Author Topic: Modern TV's and their refresh rates vs video framerates  (Read 4696 times)

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Offline mr_lou

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While working on my "8bit Stories" project, I'm planning on adding videos.

All of these videos will be 50 fps because that was the output of the machines back then, and I want to have accurate videos.

I know from experience that when connecting an Amstrad CPC to a modern 60hz TV, the result is definitely not like what a 50hz CRT produces. My smooth scroller on my 50hz CRT is not smooth at all on my 60hz TV. And switching between two colours with a SPEED INK 1,1 does not really switch colour per frame, but rather gives me two scanlines of each colour repeating down.

But one thing is connecting a 50hz output to a 60hz display. Another is displaying a 50 fps video on a 60hz display, right?
But it looks to me like even playing a 50 fps video from YouTube on my 60hz TV will have the same effect regarding scrolling. Logically because there's no automatic interpolation. I don't see how that would even be possible.

So... Now I'm wondering this:

Does anyone know what a Blu-ray player will do when playing a 50 fps video on a 60hz display? Will it give the same result, or do Blu-ray players / TV's have some automatic interpolation functionality that smoothen things out? Does HDMI contain some information about the framerate of the video currently being sent, that TV's can receive and then automatically switch to a refresh rate that matches this? Does HDMI signals someone allow syncing?

I know there are 120 hz TV's now, and even 240 hz TV's. This is preferred because it is dividable with 24 fps, which is still used in modern movies, while also dividable with 30 fps. Because it's dividable by these numbers, I'm assuming there's some kind of sync going on?

But obviously 120 hz not dividable by 50 fps. So I'm wondering if it's a stupid idea to use "clean" 50 fps, or if the best thing to do is to convert the 50 fps video into an interpolated 30 fps video due to limitations of the majority of TV's out there in people's homes?

Are there any such thing as TV's that can detect how many fps a certain video input is (not hz) and then automatically switch to an equivalent refresh rate (hz)?
Is it very rare to find modern TV's that can do 50 hz?

I've been trying to find info on this on Google, but it seems most TV manufactorers wants to keep the refresh rate of their TV's a big secret....

Any experts here at CPCwiki on this topic?

Advice?

Offline Bryce

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #1 on: 22:53, 05 October 15 »
The HDMI protocol works on a Pixel clock rate, not picture refresh rate basis. HDMI transmits pixels at anything from 25Mhz to 340Mhz (only up to 165Mhz for HDMI 1.0). The TV screen refresh rate has no real relation to the fps. The actual fps is decided by the settings in the TV. Sorry I can't help further.

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« Last Edit: 22:56, 05 October 15 by Bryce »

Offline dodogildo

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #2 on: 01:34, 28 October 15 »


All of these videos will be 50 fps because that was the output of the machines back then, and I want to have accurate videos.


US made C64 has 60hz btw. I have one and it runs (and sounds) quite strangely connected to European power grid even with a 220<>110 v converter  and full system (pal ntsc secam) sony crt

Offline TFM

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #3 on: 17:50, 28 October 15 »
US made C64 has 60hz btw. I have one and it runs (and sounds) quite strangely connected to European power grid even with a 220<>110 v converter  and full system (pal ntsc secam) sony crt


Same with the CPC6128 with CTM644 from USA, the CTM644 works with 60 Hz. The CPC6128 has a solder link on the PCB switching it from 50 to 60 Hz output. The picture looks roughly the same on 60 Hz or 50 Hz CTM644 monitors. However, using an EU CTM644 (50 Hz network) with 60 Hz looks different, it will show two black bars (beginning and end of screen). Sadly I can't provide a picture now, my CTM644 (US version) broke.



