6-Bit vs. 8-Bit....PVA/MVA vs. TN Film
Are Things Changing?


Index:

 

 

Introduction
 

Recently there has been a fair amount of talk about colour depth. It has long been said that there are two clear categories of colour depth, based on panel technology used:


Samsung 913N vs. Samsung 193P
Is it as simple as 6-Bit TN Film vs. 8-Bit PVA?


However, recently it seems things are changing. The reason that TN Film panels have traditionally been 6 bit panels is that the manufacturers used low performance A/D Convertors. This was a means to keep costs of these panels down, and also to help improve responsiveness. That's why they have been 6-bit (and fast) for many years. TN panels could just as well be 8-bit panels if the manufacturers decided to make them so. The thing with response time and 6-bit is actually pretty simple. By reducing the panel to 6-bit you generally create 4 steps between each shade. This means that the "distance" between the shades is larger and that the liquid crystals have to twist more. A larger twist requires a larger voltage and a larger voltage makes the liquid crystals respond faster. This is similar to the principal behind overdrive as well.

On the reverse side, PVA and MVA panels were always traditionally slow in comparison, and before the advent of overdrive these panels were quite a way behind TN Film in this respect. The models were priced higher and were more widely used in the "premium" models as opposed to the general, often gaming orientated, TN Film based market. As such, they didn't cut costs and didn't worry about responsiveness as much. As a result, 8 bit colour depth was offered instead.

8-Bit TN Film Panels?

Nowadays things aren't as clear cut. Many manufacturers including Samsung and BenQ for instance are listing some of  their TN Film panels as offering 16.7 million colours. BenQ's tech support have confirmed that "The FP202W uses a TN panel with 8-bit drive giving full 16.77 million colours. Dithering only achieves 16.2 million colours." Samsung list many of their TN Film panels with 16.7 million colours as well, for instance their LTM190EX TN Film panel.

The above image is taken from Samsung's monitor module PDF and shows a few interesting points. On one hand there is a 17" TN Film panel (LTM170EU-L21) which offers on 16.2 million colours, a sign that it is using traditional 262k colour depth with FRC as one would expect. On the other hand there is a 19" TN Film panel (LTM190EX-L01) which is listed as 16.7 million colours. Does this imply it uses 8 bit colour depth with no need for dithering?

While it is hard to get any confirmed information from the manufacturers, it would seem that perhaps some modern TN Film panels now offer a full 8 bit colour depth, and perhaps negate the need for dithering technology. This is a good sign, and thanks to overdrive, response times still remain very low on TN models. Discussions across the internet seem to suggest that TN Film panels are either 6-Bit with FRC, or 6-Bit extended to 9-Bit, offering 16.7 million colours on paper. Either way, it looks as if there have been changes in TN Film recently, and dithering has become far less common.

 

The Flip Side - 6-Bit VA Panels?

While the change in TN Film technology is a positive one, on the other hand it appears that some VA panels are only using 6 bit colour depth and are now using dithering and FRC technology! This is not really detailed in any of the product descriptions and in many regards is quite misleading to the consumer. Panels are still listed as having 16.7 million colours, but is this realistic? Does it imply they are using 8-bit colour depth?

Samsung are the most talked about culprit here, and their modern PVA panels seem to exhibit some disappointing artefacts and spatial dithering patterns. This article highlights the problems observed by many people on the Samsung 970P (6ms G2G PVA panel).

       
Samsung 970P. 6 Bit PVA using spatial dithering? See this article

 

Samsung have confirmed that they are using 6 bit addressing in the final step plus FRC and most importantly they utilize spatial dithering in some of their PVA panels (as above with the 970P, at least in this users case). Either in an 8x8 pixel arrangement or in a 2x4 pixels arrangement. This would mean that some of their PVA panels are not a true 8-Bit colour depth, but rather achieve this through FRC and dithering 'trickery'.

Samsung complicate things further on their panel descriptions available here. I've produced a sample of this below which shows some variations:

Some of the PVA panels are listed as 16.7 million colours, and some are labelled with the extra "True 8-Bit". This might imply that some PVA panels reach the 16.7 million colours through a degree of FRC, with only those labelled as "true 8-Bit" offering the full 256 shades per sub pixel. Rasmus Larsen over at Flatpanels.dk has stated:

"Samsung use a more complex method than just 2x2 dithering. First of all they expand to 9-bit in order to get the last shades. If you utilize the ordinary dithering and FRC you will end up with 16.2 million colours instead of 16.7 million colours because you can't reproduce the last few shades in the "dark" end of the grey tone system."

This backs up the theory that some of Samsung's PVA panels can produce 16.7 million colours, but are only doing so using FRC. Yakken at Flatpanels.dk also says: "When they say that they can produce 16.7 million colours, this is true if we look INSIDE the monitor. The signal processing is done at 9 bit, but as the panel is only addressed by the 6 of these 9 bits (3 bit for FRC) true 24 bit is not the result in the end."
 

Yakken has also compared two Samsung PVA panels, the 193P+ and 191T. On one hand you have a new generation PVA panel with Samsung's RTC application, MagicSpeed. The 191T however is an old style PVA panel from a few years ago. Yakken states here:

"The 970P monitor is furthermore not the only monitor from Samsung that lacks the ability to display true 24 bit colours - my 193P+ also suffers from this annoying defect. Yes I would really call it a defect, and so far Samsung has been ignoring my e-mails concerning the issue. My investigation is found here: http://www.flatpanels.dk/flatforum/viewtopic.php?t=553 (in Danish). My 191T does however, not suffer from this, so somewhere in between these two models, someone got a bright idea, and I'm betting my cat that it rimes with MagicSpeed..."

