This article can be found in Russian language at nvworld.ru.

11.09.02 - Current supplementation:

The test results of the Philips chips cover the SAA7102 and 7108, but not the 7104 and the 7108 'A'. Currently there are first tests running and it turns out, that the problems described below are not present anymore with this chip. The test will be completed with these chips soon.

Thinking about buying a new graphics card? Then you might also think about which TV chip should be used on it. In the FAQ you can read that the CX25871 is recommended. But is it really the 1st choice? Which pros and cons do the different TV chips have? Are there special bugs with certain TV encoders? This article tries to throw a bit light on these questions.

First, which TV chips are availabe at all? On nVidia based cards you can find encoders from Chrontel, Conexant and since a while from Philips, too. On GF4-MX cards you will also find the chip which introduces itself in the display properties as NV17. This is a development from nVidia. It´s not an external chip on the graphics card board like the other ones, but it´s integrated in the VGA chip directly. Eventually there will be a special test of this TV encoder shortly. It is very likely that the main purpose of this encoder is to be a cost effective, or just cheap solution. One can consider that the qualities and the number of features cannot compete to the other external chips. Note: The NV17 will not be supported by TVTool because of a complete lack of documentation by the manufacturer. In the following the pros and cons of the different external TV chips will be explained a bit.

NV17 in display properties

Philips

Philips offers the models SAA7102, 7104 and 7108. The 7108 is actually a 7102 which was complemented by a video decoder, in other words a video input. The 7104 is an advancement of the 7102 and can process resolutions up to 1280x1024 in contrast to the maximum of 800x600 of the 7102. But attention: This does not mean that you can use this feature, because it´s not supported by software. The 7104 has no video input. Principially the Philips chip can be programmed flexible, so different resolutions and pictures sizes could be created. But this is theory. Because there is no sufficient documentation available most features can´t be realized (at least not at the moment).

color shades SAA7102	other tv chips

Pros:

• A good text representation especially in desktop size. Therefore the chip is very suitable for presentations where the the readability of texts is more important and foto reproduction can be neglected
• It has a video input (7108 only)

Cons:

• Coarse color shades in PAL and NTSC. In contrast to Chrontel and Conexant chips the TV encoder from Philips show significant coarse color shades, which attracts attention especially when watching DivX movies, but also DVDs (s. ill). This effect seems to differ from sample to sample. With some samples you think a special video filter is enabled (solarize), with others you can hardly notice it. So this chip is not very suitable for the playback of movies and photos! (The 7104 might not have this problem, but it´s not tested 100% yet.)
• 800x600 only. So far only this resolution is supported by TVTool and for the time being there is no improvment in sight. The problem is the insufficient documentation. Beyond it Philips shows no interest for further support. So the important resolutions for 768x576 and 720x480 can´t be implemented.
• Wide Screen signal for PAL only, NTSC is not supported (by hardware).
• 7104: Is not supported correctly by TVTool. Here we have the same reason like ever, the lack of documentation.

Often you can hear that one had to reach for the 7108 because this was the only chip with video input. The question is if such an input is really suitable for your purposes. Two things should be taken into consideration: The quality of such an input will not be satisfying for engaged video processing. For example real time compression in Motion-JPEG is not supported at full resolution with this kind of chips. Special video cards like the Miro-DC series is much better for this purpose. Web cams are connected to the USB port mostly. So the field of use for such a video input is limited.

Chrontel

Chrontel Chips can be found on cheaper cards mostly. They are less complex in construction as Philips and Conexant chips and cannot be programmed as flexible. That makes the work of the developer much easier, as there is only a quarter of registers which has to be programmed. But the functionality is limited of course. There is a whole string of enocders from Chrontel, mostly the type 7103-7108 are found. On the whole they differ in details only like the capability to create the anti-copy signal Macrovision. Concerning the number of output modes and resolutions there are no differences.

possible distortions with Chrontel Chips (click it)

Pros:

• good picture quality with fine color shades
• Support for 800x600, 640x480 und 768x576 (TVTool only)

Cons:

• Significant interference-proneness: Chrontel chips seem to be very susceptible for disturbances in the supply lines. Especially in the PAL overscan modes it is a matter of luck if the chip behaves well in your PC or not. That means you can get a clean picture. But many users experience more or less distortions, started by small horizontal lines up to a completely distorted image like shown in the photo above which reminds of scrambled pay TV channels.The manufacturer has no solution to this problem.
• Only two picture sizes possible: Standard und Overscan. A desktop size is technically not possible, which was useful for presentations and gaming.
• Special modes like PAL 60 not possible.

If the Chrontel chips face a clean supply line in your PC and you don´t notice distortions, then you get on well with them. But you can´t check it before buying and trying it. So a return guarantee from your dealer is important! The picture quality is quite ok like mentioned before and the resolution 768x576 (with TVTool) ensures a good movie representation without over- or underscan. This is how a standalone DVD player works. On GF4 cards Chrontel chips don´t seem to be used anymore by the way.

Conexant

Conexant has bought Brooktree a while ago including the TV encoder line. From Brooktree came the BT869, which was continued by Conexant as CX25869. Later it was developt further to CX25871. The 871 offers the possibility to process resolutions up to 1024x768. Also it has some new or enhanced features like a better flicker filter, which has a nice text enhancement function. This function is useful even for movie playback (s. ill. below). Also the possibility to create the WSS signal for 16:9 TVs is new in the 871. Unfortunately we have the documentation problem here, too, so not all features can be implemented. There was the possiblity to create more pictures sizes to adopt the image better to certain TVs. The support for 1024x768 is not implemented in TVTool yet for the same reason. But there is hope that this situation could be improved shortly.

w/o Text-Enhance	with Text-Enhance

Pros:

• Good picture quality with fine color shades
• CX25871: Higher sharpness because of Text Enhancement (s. ill.)
• Support for 800x600, 640x480, 768x576 and 720x480 by TVTool
• Support for PAL 60 (PAL-J and NTSC-M are planned)
• Three picture sizes implemnted in TVTool: Standard, Desktop, Fullscreen
• WSS Signal for PAL and NTSC
• Fine adjustable flicker filter

Cons:

• Unfortunately no sufficient documentation. So not all features can be implemented like 1024x768, more picture sizes etc.
• The BT869 has some problems with 800x600 overscan modes in NTSC (critical timing).

All in all the BT869 and the CX25871 are the most powerful chips at the moment, which can be found on nvidia based graphics card. The Philips SAA7104 might be able to compete, but the lack of information prevent that at the moment. Also the Conexant chips don´t have bigger bugs which would spoil your fun watching your DivX/DVD movies on TV. The CX25871 even creates a sharper picture than many standalone DVD players because of its improved flicker filter, although this feature might only pay off really on high quality 100 Hz TVs. Condition is that TVTool used (of course :-) as the nVidia drivers don´t support most features. Who wants best quality and most features (without video in) should take the CX25871. But also with the BT869 you are served well.

All three manufacturer have in common unfortunately that they have no interest in making the features of their chips available to the users. There is no support for developers, usually there is no sufficient documentation and emails are not answered at all (self-evident). This situation is already known from nVidia, but for me it´s even more incomprehensible for the TV chip manufacturers because the competition in the market should ensure showing interest in making their own TV chip the best supported device with most features in the market. But obviously it is enough to integrate all kind of features in the piece of silicon to be bought by the card makers in high numbers. Let´s hope that this situation will change a bit with the growing interest in this matter!

That´s it.

(c) JB 2002