When screens were cathodic, the frequency of the screen was directly linked with the ability to redraw a screen n times every second. It was a complete redraw of the screen and there was no other way to change the image.

LCD screens are matrices where each pixel is directly addressable. It means that to change an image, only a fraction of the pixels must be changed, and they can be changed almost all at once.

Yet, LCD screens have a "frequency" in their technical definition (my laptop screen for instance is 60 Hz).

What does this frequency refers to?


Same thing: how many times per second the screen is refreshed.

Graphics cards aren't streaming single pixels that changed. They don't even have this concept because they are rendering entire screen all over again each frame. Similarly, LCDs can't receive single pixels, they are redrawing entire screen on each frame. (Why is a separate question, but I guess the answer boils down to: unnecessary complexity and cost.)

Your laptop screen is refreshing 60 times per second, in constant intervals between frames.

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They might be individually addressable, but they are not simultaneously addressable.

They still need to be set, and refreshed, sequentially. That imposes similar limits as CRT screens. Processing speed in terms of being able to get data to each row of pixels also limits how fast you can can display an image.

The CRT method of scanning across the screen also suits LCD, it is a sensible proposition to build up the image row by row because it makes everything happen predictably and consistently.

I've written about this kind of thing before at Can I set LCD/LED or OLED monitor refresh rate to around 0Hz?

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In short, the 60Hz rate refers the refresh rate which is the rate at which the entire image gets updated. Generally screens are specified for for a range of refresh rates, and informed consumers will look at the maximum refresh rate.

Gamers will be more interested in the LCD's response time, which is an indication of the speed at which a pixel goes from black to white and back to black.

An LCD response time is typically about 10ms, and it can go as low as 1ms. Frame updates of 50Hz are fast enough for human vision so at first sight there is no reason to have better response times. They are however usefull to reduce another visual effect that I worked on 25 years ago but will not disclose here).

Based on observation, your question has the following foundations:

  • LCD pixels "are" directly addressable:
  • LCD pixels have "at least" some memory (when one stops driving the LCD screen, the image remains at least partially on screen.

LCD pixels are directly addressable

Directly adressable does not mean that they are randomly addressable in practice. The frequency that you mentionned (60Hz) is the refresh rate. While it is the frequency at which the entire screen updates, it is therefore also the speed at which a single pixel updates.
The update rate must be fast enough so that human vision can not distinguish it from something observed directly under natural light.

The refresh rate is in itself not the most constraining limitation on an LCD screen. The real difficulty lies in the pixel rate.
A rough estimate for the pixel rate is the number of pixels on hte screen multiplied by the refresh rate (if we forget about vertical and horizontal blanking that still exist in these contexts for historical reasons, but also for housekeeping reasons).
Displays with 4k resolutions have more than 4000 pixels! The highest resolution ix 4096 x 2160 pixels, or 8847360 pixels. For simplicity of thought, and to consider at least some or all of the blanking, lets suppose that there are 10,000,000 pixel clocks in each image. At 60Hz, that means that our pixel clock is 600MHz. If each pixel is 24bit, our cable has to support a data rate of 1.8GB/second.

At these speeds, it is in itself already quite difficult to make sure that they pixels are correctly driven. An electrical signal travels about 15cm in one nanosecond. Which means that about one pixel clock or more has elapsed while the signals travel from the bottom to the top of the screen or from the middle to the left or the right.

At these speeds it is also easier to have a "simple" continuous stream of pixel data, rather than a stream that has to be decoded at high speed. So full frame updates are the first approach rater than partial frame updates. From what I found, HDMI does not seem to support partial updates, but the MIPI standard.

So the protocol for partial updates exists, so there are implementations where partial updates are possible. Does this mean that the LCD screen itself only gets partial updates? I do not think that this is the case today (but I can't verify).
The display will have an internal MIPI controller and memory buffer to hold at least one entire frame, perform the partial updates in this memory buffer and send the entire frame to the screen every time.

LCD pixels have at least some memory

Yes, but this memory is in majority capacitive (but the "Memory in Pixel" LCDs also exists), so the pixel information degrades over time.
So for the best viewing experience, the pixel data has to be updated all the time. It can surely be "optimised" to lower the update rate, but again that is more complex. The simpliest solution would still be to update the entire image at a lower rate when the image is stable, and increase that rate during updates.

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