LCD TV : Why 240Hz TV ? 720P vs 1080i !



Spoiler Warning: If you really want to enjoy watching movies in a TV it is better not to read this series. You know very well how boring it is to watch a magic if you know the secrets behind it! If you know more and more about this LCD TV technology you start seeing a series of frames (not so perfect) instead of the video itself. It is like a drinking a soup with a fly in it. If you happen to notice the fly you will see only the fly and unable to experience the tasty soup! In other words it is like an itch you can’t scratch!. This is my real life experience. If you still wants to go ahead, do not blame me for any after effects!

In olden days when we started with CRT based TVs, life was very simple with 30 (NTSC)/25 (PAL) frames per second and interlaced scanning to conserve the transmission bandwidth. This continued with the initial days of LCD TVs too but LCD display is inherently progressive in nature so de-interlacing was necessary. LCD is a sample and hold (SnH) device unlike CRT tubes where you need to refresh continuously. So for a DVD instead of 576i (576i/ 50 for PAL and 480i/ 60 for NTSC) in old TVs we have 576P/25 in LCD TVs which is perceived as a better picture. In the following years we had seen the introduction of HD ready and Full HD (1080P) TVs but was inherently using 50/60Hz frame rates.. People started comparing LCD TVs with Plamsa TVs and find fault with its poor contrast level, and slower response time. So they invented dynamic contrast ratio and started specifying insane numbers instead of specifying static contrast ratio. It is like specifying PMPO (Peak Music Power Output) in audio systems instead of RMS power. Audio engineers relearned from Marketing guys that music is rich in harmonics and it is better to take upto the nth harmonics while specifying power output. Once they successfully removed the reference level anybody can quote any numbers. Same story for dynamic contrast ratio.. now contrast ratio of LCD TV runs into million.

Since TV is very competitive market each one wants to outsmart the other. One fine day one of them introduced 100Hz(PAL) / 120Hz(NTSC) LCD TVs saying that motion is not smooth in conventional LCD TVs. (blame its poor response time of 8ms on those days!). So we need to have double the frame rate for not to miss any action sequence in sports. Then another has come up with 200/240Hz TV. Everybody followed. All of us forgot to do the maths . To have 240 Hz frames LCD response time should be within 2ms. IF we can have LCDs with response time within 2ms why can’t we go back to the old 50/60Hz TV? To understand this further let me explain how they cook up 240 frames when the original source have only 24/50/60 Hz frames (24Hz for movies and 50/60Hz for videos).

There are fundamentally 3 approaches to the 200/ 240Hz :
  1. Repeat the 50/60 (full frames) 4 times to arrive at 200/240Hz – Simple but worst implementation.
  2. MEMC: Motion Estimation Motion Compensation- Add 3 interpolated frames between two successive “real” frames. LCD response time should be within 2ms.
  3. Scanning Backlight 240Hz – Use MEMC to achieve 120Hz and simply strobe the backlight LED on and off!
3rd one of strobing the LED backlight is introduced by LG and Toshiba who were in a hurry to get into 240Hz bandwagon. 2nd one is the best among this and Sony, Samsung etc are using this from beginning. Now others too, including LG.

Each LCD manufactures call 240Hz technology by different names, Motion flow, Smooth Motion. True Motion, Auto Motion Plus etc..

LG has gone one step ahead of announcing 480Hz TVs. Use MEMC to 240Hz and use Scanning Backlight to extend this to 480Hz.

Again for to display 24Hz movies conventional LCD TVs use a feature called “2:3 pulldown” which means to make up the difference between 24Hz Movies and 30Hz TVs for every 5 frames they repeat two frames from the 3 “True” frames. Remember that 240Hz is multiple of both 24 (movies) and 30. So no pull down is needed. You would have seen recent TVs support 1080P/24Hz for movies so that movies can be shown in its original sequence of frames.

So back to the old question do you really need 240Hz. Many are able to see the difference. But Video Industry expert Dr. Raymond says that there is no perceivable difference.
http://www.displaymate.com/LCD_Response_Time_ShootOut.htm Since I bought a costly 240 Hz Sony KDL-52NX800 TV recently I am trying my best to imagine some difference. I cannot say I am very successful but I am blaming it on my fading eye sight.