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Offline dodogildo

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #4 on: 22:38, 28 October 15 »

Same with the CPC6128 with CTM644 from USA, the CTM644 works with 60 Hz. The CPC6128 has a solder link on the PCB switching it from 50 to 60 Hz output. The picture looks roughly the same on 60 Hz or 50 Hz CTM644 monitors. However, using an EU CTM644 (50 Hz network) with 60 Hz looks different, it will show two black bars (beginning and end of screen). Sadly I can't provide a picture now, my CTM644 (US version) broke.
Too bad there's no CPC hardware made for using in the States. For C64 we can at least buy the proper version to use either in EUROPE or US...

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Offline Fessor

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #5 on: 23:19, 28 October 15 »
The 6128 was made for the States and shown initially at the CES in Chicago, but failed at the market as the 464 and 664 before.


Offline Bryce

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #6 on: 23:46, 28 October 15 »
Too bad there's no CPC hardware made for using in the States. For C64 we can at least buy the proper version to use either in EUROPE or US...

Amstrad did it much better than Commodore. With a CPC you only need to change a link on the PCB to choose 50hz/60hz. With Commodore they are two completely different machines, so conversion isn't (reasonably) possible.

Bryce.

Offline dodogildo

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #7 on: 23:48, 28 October 15 »
Amstrad did it much better than Commodore. With a CPC you only need to change a link on the PCB to choose 50hz/60hz. With Commodore they are two completely different machines, so conversion isn't (reasonably) possible.

Bryce.
Ahh that's very cool!

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Offline arnoldemu

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #8 on: 11:11, 29 October 15 »

Same with the CPC6128 with CTM644 from USA, the CTM644 works with 60 Hz. The CPC6128 has a solder link on the PCB switching it from 50 to 60 Hz output. The picture looks roughly the same on 60 Hz or 50 Hz CTM644 monitors. However, using an EU CTM644 (50 Hz network) with 60 Hz looks different, it will show two black bars (beginning and end of screen). Sadly I can't provide a picture now, my CTM644 (US version) broke.
@TFM: I don't fully understand what the images look like. Please would you draw some diagrams to explain?

Is the CTM644 (US Version) different to the EU CTM644 version?

I would be interested to see:
- what a 50Hz CPC image would look like on a CTM644 US version.
- what a 60Hz CPC image would look like on a CTM644 EU version.

And I would be interested to know if the US/EU CTM were different. i.e. was there a different Sync "converter" IC?

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Offline gerald

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #9 on: 11:38, 29 October 15 »
@TFM: I don't fully understand what the images look like. Please would you draw some diagrams to explain?

Is the CTM644 (US Version) different to the EU CTM644 version?

I would be interested to see:
- what a 50Hz CPC image would look like on a CTM644 US version.
- what a 60Hz CPC image would look like on a CTM644 EU version.

And I would be interested to know if the US/EU CTM were different. i.e. was there a different Sync "converter" IC?
The main differences in the two monitor version is the vertical oscillator frequency. The US one has to scan the whole screen in 1/60th of second, while the european one has 1/50th of second.
The horizontal frequencies are close (15.750/15.625), and I a m not sure the it makes a big difference on display.

The effect of displaying a 60Hz screen on a 50Hz scanning display is compressing height of the picture, a bit like rescaling a 4:3 picture on a 16/9 display.

Also, a 60Hz screen will not have mode 1 square pixels, but a bit higher that wide.

Offline pelrun

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #10 on: 14:59, 29 October 15 »

Image differences are purely because the V.Size pot (VR406) is calibrated for one refresh rate and therefore is wrong for the other one; going through the alignment procedure in the service manual should give you the same image regardless of the refresh rate setting.


The only hardware differences between the monitors should be the input voltage stage, since the US is on 110V.



Offline arnoldemu

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #11 on: 15:18, 29 October 15 »
Image differences are purely because the V.Size pot (VR406) is calibrated for one refresh rate and therefore is wrong for the other one; going through the alignment procedure in the service manual should give you the same image regardless of the refresh rate setting.


The only hardware differences between the monitors should be the input voltage stage, since the US is on 110V.
The tube dimensions will be the same?
The thickness of the "visible" lines (lines with cpc graphics on them) is the same?
The spacing between lines will be different... so on a 60hz monitor there are thicker "scanlines"?

