Why back to TV display ?
There are many reasons why there is needs to connect PC to TV:
- Programs like Power Point and Harvard Graphics made it easy for anyone to create good presentations
- Today's TVs are better than old TVs
- TVs are generally bigger than computer monitors and give bigger picture
- Nearly every home and many business meeting rooms have a TV
- Converter technology (both hardware and software) has improved
VGA to TV converters are also needed by computer professionals who make computer animation and computer educational video tapes. The makes for small web browsing computers need techniques for efficent VGA to Tv conversion to be able to use inexpensive standard PC graphics technology on their products.
Does the image look in TV as good as my VGA Monitor?
The answer is no. Once the image is converted, its quality will depend upon the quality of the converter, what video interface is used for connection and the quality of your TV. Generally, the better the TV you have, the better the image will look. TV will never be as sharp as a VGA monitor because the technology is different and converters must do some filtering to the picture to avoid flickering in the TV screen.
Theoretically many scan converter products will display as many colors as your VGA card supports. There are although some converter products in the market which support less than the maximum number of colors. TV composite video signal can't display all colors as well as the VGA card RGB signal, so some colors just don't look so nice in TV screen as they look in VGA screen (especially bright pure colors).
What are the limitations of TV technology ?
TV picture is composed of luminance (brightness) and chrominance (color) signals shoved together with timing information into what is known as a Composite Video signal. There are technical trade-offs in this economy of bandwidth. Composite Video just can't deliver the horizontal resolution or bandwidth that VGA enjoys. Thus your crisp little graph lines, even the outlines of your font appear smeared.
While the 525 lines in a complete NTSC television picture may seem impressive (after all VGA has only 480) there's a catch. Approximately 100 lines are lost to timing information and retracing. Only about 425 lines make it to the screen, and NTSC is not completely clear about which 425. On PAL TV there is 625 lines and 576 of them are visible. The horizonal scanning frequency of TV signal must be exactly fixed (15625 Hz for PAL and 15750 Hz for NTSC). The horizonal frequency of TV signal must be accurately right or the result is smeared colors and syncing problems. One big problem is also TV picture flickering caused by low refresh rate and interlacing.
Almost all TVs overscan the image, placing parts of the picture off the screen, and different manufactures have different ideas on how much of the top, bottom or sides should be kept. Television broadcasters (but not sub-titlers of foreign films) compensate by keeping important information in the central or "safe" area of the screen.
What is a scan converter?
A scan converter converts your VGA or Mac display out to TV or VCR. Using a scan converter, you can display your computer monitor's display simultaneously on a television or record it to video tape. This is particularly convenient in business presentation, as you can hook your laptop to a scan converter unit and display your presentation on a large screen TV. It is also great for building computer program training video tapes. The uses are wide and varied. Remeber, the bottom line is that a scan converter will take whatever is on your computer and put it on a TV or video cassette.
There are many different typies of scan converters on the market and some comverter do this better that other. Typically there are stand alone scan converter which are very easy to use (just plug them to computer and no drivers needed) which are very suitable for busness presentations but quite unsuitable for outputting quality computer animation. Some converters are more optimized for good quality animation output and gaming.
Will the scan converter fit the whole computer image on the TV?
Since TV and VGA are different technologies, their screen resolutions don't match exactly. A VGA resolution of 640 x 480 will cause "overscanning" on the NTSC TV. On hte other hard 640x480 resolution does not fill the whole PAL TV screen and 800x600 will overscan on PAL TV. This means that a few lines will go off the visible edges of the TV screen resulting in either the top or bottom of the image being cut off.
What converter must do ?
VGA to TV converters have to do quite a lot of thing to be able to show VGA monitor picture on TV screen:
- Match VGA horizonal sync frequency to TV sync frequency
- Match the VGA display frame rate (60 or 70 Hz) to TV freame rate (50 or 60 Hz)
- Convert non-interlaced VGA picture to interlace format for TV
- Convert VGA picture signals to RGB+Sync, PAL/NTSC composite video or S-video formats
Conversion is not very easy task. Currently there are three techniques used for the conversion. Each of the techniques have their advantages and disadvantages. You have to carefully think what features you need in your application and then select product type which matches your needs.
Converter connectors
There are lots of ways to connect VGA to TV converters to TV. The best picture can be achieved using direct RGB connection in SCART interface available on almost all modern European PAL and SECAM TVs. The next best interface is is S-video (also called S-VHS and Y/C connector), which is available in modern high end televisions. Then comes composite video interface and the worst one is RF connection.
If you are planning to save your graphics to consumer VCR the only possible selections are composite video and RF connection (S-video is also possible if you have S-VHS video). The selection of interface does not have much effect on consumer VHS video picture quality, because the picture quality of VHS VCR is very poor compared to bradcast quality TV picture.
Connector types and their characteristics
Composite video
- Practically every VGA to TV converter has this connector
- Can be connected to any VCR
- Can be connected to modern TVs with composite video input or SCART AV connector
- Connector is typically RCA or BNC connector
- Better picture quality than RF connection
RF connector
- Every consumer TV and VCR has this input
- The picture quality is not very good because cheap RF modulators in converters
- Connector is TV antenna connector
- You have to tune the TV to the channel the connector outputs
S-video connector
- Connector used in S-VHS and Hi-8 VCRs
- High end TVs have this connector
- Quite many VGA to TV converter have also this connector
- Better colors and picture details than composite video connector
SCART RGB connector
- Can be used only with TVs that have SCART connector
- Best picture quality you can get out of TV, no color smearing
- Connector is 21 pin SCART connector
- Only quite few converters have SCART RGB output
SCART AV connector is a standard 21 pin AV connector which cna be found from practically all modern TVs and VCRs made for European market. SCART connector is a standard interface for combining audio, composite video, RGB video and some control signals to one connector. The connector standardizes the pinouts and signal levels, but does not demand that all equipments have to impelemt every fucntion. Typically modern TVs implement every function, but VCRs use only composite video and audio I/O of the connector. You can find technical details of SCART AV connector from http://www.epanorama.net/counter.php?url=http://www.bbc.co.uk/aberdeen/eng_info/scart_connector.html.
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