Display Story2. What's Difference between LCD and PDP

I wrote an article about a display focusing on smart revolution. From now on, I am going to make a posting focusing the display by itself. There were many considerations what is selected to the first subject. And it was decided as first subject to difference driving method between LCD and PDP, which was asked at an education for incoming employees.

What's difference between LCD and PDP

What come to your mind when you have question about a difference between LCD and PDP? If you were to consider selecting a TV between LCD and PDP, you might come to mind responsive time, luminance and power consumption. When these displays were in keen competition, there were some words that if you want to see a movie, select PDP and else if you want to see a drama, you should buy LCD. Although PDP are dusky comparing LCD, PDP has merit to motion blur and it is effective when you see action movie switching scenes quickly. LCD has advantages bright luminance and low power consumption.

Why are happened these differences? It is because there is a difference of driving method between two displays. How about watching a movie comparing LCD and PDP before giving a detailed explanation? It is a movie which film a screen of LCD and PDP directly and this movie shows wide difference between two displays.

- This movie is directly captured to Screen of PDP and LCD by Slow-Motion of iPhone 5s

Did you catch a difference between PDP and LCD in the movie? Is there flicker in PDP and LCD is not? And is there motion blur in LCD more than PDP? Surely, this difference doesn't mean a gap of performance fully. When you watch a TV directly, you might not catch a flicker on PDP. But, this difference of driving method is important factor to make a difference of performance between two displays. Above movie is filmed at 1/4x speed and it was possible to be supported 120FPS which mean to film 120frames per 1s. That is why you can feel a difference between these displays. If you might not catch a difference when you see it directly.

Difference between driving method of LCD and PDP

Like aforementioned, this difference is caused by different driving method. It can't help beging different driving method because these displays have different basic structure making a light source. When they had in competition, they have clear merit each other. But at now when I am writing this article, LCD seems to win on tight race with PDP. And many companies have dropped to develop PDP TV.

There was surely different and each had merit to appear to consumers. But, LCD could get a dominant position in competition with PDP because LCD had different structure and driving method which has advantage to revise its problem more easy than PDP.

Different driving method by difference of basis structure.

Then, I want introduce an elementary difference between two displays. LCD used to be called AMTFT (active-matrix thin film transistor) - LCD. This has a feature to adjust amount of light from backlight unit through tilted liquid crystal between two polarizing plates.

PDP(Plasma Display Panel) is a display making light using plasma effect. You can easily understand this technique based on a technology of fluorescent light. The display panel utilize small cells containing electrically charged ionized gases.

-Left is PDP's basic structure, right side is LCD's basic structure.-

Actually direction of this article is not detailed explain about difference of structure above mentioned because I want show display's difference from my view with different driving method. But for this explain, their basic structure has to be explained first briefly.

AMTFT has active-matrix structure by using TFTs. This structure makes to be possible to maintain a voltage as a pixel's need during over schedule time. The voltage supplying a pixel is maintained during a frame time and it tilts liquid crystal according to supplied voltage. The liquid crystal has interesting feature that a degree it is tilted adjust amount of light on a pixel. To add detailed explain, two polarizing plate play a role blocking light and tilted liquid crystal make to be possible to pass their block.

But, there is a problem to use liquid crystal it is core of LCD. When supplying voltage it, so many time is needed to move we want. That is why LCD driving needs to secure time to maintain supplying voltage and TFT and Cap play an important role on this driving method. This method use to be called 'Analog Driving' because each pixel has a different voltage to display gray-scale.

PDP is made of 2 sheets of glass with 2 gases stored between the sheets. Its driving consists of addressing and sustain. Each row of a frame needs addressing and the frame makes light after finishing whole addressing of rows. PDP's structure should need sustain which is similar way to fluorescent tubes used for lighting and it makes to be hard using method to maintain a voltage for a frame time like LCD's one. Also it needs very high voltage and a different voltage doesn't make a detailed difference of light like LCD's one. That's why PDP adapt different driving method which makes a light by fast repetition of addressing and sustain. At this time, amount of light is same every sustain and amount of light is decided by a number of sustains. Therefore this method used to be called 'Digital Driving' because amount of light is divided by time repeating number of sustain.

Let's compare driving methods

There is difference the number of image for a frame time. PDP needs 8 subframes, LCD needs just 1 frame.