 

This situation carries on outside of Samsung's panel catalogue and there are reports that other VA manufacturers like CMO use 6 bit + FRC on their panels. AU Optronics, the leading manufacturer of P-MVA panels might also use this method of colour depth. The official spec for the Viewsonic VP930 for instance has always been listed as 16.7 million colours.....on the US and UK sites that is! On the German site, the spec was listed as "6 bit +FRC" but has since been changed. Some users of the VP930 have reported noticeable dithering patterns, including xtknight over at Flatpanels.dk: "On my VP930b (AUO P-MVA), the dithering is extremely hard to notice and only occurs in a very few select tones. There are a couple of tones that are really bad, but just about 2 or 3, so it doesn't bother me."

 

VA + Overdrive = 6-Bit?

This change seems to have come about with the conception of overdrive technology. Is this a coincidence? The VA models which seem to exhibit dithering artefacts seem to only be those which use the modern overdrive / RTC / MagicSpeed technologies to boost grey to grey response times. Since 6-Bit +FRC was originally used to help improve pixel latency on TN Film panels, it seems logical that perhaps with the push on VA response times, they manufacturers have also taken a hit in colour depth to help achieve the low response times that we see from modern PVA and MVA panels. Both modern PVA and MVA panels seem to show dithering and match up with models using RTC technology as well (e.g. Samsung 970P and Viewsonic VP930).

As with the control on overdrive impulse to their panels, Samsung seem to perhaps have done the worst job at hiding this situation. There are a lot of reports of issues on the 970P and 193P+ for instance where dithering artefacts are pretty clear. Perhaps Samsung have not managed to work this dithering on their PVA panels as well as they could have, and dithering is more obvious as a result. AU Optronics seem to have perhaps done a better job on their P-MVA panels though. They have controlled the overdrive application better, and their dithering seems to be less obvious to the end user. Either way, it does seem that with the arrival of overdrive technology, manufacturers have also decided to opt for 6-Bit +FRC to help improve response times further.

There haven't been any reports of issues with S-PVA panels, Samsung's "new generation" of PVA. Perhaps they have retained the true 8-Bit colour depth on these "Super" PVA panels, and remain more for the premium market.

 

What About IPS Panels?

So does this "problem" occur with IPS panels? IPS has always been known to offer very good colour reproduction, and until recently, overdrive technology has not been used. There hasn't really been any talk of overdrive from the main IPS manufacturers like LG.Philips, but with the arrival of G2G quoted response times and 6ms / 8ms specs, it seems likely that this is what they are now using. Perhaps we need to worry about whether the panel manufacturers have sacrificed colour depth as well to help boost response times.

The new AS-IPS technology being used in new models like the NEC LCD20WGX2 doesn't seem to suffer from these problems. There haven't been any reports of poor colour gradients and in fact the colour quality on this screen has been widely praised. Other models like the Philips 200W6CS have not been noted as having any issues, and while the Dell 2007WFP initially looked suspect, it was revealed the Faroudja Video Processing was to blame instead.

 

What's The Pattern? - Summary

After speaking with Rasmus Larsen from Flatpanels.dk further about this, we tried to identify a pattern in this whole situation. From user discussions across the internet and from panel specs, it seems like perhaps the following best describes what we are seeing from modern panels:

 

What Does This Mean In Real Use - Does It Matter?

So how can you tell what your TFT is using? Perhaps the best way to check is by using this Colour Gradients test. Every second line should show a smooth gradient with no obvious stepping visible. On an 8-Bit panel, the colours should blend smoothly into one another, but on 6-Bit panels, there are clear gradients. Take these gradients for instance (click for full size versions):

Aside from the obvious colour blocks which are supposed to be like that, you can see clear stepping in the colours which are supposed to show a smooth gradient. This is a particularly obvious case used to demonstrate my point, but some other panels using dithering show similar situations. These cases show poor 6-Bit + FRC colour depth and this really can be a problem to the user. However, don't be too alarmed by this as in many cases these problems aren't immediately obvious and to many people not noticeable or a problem.

Aside from colour gradients being poor, dithering can sometimes show some nasty artefacts and the article linked to above about the Samsung 970P goes into more depth about this. On the other hand, sometimes the dithering is very hard to notice and doesn't really impact the end user's enjoyment of the screen very much. As you can see from the comments about the Viewsonic VP930 it is only noticeable on a few shades and not something which an average user will pick up. Colour gradients are also a lot better, and again might indicate that some manufacturers have done a much better job with their dithering than others.

In terms of colour accuracy and colour reproduction, the use of 6-Bit + FRC doesn't make a massive difference. Modern TN Film panels for instance can offer very good colour reproduction qualities and DeltaE can be very low on colour calibration graphs. To the average user, colours still look very good and it might well be very hard to tell any real difference between a 6-Bit and an 8-Bit screen.

 

Credits and Further Reading

A big thanks to Rasmus Larsen and Yakken over at Flatpanels.dk for their input with this discussion. Also to xtknight for his involvement and assistance. Thanks to Daniel Dekany for some corrections about the 970P and its dithering patterns.

Please be aware that since there is very little official information available online about this situation, this article may be updated from time to time with any changes. It is not an official answer to the situation, but hopefully serves as a guide to what seems to be observed by many end users.
 

Related Links:

Panel Technologies Explained at TFT Central
Discussion of 6-Bit vs. 8-Bit over at Flatpanels.dk Forum
Widescreengamingforum Discussion of 6-Bir PVA/S-PVA
Samsung 970P dithering issue article
Hi-FRC Whitepaper from CMO