A trip down the insides of a modern monitor
The task of a monitor can be described easily:
- Receive a video signal from a source. (Computer, DVD player, PlayStation...)
- Display said video signal on the monitor output. Right now, this is almost always an LCD panel.
As is always the case in with today's technology, things that seem to be so easy, require a lot of complex processes under the hood to make it happen.
So let's have a look at the different building blocks that live inside a monitor.
In the block diagram above, you see the most important components of a contemporary monitor.
There are 3 major blocks:
- The scaler unit
- the timing controller (TCON) unit
- the LCD panel
In the most typical configuration, the scaler unit will be a separate PCB. The TCON unit will be on its own PCB as well, but the PCB will be attached to the LCD panel: the LCD panel with TCON is something you buy together from the panel vendor. The scaler board is designed separately by the monitor ODM.
Some monitors don't follow this block diagram, and leave some things out. For example, direct drive monitors don't have a scaler chip at all, though they will still have a small PCB with some controls.
Now let's dive a bit deeper into the this block diagram.
The Scaler Board
The scaler board has 2 major functional blocks: the scaler chip, and the backlight driver. Many monitors have some additional units that are not display related. A USB hub is most common, a webcam is another one. Something really exotic would be an infra-red transmitter to control 3D stereo glasses.
The Scaler Chip
The scaler chip does all the heavily lifting to make sure that the desired final image will be displayed on the screen. The actual functionality differs from chip to chip and from monitor to monitor, but is a very typical block diagram:
In this diagram you see a bunch of units:
Video input selection
Most scaler chips support a whole bunch of inputs, and different input formats as well: HDMI, DVI, VGA, DisplayPort are the most common. The scaler knows how to decode all these formats back into pure digital pixel format, select one of the ports and sends the pixels into a pixel processing pipeline.
The sky is really the limit here: there are lots of algorithms that can be used to improve image quality. But some of the more usual ones are image contrast adjustment, resolution upscaling, gamma correction, six color component adjustment and so forth.
The On-Screen-Display unit generate images that need to superimposed on top of the incoming image. It is used to display menus and monitor manufacturer logos.
This is the final processing step before sending the image to the TCON. It adjusts the pixel values that are send to the LCD panel to compensate for dynamic behavior of the LCD elements on the panel.
TCON output interface
This converts the pixel values back to some serial high speed interface format that is compatible with the TCON board. Protocols that are often used are Panelink, V-by-1, and eDP.
The backlight is the light inside the LCD panel that makes things light up. Today, it mostly consists of high power LEDs. The backlight controller creates the necessary signals for the backlight driver, also the on the scaler board, which, in turn, generates the necessary current to light up the LEDs.
Somebody need to make sure that the pipeline is set up correctly. That a button press will result in the right menu to pop up or the backlight intensity to go down etc. So a small CPU system is essential to make that happen.
A scaler has various memory needs: the OSD data (logos, menus) need to be stored somehwere, the CPU needs a place to store its program and data, the previous frame needs to be stored for overdrive processing. Especially that last part needs quite a bit of memory, and since the cheapest memory around is DRAM, most scalers use that to store all of that.
The Backlight Driver
The backlight driver can be best described as an advanced power or current supply. It receives a control signal from the scaler, often in pulse width modulated (PWM) form that indicates the strenght of the current that needs to be sent through the LEDs that are built into the backside of the LCD panel.
Older monitors can only set one fixed current and vary the intensity of the backlight by switching it on and off at a very fast rate. Some people get headaches by this, so many newer monitors now send a variable current through the LEDs without ever switching off.
There may be more than just one backlight driver on the scaler board.
The timing controller (TCON) chip on the TCON board receives the pixels from the scaler in serial format and does whatever is necessary to drive the individual pixel components of the LCD array.
The interfacing between scaler and the TCON is an entirely internal affair that's invisible to the user, so there's quite a bit of freedom in selecting the number of connections. While the input to the monitor for a source is almost always just one cable that needs to carry the whole image, this is usually not the case for a TCON: there are often many parallel links between the scaler and the TCON.
A good example would be a 4K resolution at 60Hz: it requires a DisplayPort link that runs at HBR2 speeds (4 data lanes at 5.4Gbps each). However, the TCON may only support eDP HBR1 (4 data lanes at 2.7Gbps), but have 2 eDP ports, so 8 data lanes at 2.7Gbps instead.
It is even possible, not quite uncommon for state-of-the-art LCD panels, that there is no TCON chip on the market that supports all the pixels of the LCD panel by itself. This is not a problem either: the TCON board will simply have more than 1 TCON chip, where each chip services a vertical strip of the LCD panel. For example: TCON0 services the left part of the screen and TCON1 the right part.
In some rare cases, the TCON chip also does overdrive, in which case it needs memory to store the previous image, though that functionality is usually disabled when it's connected to a scaler that already has a overdrive capability.
The TCON sends pixels to the source or column drivers of the panel. These are the chips that will finally drive the TFT matrix that is used to make pixels do its magic.
The LCD panel itself has the active thin-firm transistor matrix, the source drivers, and the backlight LEDs.
Each source driver chip can drive a bunch, e.g. 240, of columns in parallel. Since most LCD panels have more horizontal pixels than that, there are multiple source drive chips.
A digital to analog convertor generates the right analog voltage to pump the right charge into each capacitor that's connected to pixel component.