per second).
- Number of visible rows/lines of video ??? The remaining lines are used for vertical blanking which is the return of the gun back to the top of the image. Some of these lines can be used to store cueing, testing and other information.
HDTV
HDTV stands for High Definition Television. This is a newer standard that will allow for greater resolution (larger number of lines and greater sampling of the lines). Just like your TV can not recieve PAL or SECAM signals, it will not be able to accept HDTV...so, get ready to buy a new TV! There are a number of competing HDTV standards. They all roughly double the width of the screen. For example, one resolution used is 1920 pixels x 1080 lines. To pump this additional data through the narrow TV channels, images are digitized and then compressed before they are transmitted and then decompressed when they reach the TV.
Some Links to check out:
A Comparison
|
Standard |
HDTV | NTSC |
PAL |
SECAM |
| Property |
||||
| images / second |
25 to 30 | 29.97 |
25 |
25 |
| lines / image |
1080 | 525 |
625 |
625 |
| (horiz./vert.) = aspect ratio |
16:9 | 4:3 |
4:3 |
4:3 |
| interlace |
2:1 also progressive, non-interlace mode |
2:1 |
2:1 |
2:1 |
In an interlaced image the odd numbered lines (1,3,5,...) are scanned in half of the allotted time (e.g. 20 ms in PAL) and the even numbered lines (2,4,6,...) are scanned in the remaining half. The image display must be coordinated with this scanning format. (See Section 8.2.) The reason for interlacing the scan lines of a video image is to reduce the perception of flicker in a displayed image. If one is planning to use images that have been scanned from an interlaced video source, it is important to know if the two half-images have been appropriately "shuffled" by the digitization hardware or if that should be implemented in software. Further, the analysis of moving objects requires special care with interlaced video to avoid "zigzag" edges.
The number of rows (N) from a video source generally corresponds
one-to-one with lines in the video image. The number of columns, however,
depends on the nature of the electronics that is used to digitize the image.
Different frame grabbers for the same video camera might produce M = 384,
512, or 768 columns (pixels) per line.
Kinds of Signals
- Composite = Contains both timing and Image Information on one signal. An example is given by the RS-170 Sspecification. This is typically used by TVs, and VCRs.
- Non-Composite = Seperate signals are given (and hence need a physical cable for each signal) for the greyscale or color information and the timing information. This is sometimes used by Industrial Cameras, etc. (Our lab is an example of this). Note that the synchronization of these signals is critical.