How did LCD TVs do 24 Hz?

[Sunspark]

I'm using an old 1366x768 panel LCD TV here.

I can select 23.976 Hz and 24.000 Hz if I use a resolution of 1920x1080 and let the TV do the downscaling to the panel resolution.

If I use a pixel correct 1360x768 resolution I am only able to set a refresh rate of 59.91 and 60 Hz. I have tried using CRU, etc. to set a custom refresh rate of 24 at that resolution. It rejects it entirely. I've looked at the mainboard in it, it's just a single CPU that does everything.

I don't know why it accepts 1920x1080 @ 24. Maybe custom EDID work? But beyond that, how did the LCD panels work with that anyway? Is it actually running at 24 Hz, or is it doing software tricks to make it look that way?

I've set up madVR smooth motion to see how it looks, and yes, it is smooth.. when I change it to 1920x1080@24, smooth motion is then off, but the motion is still very similar.. so that tells me that it is not dropping frames and might actually be at 24 (partly since that appears in an official spec sheet, but a reviewer said it didn't work).

Which separately, is another oddity.. desktop, etc. at 24 Hz is very choppy even with 24 Hz video, but all of a sudden the second the player goes into exclusive mode it becomes smooth.. why wouldn't it be smooth in a window on the desktop if the fps of the video is 24?

So, is the LCD literally running at 24? Or is the CPU in the mainboard just doing software-based smooth motion?

Many monitors won't let you because they can't support a pixel clock that low or something.

[QBhd]

You probably need to find the pixel clock... usually in the EDID. Try this:
https://www.entechtaiwan.com/util/moninfo.shtm
And then use something like this:
https://www.monitortests.com/pixelclock.php

You may have to try many many different results before you find one that sticks...

QB

[Sunspark]

I do have that installed already, so let's see..

Horizontal scan range 15-68 kHz
Vertical scan range 23-61 Hz
Video bandwidth 150 MHz

But then what? The other link says I need to know blanking intervals..

[Emulgator]

Most old (my history with externals TFTs is from 2004) panels I came across expect only 60fps native timing,
so one would expect a straight pass-through only for 60.000fps

That means for anything coming in not at native 60.000fps their controller would hard pulldown to 60fps.
so already a 59.94 source would also get that hard pulldown applied, just for one each 1000 frames.

(And maybe passing 59.94 fps inputs ? If such pixelclock shift is implemented, yes.)

For 24fps 2:3 odd frames are projected 2 times, even frames are projected 3 times, leading to distinctive judder if you circle your cursor.
I may be wrong though...

The newer 4K AUO278E and 309B I have here at hand can additionally handle 48fps through their driver.
This makes 24fps possible in 2:2 pulldown.

From my first Laptop-internal TFT from 2001 no tool can report anything.

[wonkey_monkey]

Which separately, is another oddity.. desktop, etc. at 24 Hz is very choppy even with 24 Hz video, but all of a sudden the second the player goes into exclusive mode it becomes smooth.. why wouldn't it be smooth in a window on the desktop if the fps of the video is 24?

That's just how GPU exclusivity works. Unless your program is fullscreen, you can't be guaranteed to hit every refresh.

And (from what I've gleaned) if you're redrawing at less than half the framerate, Windows may only check your window for refreshes every two refreshes, worsening stuttering. So it can be a good trick to repaint once per display frame, even if you don't need to.

[Sunspark]

It's interesting because not every manufacturer used the same approach. Some might make the panel work at 48 Hz instead.. but I am similarly unable to set a custom refresh of 48 and 50 Hz. I do know how to use CRU. Apparently sometimes it is just never going to work at all.

My 60 Hz monitor will accept 24 Hz and this number will show up on software HUDs like madvr or mpv, but now that I see the TV in motion, the monitor definitely does not move that smoothly.. so I am speculating it is accepting and displaying the input but not doing anything special with it.

Reading reviews of similar TV models from the manufacturer they say that the 24 signal is being processed.

It might be doing some kind of inverse telecine? Hmm. Another possibility is that it runs the panel at 48 Hz (even though the stats screen in players reports 24 Hz) but just doesn't allow receiving inputs of non-approved resolutions/refresh rates.

Maybe the video processor in it is implementing the same thing as software-based smooth motion? Temporal frame blending.

I suppose I won't ever really know since apparently different manufacturers did different things.

[huhn]

most old devices can not do 24p at all they are doing 60. just like a CRT would with a dvd. at best they can natively do 50/60 on Samsung this problem was very common even with UHD devices and Samsung TV usually needed a native 120 Hz screen to make it work. with sony it worked on most expensive TVs only the entry class where incapable of that.

if it doesn't do 24p with native clock it will not do it at all. doing 48 is very unlikely to work.

[DJ Bobo]

They won't do 24Hz at lower resolutions than 1080p, because it was meant exclusively for Blu-ray video. Hardware chips have a table of allowed resolutions and their respective refresh rates. If it is not in the table, it won't display. It's that simple.

[Sunspark]

Yes, what I ended up doing is, because I don't want to use the igpu for both downscaling and the smooth motion setting, I am just sending it 1920x1080 at various refresh rates that it accepts, and letting it handle the downscaling and internal refresh changes. My assumption that for 24 it is changing the panel to 48. I haven't noticed hitching.