What is LCD?

Posted Wed Aug 9, 2017 at 08:25 PM PDT by
TCL P series 4k TV

Welcome to HDD's Home Theater 101 articles, where we explain emerging technologies while also recommending the very best possible A/V gadgets n' gear you can buy at your particular budget level. 

When it comes to new display and TV technologies in 2017, there are two options: LCD and OLED. The most common panel type in the Ultra HD TV market remains LCD.

But just what exactly is an LCD TV? And what's the distinction between the tech's varying panel types and LED backlight options? Fret not! If you don't know the difference between edge-lit LED panels and full-array local dimming panels, this guide is here to breakdown the ins and outs of LCD. So, without further ado, let's dive right into...

HOME THEATER 101
WHAT IS LCD?

THE BASICS


A Liquid Crystal Display (LCD) is a type of panel technology frequently used for televisions, computer monitors, laptops, and mobile devices. Though the first commercial LCD TVs were launched in the late 80s, the tech did not start to enter the mainstream market until 2004 when HDTV LCDs began to catch on as an alternative to plasmas, CRTs, rear projection, and other display types. By the end of 2007, LCDs rose to the top of the TV sales chart for the holiday season, besting the competition and cementing itself as the most popular TV tech.

And unlike those other now defunct display formats, LCDs have been able to keep up with evolving picture quality standards and manufacturing costs, enabling them to support next-gen features like 4K Ultra HD resolution, high dynamic range, wide color gamuts, and 10-bit processing at mass market prices. 

As the name implies, LCDs use liquid crystals in order to display pixels. In simplest terms, a Liquid Crystal is essentially a substance that has some properties of a solid and some properties of a liquid. When integrated into a display panel with a light source, these liquid crystals are capable of rotating polarized light to switch an LCD TV's pixels on or off. Typically, this is accomplished by placing a polarized glass filter in front of and behind the liquid crystal used in each pixel. 

(Image via Wikimedia Commons)

When the pixel is switched off, electricity flows through the crystal causing it to straighten. In this form, most light is prevented from exiting the second polarizer, creating a dark pixel. Meanwhile, to switch the pixel on, the flow of electricity is stopped, causing the liquid crystal to twist. This structure then enables light to flow through the two polarizing filters, resulting in a bright pixel. Subpixels with a red, blue, or green filter then give the pixel the correct color for the content being displayed. 

LCD PANEL TYPES


Though the basics of LCD tech are similar across the board, specific variations on the process are used to achieve slightly different results, leading to a few key LCD panel types with their own strengths and weaknesses.

TN (Twisted Nematic) are the most common and cheapest LCD panel types to produce and essentially work through the twisting crystal process described above. Unfortunately, these displays typically offer the weakest performance when it comes to viewing angles and contrast. With that said, they do provide some of the fastest response times, making them well suited for gaming.

Meanwhile, VA (Vertical Alignment) panels use crystals arranged vertically and perpendicular to the display. This then causes them to tilt horizontally when electricity is applied allowing light to pass. VA panels feature the best contrast ratio and black level performance of any LCD panel type, making them the ideal fit for home theater use in a dark room. However, while they boast improved viewing angles compared to TN screens, color and contrast suffer at off angles so you'll want to sit directly in front of them. Also, response times tend to be slower than other LCD options.

Finally, IPS (In-Plane Switching) panels feature liquid crystals arranged horizontally in a manner which causes them to turn by 90 degrees in parallel with the display when voltage is applied. This process offers superior color reproduction and the best viewing angles of any LCD type while providing response times almost on par with TN panels. With that said, these displays have weaker contrast compared to VA screens, limiting their effectiveness for home theaters.    

BACKLIGHT METHODS & LOCAL DIMMING


Beyond primary panel types, different LCD displays can also use different methods of lighting in order to illuminate their pixels. Initially, most LCD TVs used a CCFL (cold cathode fluorescent lamp) backlight with an array of parallel CCFLs positioned behind the panel. This tech worked well but limited how thin manufacturers could make their displays. Likewise, these backlights were not as energy efficient as modern methods.

Today, most LCD TVs use a form of LED (light emitting diode) lighting to illuminate the display's pixels. In fact, this method has now become so prevalent that most manufacturers actually refer to their LCD display models as LED TVs now.

