About LCD technology...

[Ahakim]

I'm not sure if this post should be in this section!
I want to understand (in depth) how digital video signals are oriented to the right cells/pixels in LCD TVs to produce the color image. I read Jestine Yong's books on the matter but I still have many (unsolved) questions about active-matrix (TFT technology) inside theTV panel. For instance: does each cell in the pixel work in a digital way 0-1 or does it receive (via TFT switching) a controlled amount of voltage to make the cell crystal twist only enough to let white light pass through..(depending on the color wanted to give the right image)?
ANY other books on the matter? THANKS.

[tom66]

Re: About LCD technology...

Each cell is a thin film transistor which controls the orientation of the LC (liquid crystal.) The LC rotates the polarised light to allow varying levels of light through the front polariser.

Each TFT has three external terminals. You have the SOURCE, GATE and DRAIN. It's just a NFET (or PFET? Not sure.)

The drain (backplane) is tied to the VCOM for the LCD panel, which is placed at approximately half AVDD. AVDD is the maximum swing of the source output. This allows for easy AC inversion of pixels (more on this later.)

The gate driver is found alongside the rows of the panel and controls which row receives data from the source drivers. The gate driver uses the VON and VOFF voltages (multiple names: TFT_ON, TFT_OFF, VGH, VGL, etc.) VON is around 20V, which turns on each of the TFTs in the appropriate row. VOFF is around -8V, which turns off all of the TFTs in that row. Each gate driver has multiple outputs, there might be four on a 720p panel giving 180 channels per IC. Each driver is usually a shift register as that's typically the easiest method for selecting rows one at a time. Selecting each row one at a time through the shift-register gate driver allows data to be loaded into the panel, through the source drivers, which allows for an image to be stored on the panel. However, unlike a plasma panel, once the data is loaded it need not be sustained. It simply appears once light shines through.

The source driver is what controls the intensity of each pixel. Each source driver has many DACs across many outputs. Typically, for a 720p panel using 6 drivers, each source driver will have 360 output channels. Each output channel has a 4-bit DAC plus 4-bit mux with a range from 0V to AVDD. The mux selects between gamma levels, and the DAC interpolates between two levels that the gamma IC produces. DAC implementations vary. Cheapest is charge-pump using time and capacitor, however, it is generally quite inaccurate. Common type is a common resistor ladder, with each DAC being a 16 way mux (so there are two muxes.) Typically 16 gamma levels are used, giving a total of 256 levels (16 DAC levels * 16 gamma levels.)

The output from the gamma IC is a nonlinear curve which allows translation from the nonlinear response curve of the TFT and LC to a more linear response which closely matches that of the human eye and the defined gamma properties of the LCD.

If you charge a pixel with a constant brightness then after some time it will permanently take this state; this is known as burn in and can occur on LCDs which do not use a VCOM inversion circuit. The VCOM set at mid AVDD allows the LCD drivers to alternately drive a pixel negative and positive, with the same apparent brightness level in each state, giving an average zero DC level, minimising chance of burn in. If VCOM were not used, then each driver would need twice as many output levels and a bipolar swing, much more expensive.

That's about all I know from a bit of research on the subject.

[budm]

Re: About LCD technology...

Read more about it here:

[shovenose]

Re: About LCD technology...

Holy cow, I know this was not my thread but I finally (kind of) understand how an LCD works. Thank you!

[Ahakim]

Re: About LCD technology...

Many thanks for the explanation & the time you gave(veterans)...As well as for the very helpful/useful Sanyo manual. It really helps.