I believe the cpc monitor does have "scanlines" (i.e. non-visible lines between visible lines) because it's a progressive signal and not interlaced. I think on the green screen it's more noticeable, but on colour it's not so easy to see, but on a 60hz display it should be more noticeable?
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Offline pelrun

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #12 on: 16:22, 29 October 15 »
There's nothing in the video signal that requires a different type of tube. Scanline width and spacing will be identical - that's purely down to the calibration of the pots. The only major difference is that there are fewer total scanlines in a 60Hz signal (each scanline takes the same amount of time, but there's less time per frame) - but that's swallowed up by the border anyway.

Offline arnoldemu

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #13 on: 19:40, 29 October 15 »
There's nothing in the video signal that requires a different type of tube. Scanline width and spacing will be identical - that's purely down to the calibration of the pots. The only major difference is that there are fewer total scanlines in a 60Hz signal (each scanline takes the same amount of time, but there's less time per frame) - but that's swallowed up by the border anyway.
Would the spacing between lines be different on 50hz vs 60hz?
EDIT: Here I mean CPC set to display 50hz on monitor calibrated for 50hz and CPC set to display 60hz on monitor calibrated for 60hz.

If the dimensions of the tube are the same, and the thickness of the line drawn by the monitor is the same, then if there are less lines they need to be spaced out more vertically in order to fill the display in the same way otherwise you see letter box display?

EDIT: Letter box is often seen if 60Hz native is viewed on 50hz native (using vhold)
« Last Edit: 19:48, 29 October 15 by arnoldemu »
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Offline TFM

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #14 on: 00:05, 30 October 15 »
Would the spacing between lines be different on 50hz vs 60hz?
EDIT: Here I mean CPC set to display 50hz on monitor calibrated for 50hz and CPC set to display 60hz on monitor calibrated for 60hz.


For CTM644 IMHO yes, but maybe not significant. I guess about 5%-10% only. That could be about the same then different CTM's of different users.
« Last Edit: 00:14, 30 October 15 by TFM »
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Offline TFM

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #15 on: 00:07, 30 October 15 »
Too bad there's no CPC hardware made for using in the States. For C64 we can at least buy the proper version to use either in EUROPE or US...

Sent from my LG-H815 using Tapatalk

Ain't true. I got f.e. the US version of the CTM644 (sadly it broke last year). There are keyboards too of course. The CPC6128 was presented first at an US computer fair iirc.  :)


EDIT: Fessor and Bryce explained that all better.  ;) :)
« Last Edit: 00:11, 30 October 15 by TFM »
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Offline TFM

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #16 on: 00:11, 30 October 15 »
Is the CTM644 (US Version) different to the EU CTM644 version?


The are almost identical. Notable differences:
- Label on the back
- Needed power and frequency
- US CTM644 got two PCBs in the case, the big one and a 2nd small one at the side. (Guess it does something to the incoming power).

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Offline pelrun

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #17 on: 04:57, 30 October 15 »
Would the spacing between lines be different on 50hz vs 60hz?
EDIT: Here I mean CPC set to display 50hz on monitor calibrated for 50hz and CPC set to display 60hz on monitor calibrated for 60hz.

If the dimensions of the tube are the same, and the thickness of the line drawn by the monitor is the same, then if there are less lines they need to be spaced out more vertically in order to fill the display in the same way otherwise you see letter box display?


No, because you don't calibrate the size of the full video frame, but just the default non-border area. That's always the same number of scanlines on a CPC, so you'll have identical scanline spacing. On 60Hz you'll have a few less border scanlines, but you won't really notice those.

Offline arnoldemu

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #18 on: 10:06, 30 October 15 »

No, because you don't calibrate the size of the full video frame, but just the default non-border area. That's always the same number of scanlines on a CPC, so you'll have identical scanline spacing. On 60Hz you'll have a few less border scanlines, but you won't really notice those.
how can you calibrate only the border area?
does the pot not calibrate the whole spacing?

if it calibrated just the border and i changed the graphics area with the crtc  the screen would stretch?

the firmware programs a different frame length for 60hz compared to 50hz.

r4 is 38 vs 31.
r5 is 0 vs 6.