1 frame time means that the time need to drive display for a frame. PDP drives a frame including 8 subframes at 60Hz and then it needs 16ms to show a full frame. And LCD changes a frame every 120Hz and it means frame time is 8ms. On this basement, PDP expresses 1frame by dividing with 8 sub frames. At example, there is a image having 8bit resolution, 256 grayscale (normally, PC generates 8bit, 256 grayscale). and these subframes make a whole frame by showing them during a frame time. Eyes used to be known to divide a image every almost 45Hz, which means that light of images would be accumulated and be combined as 1 image in this period. That is why people don't recognize 8 other images when they watch PDP. You can check this factor at an above figure. At this one, PDP has 8 different subframes but LCD has just 1 frame.

LCD can show a frame image using just one frame like aforementioned. This display can support different voltages to each pixel and it makes different light according to a voltage of a pixel. It means that LCD can control every pixel in a frame time. That's why a image which it want show corresponds to a displayed image like above figure. This different method showing a frame image between PDP and LCD aforementioned is by the difference of driving method, that is a difference between digital driving and analog driving.

What is a difference of feature between two display.

Did you remember a difference feature between PDP and LCD aforementioned in front of this article? At repetition, almost TV seller would compare lists like responsive time, luminance, power consumption and etc to show a difference.

	PDP	LCD
Responsive Time	●
Luminance		●
Power Consumption		●

- Each merit s of PDP and LCD

I have explained that different driving method cause difference of display's feature. And from now on, I want to focus why different driving method makes other results.

1. Responsive Time : In the display, responsive time means how much fast showing a image. Aforementioned, eyes have feature to remember before image according to its duration on display. If a image's switching time were slower and slower, it would to be increase probability to be happened as blur. It used to be called motion blur and PDP has a merit this blur because this display shows 8 different sub frame with over hundreds sustain during a frame time. It isn't to compare that LCD shows just 1frame during same time. These difference makes different responsive time between them and it makes PDP is better than LCD when watching action movie passing scene quickly.

2. Luminance : Luminance means brightness on the display. When you drop by shop for comparing TVs, you may easily recognize display's difference through their brightness. The brighter display tempts consumers effectively and LCD has a strong merit at this point. These difference of brightness is hard to see it's by only difference of driving method because they are have different basic structure to make a light. But, it is fact that the difference of driving method make a different time showing light during a frame time and it affect their brightness. As aforementioned, eyes have a feature to recognize something by accumulating light in specific period. For example, if there are 100nit (luminance's unit) for a 8ms and 200nit for 4ms (0nit for other 4ms), they would be recognized as same luminance. PDP has a relative short time to make a light because it should be initialize voltage every addressing of a subframe. The limitation about amount of light during short time can't help being existed and this factor makes PDP can't make brighter screen than LCD.

Amount of light which eyes can recognize is decided by time and its brightness 

3. Power Consumption : Power consumption is on extension line of the luminance. First, there is a huge difference using voltages. PDP use 60V at data sourcing part and us 180~200V at gate addressing part. On the other hand, LCD use 24V at backlight part and use 10V at data sourcing. Already there is difference between voltages to generate default structure of each displays and beside, LCD use hold-type driving method which needs low voltage to maintain voltage during a frame time. For example, if LCD have a just 1/10 frame time to make a luminance according to 1 frame, there would be huge difference of voltage and current comparing conventional method.
( The luminance of backlight is proportional to current, which mean that 10x current is needed to make same brightness like hold-type method and it require high voltage according increase current. These would reduce power efficiency and need a high performance power system. PDP use digital driving method which have a short time to make a luminance and it is one of reasons that PDP get worser power efficiency than LCD.

How can walk ahead LCD on intense competition with PDP.

The display has taken innovation to give an experience like seeing real things to people and it is necessary to natural movement and bright screen like on genial weather. In order to implement these feature, PDP has been also to make an effort for bright screen and low power consumption but it couldn't page off its limitation. Contrastively, LCD's effort which change backlight unit to LED make its merit more strong and widen a gab with PDP. Also its demerit which is response time can be overcame by revising driving frequency (60Hz → 240Hz) and overdriving technique. It could be possible that LCD have different basic structure and driving method with PDP.

Bottom Line

I organize contents which have remained in my brain through this article. This subject can be interesting because it reflects many techniques to drive a display circuit and gives other point of view as driving method comparing between LCD and PDP. Postings which is going to be wrote would connects up this subject because this posting include overall concept related on from techniques fixing aforementioned problem to novel 3D driving and OLED driving method.

In above mention, there seems to a next item. I have a plan to write about motion blur which is demerit to LCD. Then I want finish this article now and I wish meet you with better posting.

Best Regards
gungs