LED/LCD - TAKE ME TO SCHOOL...

[Alec]

I was recently trying to explain these samsung LEDs to a mate. I did the best i coudl but id liek ti understand it better.

We currently have Samsung doing LCD TVs woth LED lights around the periphery of the screen. We also have another company bringing out LCDs which use entirely LED ligting.

What, tho, makes these LCDs, and a "proper" LED (dont even know if theyre on the market yet) an LED?

I'm assuming its something in the panel design but have no idea really about these things, If someone could explain or point me somewhere that'd be grand.

 

[Andrew Everard]

In all these TVs, both LCD and 'LED', the picture is actually created by an LCD panel; the differences are in how that LCD panel is backlit.

Conventional LCD TVs have long used tubes to backlight the panel; an array of strip-lights. Trouble is, these use quite a lot of power, and can only be dimmed en masse: in other words, the back of the LCD is illuminated all the time the TV is on, even when it's showing an image which is all black. Were you to want to watch such a thing.

LED edge-lit TVs use an array of LED light-sources around the edge of the LCD panel, these shining into some kind of reflector/diffuser arrangement to give even lighting across the screen. The main advantages here are lower power consumption and the ability to make the TV slimmer.

Full LED TVs use more LED lightsources, arranged in a matrix behind the screen. They still have the benefits of edge-lit designs, LEDs taking up much less space than tubes.

If there are sufficient LEDs they open up the possibility of local dimming, in which a processor driven by the same video signal as that controlling the LCD panel can adjust the brightness of individual LEDs, or clusters of LEDs. This enables those LEDs lighting areas of the LCD panel showing a dark image to be dimmed right down, enhancing contrast.

RGB LED TVs - the name varies from brand to brand - will light sections of the LCD display panels not with the white LEDs used in the types mentioned above, but clusters of red, blue and green LEDs. These can be controlled to alter the colour of the backlighting in specific areas of the screen and this, combined with local dimming, theoretically improves colour fidelity as well as contrast.

It's important to reiterate that there's actually no such thing as an LED TV - well, not on the domestic market - and that all these 'LED TVs' don't light each pixel of the screen with its own LED, unlike those huge TV screens you see at concerts and sports events.

Those Jumbotrons/DiamondVisions and so ons really do create the picture with a matrix of lights, or rather a matrix of clusters of lights to create the colours, and rely on you being far enough away from them not to 'see the joins'.

As yet this technology is impractical in the much smaller screens we use at home, as is mapping each picture element, or pixel, of the screen to its own LED backlight. After all, in a typical 42in screen, you'd need just over two million lightsources, while for an RGB LED backlit system you'd need three times as many. And they'd have to be microscopically small.

Then again, if you could one day make a matrix of LEDs that small, and enable each and every one to be dimmed individually and adjusted in threes to provide any colour required, then you really would have a true LED TV...

That's what an OLED (organic LED) TV is: self-illuminating, with each picture element, or pixel, lit according to the subject-matter being shown. It has no backlight.

Sony sells an 11in version, but it's fiendishly expensive, and while others have shown larger prototypes, they're reportedly struggling to make these a viable commercial proposition.

 

[Alec]

Thankyou Andrew.

So, we have LED displays but theyre massive? Are they LED all the way or are they in some sense still LCD like the others you mention? And what defines a pure 100% bona fide LED?

Are there examples of each of the "semi LED (really LCD)" TVs you mention on the market, or are some just in development (or even just ideas up to now)?

 

[Andrew Everard]

No, the pure LED displays I was referring to are mahoosive: this one is 31.04x14.4m, and if it were a conventional TV aspect ratio would be a 1340in screen!

And yes, they are pure LED: the image is formed by a matrix of clusters of LEDs: read more here

 

[Alec]

al7478:

So, we have LED displays but theyre massive?

Yep, got that. Thanks for the link I'll have a look.

 

[Alec]

OK. So even Full LED TVs and these RGB ones both mentioned in your first post are still LCD?

I'm just trying to get to what makes even the one that uses only LEDs an LCD, and what makes an LED an LED.

I'm guessing its something else about the panel and not entirely to do with the lights used (for if it were we'd call those fully LED lit LCDs proper LED and have done with it).

 

[Andrew Everard]

al7478:OK. So even Full LED TVs and these RGB ones both mentioned in your first post are still LCD?

Yes.

al7478:I'm just trying to get to what makes even the one that uses only LEDs an LCD, and what makes an LED an LED.

An LCD TV has a liquid crystal display panel forming the image: all the current consumer TVs being described in the marketing hype as LED are actually LCDs with one of the varieties of LED backlighting I explained in my original answer.

Real LED displays, such as those huge arena display panels, have no LCD panels, but create the image purely by the illumination of arrays of light-emitting diodes.

Paradoxically, it's easier to make screens this big using pure LED technology than it is to attempt to do the same thing for domestic displays, because the pixel pitch can be larger in these huge screens, designed to be viewed from significant distances.

Some of these arena/advertising displays use a 2.5cm pitch, while the latest HD models use 1cm, as in this biggie, which is over 20m by getting on for 25m.

By contrast, the pixel pitch on a domestic TV is tiny, usually in the low tenths of a millimetre. And therein lies the problem with making pure LED work in a domestic TV, or even mapping LED backlighting closely to the image-forming pixels.

BTW, could help but smile at the specs on the new screens Mitsubishi has installed at the Dallas Cowboys' new stadium:

48m wide and 21m tall

10.5 million LEDs

Screen area of just over 1000 sq m

Resolution 2176x4864 pixels

Power consumption 635kW

Weight over 77 tonnes

And they have two of them, plus two smaller panels, all in one huge unit suspended over the centre of the pitch! The entire assembly weighs getting on for 200 tonnes, and is hung 33.5m up in the air.

I guess up there it doesn't matter if it runs a little hot...

 

[Alec]

Thanks Andrew, its getting clearer.