60hz has 262 lines.
50hz has 312 lines.

vertical displayed is the same.

yes border will be smaller on 60hz.

60hz (on cpc) is 240p. 240 lines progressive.
50hz is 288p. 288 lines progressive.

if we assume monitor displays this exact, then 44 lines each for top border and lower on 50hz.

20 lines each for top border and lower on 60hz.

with the same sized tube and perfect calibration the spacing between lines is 20% larger than 50hz.

to close the gap, i suppose you could adjust the vsync response somehow to get more lines on the screen and shrink the spacing....?

maybe it doesn't work as i think?

i'll draw some diagrams of what i think.
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Offline pelrun

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #19 on: 10:59, 30 October 15 »
how can you calibrate only the border area?
does the pot not calibrate the whole spacing?


Border area? I think you misunderstood what I said - I said you calibrate everything *but* the border - i.e. the active screen area, and you can do that just by changing the border colour to black so that the screen stands out. Then you physically hold a ruler up to the screen and adjust the pot until the paper area is the right size.

The 200 lines of the default CPC screen resolution should be 14.5cm high on the screen according to the service manual, which is about 13.8 lines per cm. That's constant, regardless of the refresh rate!
« Last Edit: 11:01, 30 October 15 by pelrun »

Offline pelrun

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #20 on: 11:07, 30 October 15 »
I think you're expecting calibration to be aiming the electron beam so that only one scanline activates each physical pixel row, which is either on or off, but that's not the case - it's all analogue, and you can adjust the dot position very finely.

There aren't actually physically distinct pixels on a CRT, unlike an LCD. The pixel size is determined by how well focussed the electron beam is and the physical properties of the phosphor coating.
« Last Edit: 11:10, 30 October 15 by pelrun »

Offline arnoldemu

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #21 on: 11:07, 30 October 15 »
I did misunderstand.

ok yeah I understand, so calibrating it will change it over the entire screen, but done to ensure the visible area is the same height, the border then becomes taller or shorter.

ok cool.
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Offline pelrun

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #22 on: 11:12, 30 October 15 »
Yeah. You'll have trouble if you try to run an bunch of demos at 60Hz, because they'll probably run out of scanlines (if they use overscan) or processing time (if they're using all the frame time). But normal stuff probably won't seem any different.

Offline arnoldemu

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #23 on: 11:15, 30 October 15 »
Thank you.

I am trying to work out how I can best simulate this for an emulator (and to produce nice accurate screenshots).

I am also interested in what differences there were between CPCs for 60hz and CPCs for 50Hz.

I know that the firmware will detect the link on the pcb and it adjusts it's timings for 50hz vs 60hz.
Also there is a 60hz version of the monitor, and according to TFM, it has an additional PCB for power, and from what you say the pot would have been adjusted to ensure the non-border part of the display is a specific height.

I can't see where in the service manuals it actually mentions 60hz or US or any difference. I was expecting it to say something.
But then if the 60hz 6128's were not actually from Amstrad themselves, but from a registered importer, then it's possible it would not mention it.

I still think the actual facts about the American cpcs is quite minimal. I would love to see photos of monitors and documentation that back it up.
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Offline arnoldemu

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Re: Modern TV's and their refresh rates vs video framerates
« Reply #24 on: 11:16, 30 October 15 »
Yeah. You'll have trouble if you try to run an bunch of demos at 60Hz, because they'll probably run out of scanlines (if they use overscan) or processing time (if they're using all the frame time). But normal stuff probably won't seem any different.
Almost nothing, except the firmware, checks these links, so most programs will force a 50hz display timings. If the monitor is calibrated for 60hz then the picture will probably roll.


I've hijacked this topic to discuss the American cpcs here.
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