What interface to use to connect VGA to TV converter to TV

VGA to TV converters typically come with many different video output connectors. The most typical connectors are S-video and composite video connectors.

Tips for choosing the best connector to use

Cable lengths in S-video connections

Cable (SVIDEO and RCA) lengths up to 6 meters are not unreasonable and the signal quality should not be adversely affected. Keep in mind that a LONGER CABLE will incur more signal degradation. Quality of the cable and connectors can also contribute to signal degradation, so if you plan to use long cables get good cables (contact a local AUDIO/VIDEO/HOME THEATRE outlet for professional advice).

Connector typies

The video outputs in computer to TV products have typically S-video and composite video outputs for Tv connection. S-video output is typically 4-pin minidin connector and composite video output typically uses RCA connector, although professional equipments quite often use BNC connectors for composite video output. TVs typically use those same connectors for thei signal inputs, so in most cases interfacing does not cause any problems.

If the cables which come with the card do not fit to your TV or other equipments then it is best to contact a local AUDIO/VIDEO outlet for professional advice on getting the right cables and/or connector adapters.

RF modulator

AN RF MODULATOR transforms a COMPOSITE VIDEO signal and possibly AUDIO into a STANDARD broadcast type signal (typically on CHANNEL 3 or 4). This comversion allows the video signal to be viewed with any TV which has antenna connector. The OUTPUT of the RF MODULATOR (for North American NTSC) is a standard "F" connector and European modulators typically use IEC antenna connectors. Those connectors are the same as used in your TV and can be wired to your TV antenna connector directly.

Normal 75 ohm CO-AXIAL cable can then be used to deliver the signal to a TV set (tuned to channel 3 or 4) up to around 30 meters distance. This method is probably the least expensive way to deliver the video signal when long cable runs are required.

RGB connections

Some professional VGA to TV converters have also an RGB output to interface the converter to TVs or professional video monitors/projectors. RGB interface provides superior picture quality compared to any other interface, but it is more problematic than those other interfaces. RGB interface is not found in many equipments and you need a special cable between your equipment and the VGA to TV converter.

Why there are differences between different connections in picture quality ?

The picture quality depends on how much signal conversion is done on the way from the computer screen to the TV screen. Less conversion gives better picture quality, because the conversions are lossless, so every time a conversion is done some picture details are lost.

Composite video connection

In composite video connections all RGB components are mixed to one video signal and the extracter form it using the following block diagram:

--------------------------------------------------------------------------
 Converter    |     Cable       |                      T.V               |
--------------------------------------------------------------------------
                                -----------------           -----------
---R(ed)-----}|\                |               |--chroma--}|         |
---G(reen)---}|----composite---}|T.V comb filter|           | Picture |
---B(lue)----}|/                |               |---luma---}|         |
                                -----------------           -----------
Converter has to encode the RGB image into a single signal. That signal is sent to the Television, where a device called a COMB FILTER in the TV separates the composite signal into Chroma & Luma. Because there is "separation" AND "encoding" involved, a major loss of reproduction quality is observed. T.V. with a digital comb filter will look significantly better if you're using composite, but are not a perfect solution.

S-video conversion

S-Video is "essentially" the same as Chroma & Luma, Brightness & Color, or y/c. They all mean the same thing. A Television set needs a signal in the form of brightness & color, or Chroma & Luma, to display a picture. You want to keep encoding/decoding to a minimum and retain the original signal as much as possible without modification.

The VGA to TV converter has to encode the RGB image into two separate signals, Chroma and Luma. The whole process is done as described in picture below:

--------------------------------------------------------------------------
 Converter    |    Cable       |                      T.V               |
--------------------------------------------------------------------------
                                -----------------           -----------
---R(ed)-----}|------chroma----}|               |--chroma--}|         |
---G(reen)---}|                 |T.V comb filter|           | Picture |
---B(lue)----}|-------luma-----}|   (not used)  |---luma---}|         |
                                -----------------           -----------
The only loss in quality comes from the "encoding" of RGB -> y/c, and this results in a minor loss in reproduction quality. Note how the comb filter is NOT used for S-Video. This means that T.V's with analog comb filters look just as good as those with more expensive digital comb filters (ie Sony XBRs) IF you're using S-Video.

RGB connections

RGB signal is a direct feed frtom VGA to Tv converter to the TV picture driving electronics.

------------------------------------------------------------
  Converter   |     Cable       |        Monitor           |
------------------------------------------------------------
                                       -----------
---R(ed)-----}|----R(ed)--------------}|         |
---G(reen)---}|----G(reen)------------}| Picture |
---B(lue)----}|----B(lue)-------------}|         |
                                       -----------
As you can see no unnecessary conversions between different video formats is done on the way. Usually there are not these bandwidth limitations is RGB signals like in composote video, so the picture signal can be much sharper those TV broadcasts and have extremely good colors in it.

In europe RGB interface is very common in TVs which have SCART A/V connector in them. Typically all TVs with SCART connector have RGB input capability and there are only quite few exceptions to this rule.

In USA it is quite hard to find TV wit RGB input. Years ago some very high end DirectView and Rear Projection T.V's (r/p's for short) had BNC inputs, but manufacturers realized that this was a waste even on high-end T.V.'s, since less than 1% of the population who bought them were using it. The other 99% would probably want to save the extra cash by dumping the BNC input. And so this is what happened. Today, only some Front Projectors (f/p's for short) have BNC inputs. So in USA you are limited to expensive Front Projector ($5000+) TV, tiny Commodore Amiga monitor, hard to get Arcade RGB monitor or special professional monitor TV.

Information sources


Tomi Engdahl <Tomi.Engdahl@iki.fi>