Two primary types of LED arrangements are currently used: direct-lit and edge-lit. In a direct-lit configuration, an array of LEDs is placed behind the panel across the entire screen. In an edge-lit system, a row of LEDs is positioned on one or more edges of the panel. Because of their size, LEDs enable manufacturers to produce thinner LCD screens. With that said, some edge-lit displays tend to produce inferior light uniformity compared to direct-lit LED or CCFL backlights.

And in addition to improvements in size and power consumption, LEDs also give LCD display manufacturers the option of implementing a useful feature called local dimming. In a traditional LED LCD, some light still passes through to the pixels even when the liquid crystals are switched on to block it. This prevents typical LCD TVs from ever being able to produce true black levels. With local dimming technology, however, manufacturers can actually dim and completely turn off individual LED light sources, allowing the screen to brighten or totally darken in selected spots to coincide with the content being played. This allows the local dimming sets to create deep, inky black levels.

Local dimming can be implemented with both direct or edge-lit LED panels, but direct-lit local dimming -- also referred to as full-array -- produces the best results since the LEDs are positioned behind the TV's pixels in multiple zones across the entire screen, not just strips on the sides. Local dimming performance is also affected by the number of LED zones used in a display (in general, the more the better) and the specific dimming algorithm the manufacturer has developed.

QUANTUM DOTS


Outside of panel types and lighting methods, other new enhancements continue to be used with LCDs, helping to keep the tech relevant and up to next-gen standards in the 4K HDR market. And that's where Quantum Dots come in.

Typically used in higher-end LCD displays, quantum dots are nano-crystals that are capable of emitting different colors when exposed to light depending on their size. When quantum dots are placed on a film over an LCD backlight with blue LEDs, they convert the light to produce higher brightness levels with more accurate and realistic colors. This lets TVs with quantum dots support wide color gamuts beyond the traditional Rec. 709 color space along with higher nit output for optimal high dynamic range performance.

Sony has been using a form of quantum dots in select displays under the company's Triluminous branding for several years. Likewise, Samsung currently uses new metal quantum dot technology in its flagship Q7, Q8, and Q9 display lineup branded as QLED TVs. With that said, all current LCD quantum dot displays simply use their nano-crystals to convert light from traditional LEDs for enhanced color performance rather than using the quantum dots to completely emit their own light -- but future QLED display technologies could result in emissive quantum dots without the need for an additional backlight.

RECOMMENDED GEAR


55" TCL P-Series Ultra HD TV ($680) - In contention to become the top bang-for-your buck LCD Ultra HD display of 2017, this model features an LED VA panel with a direct-lit local dimming backlight and 72 zones. Other key specs include Dolby Vision and HDR10 support, and wide color gamut capabilities with NBP Photon technology. Likewise, the set uses the Roku OS smart TV platform with access to more than 4,000 streaming channels and 450,000+ movies and TV episodes.

VIZIO P-Series Ultra HD HDR Home Theater Display ($1,700) - VIZIO's flagship 4K display features a VA LED panel with full-array local dimming and 128 zones. With that said, the 55-inch model uses an IPS panel, resulting in a lower contrast ratio. Other key features include Dolby Vision and HDR10 support and wide color gamut support. It should also be noted, however, that the display does not include a TV tuner.

65" Sony XBR55X900E LED Ultra HD TV ($1,998) - This Sony model features a VA LCD panel with a full-array LED local dimming backlight. Other specs include wide color gamut support with Triluminos tech, HDR10 support, and a firmware update planned for HLG playback (but sadly not Dolby Vision). Finally, the set uses the Android TV platform for smart TV apps and streaming. 

Samsung QN65Q9 Ultra HD QLED TV ($3,998) - Samsung's flagship 2017 Ultra HD TV uses a VA LCD panel with new metal quantum dot technology to deliver wide color gamut support and 100 percent color volume. Meanwhile, the set uses an edge-lit backlight with local dimming. Other specs include HDR10 support and HDR10+ support (but not Dolby Vision).


So, there you have it. Those are the basics of LCD TVs. If you still have any questions about LCDs, please let us know in the forums! 

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Sources: LCD TVs (Wikipedia), Quantum Dots (Wikipedia), Explain That Stuff!, PC Monitors 

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