Which is better 720P or 1080i?

You must be aware that some of broadcasting channels use 720P (mainly sports channels in US) whereas most of the DTH providers use 1080i. Which is better? This was a big debate in a couple of years back.

To save bandwidth, DTH providers use either 1080i or 720P instead of 1080P. Each field in 1080i has only 540 lines and two fields are necessary to show one frame. Sports enthusiast argued that they are affected by the motion blur and their heroes can even score a goal during this interval of scanning two lines in their TV. So sports channels in US introduced 720P where the scanning is progressive. To save storage space and to reduce processing power games (PS3/ Xbox) use mainly comes in 720P instead of 1080P.

Going back to the basic question which is better (720P or 1080i) people forget that LCD screens are inherently progressive in nature unlike CRTs (which uses 2 interlaced fields to come up with a frame) so the answer almost tilt in favour of 720P in the case of CRTs. but it is very tough to arrive at a conclusion in LCDs. Why?

LCD uses de-interlacing and all kinds of picture enhancement processing before painting the picture on to the screen. As you know 1080i DTH transmits 1080P frame information in two fields of 540 lines. So in a way no information is lost and the current set of LCDs de-interlaces this and shows it as a 1080P picture with some milli seconds delay. So in a way there is no compromise on resolution but anything happens between the two fields are lost.

In short 1080i does not compromise on resolution but compromise on some very fast action sequence and 720P does compromise on resolution but capture some fast action. It is upto you to decide on which one is better. For me 1080i is better any day since it does not compromise on resolution and I can live with what action is lost in between two frames. The great news is 720p/1080i sets are usually quite a bit cheaper than their 1080p counterparts on sites like the Dell TV deals page, so if you're limiting your viewing to HD satellite/cable service, you won't see a difference.

CRT vs LCD Technology

Somebody forwarded to me a posting found in BB where it is mentioned that CRT technology is matured and LCD technology is not yet matured and CRT picture is very good so why do you want to spend on LCD TV:

It is as absurd as one saying that technology used in Ambassador car is matured and recent cars are not yet matured (see everyday new models are coming out, claiming so many new features!) so let us use Ambassador, it is better for our Indian village roads!

Nobody denies that CRT tubes are brighter and can have better contrast ratio since it emits light and LCD does not emit light by itself. But these two alone does not make the video any better. How many CRT TVs you have seen with a flat screen like LCD, HD resolution and offering multimedia features. On the top of all these it consumes insane amount of power for a large screen and too bulky. CRT served its purpose in history and is now extinct and gone. Please do n’t dig its grave.

When will you say any technology is mature? I think it very relative. CRT based Color television got introduced in US in late 50s. Do you think it took 50 years to mature? Technology used in basic CRT tubes seldom changed after 15 years of its introduction. But it was surviving on its extended life by adding more and more features to the electronics around it till a viable alternative came up, in terms of Plasma and LCD. You will very well appreciate this if you remember the introduction of 100Hz televisions based on CRTs in 90s. These trends would have continued till an alternative comes up since this is the marketing strategy of TV manufacture, they need to sell and make money. Datacom engineers will remember IPV4 to IPV6 story. Industry has come up with lots of workarounds and extensions (NAT/ CIDR etc..) to IPV4 to postpone the introduction of IPV6. Anybody dare to say IPV4 is not matured? Same is happening in LCD TVs now. The only difference here is that next viable alternative to LCD is not yet ready to face the mass market (unlike IPV6 case which is more political!).

LCD Technology is matured since there is hardly any change in the basic LCD display technology itself and the recent improvements are in the electronics circuitry surrounding it. But the next technology for the display (OLED) is not yet viable. So to keep the pot boiling, TV industry is cooking up so many new things. Infact I firmly believe 3D TV would not have come up as a big feature if OLED tech were mature enough to produce large screens. I am not under estimating the influence of Avatar movie’s success. . Industry has experimented with OLED for so many years without much success. I think AMOLED (Active Matrix OLED) may become common handheld devices but industry is waiting for next breakthrough in display technology. The moment it comes LCDs will be dumped.




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