Author Topic: How many monitor lines does a real CPC show?  (Read 8796 times)

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Offline Devilmarkus

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How many monitor lines does a real CPC show?
« on: 15:51, 02 October 13 »
Hello Hardware Freaks,

TotO and I are actually discussing the question:

How many monitor lines is a CPC able to show?

As you all know, most emulators provide only 272 monitor lines.

(A few only 270)

Now, as TotO and I have seen in Grim's Docs, the CPC should be able to show around 288 lines.

But, is that true?

I made a little experiment:

- Emulating the CPC with: 288 lines and with 280 lines.

Result:
Synergy with 288 lines:


Well, as you can see, the last 8 lines are not used. (Few are blue, because a vertical rupture between switching from regular CRTC registers to Synergy's overscan format, my emulator only handles visible pixels)

Ok, now we have TFM here, who says, that his Giana Sisters takes full advantage of 288 lines. (Don't get me wrong, this is just for fun!)

Let's take a look:


I see: 280 drawn lines, where the last 8 repeat. Rest is "crap"

So now my theory is:
The CPC can show up to 280 lines.

Let's proof that:

CPC power on message with 280 lines:


Okay, let's check TFM's Giana Sisters now with 280 lines and ingame:


Oh... last lines are weird? Well, perhaps my display vertical adjustment is bad... let's move the screen down about 8 lines:



Ahhhh... feeling better now?!?
Now I'm sure, we see full 280 (Yes, 280 lines, because 288 lines we don't really need, or do we?)


Hey... Wait!!!
Where are TFM's promised 288 lines?

 :'(

So, really:
Who can give me a test disk (DSK) where the CPC takes full advantage of 288 lines?

My max. visible result has been @ 280 lines.



« Last Edit: 16:02, 02 October 13 by Devilmarkus »
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Offline Devilmarkus

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Re: How many monitor lines does a real CPC show?
« Reply #1 on: 16:47, 02 October 13 »
Let's take a look @ Grim's "800x600" interlace piccy when emulating 768x576 (288) pixels:



So, or my emulator cannot handle 288 lines (I don't think so, as it should be accurate enough for that) or the interlaced screen is only 280 lines height?

Any suggestions?
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Offline arnoldemu

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Re: How many monitor lines does a real CPC show?
« Reply #2 on: 17:33, 02 October 13 »
monitor safe region

I asked this same question ;)
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Offline Devilmarkus

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Re: How many monitor lines does a real CPC show?
« Reply #3 on: 17:44, 02 October 13 »
I know...
But your DSK has gone... It's not on the server?!?
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Offline TFM

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Re: How many monitor lines does a real CPC show?
« Reply #4 on: 18:29, 02 October 13 »
Hi there. Well, for Giana Sisters or Gerelakos I use indeed 18*16 = 288 lines. And they all contain valid GFX, which can be seen on a real CPC. I don't know which problems you have here. Maybe you cut off lines at the top of the picture?



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Offline Devilmarkus

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Re: How many monitor lines does a real CPC show?
« Reply #5 on: 18:33, 02 October 13 »
Thanks for the information.

I already tested moving down the Y-screen offset about 8 lines.

The result is: on top we have 8 lines of vSync (Or black, nopixels area) and then followed by 272 accurate lines.

When I increase the height to 288 or even 296 lines, all after 280 lines (Giana Sisters: 272 lines) looks weird.
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Offline TFM

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Re: How many monitor lines does a real CPC show?
« Reply #6 on: 18:40, 02 October 13 »
This should be done on a real CPC.  :)
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Offline arnoldemu

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Re: How many monitor lines does a real CPC show?
« Reply #7 on: 19:22, 02 October 13 »
CPC can show more lines. The problem is can the monitor?
And not all monitor's are the same, so some will show less lines, others more.
Same with the hsync position and the width.

A friend's monitor was taken for repair. When it came back the image was shifted. He could see more on the left and less on the right compared to my monitor.
I know that on some green screens, they show less lines vertically.

So perhaps add some options so the user can tweak the monitor settings to see more/less of the cpc screen?

Of course there is a limit with 50Hz, in terms of how much is seen, and we know the vblank on the monitor can be adjusted.
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Offline TotO

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Re: How many monitor lines does a real CPC show?
« Reply #8 on: 19:32, 02 October 13 »
768x576, is a standard PAL 50Hz analog display.
A CPC can properly display all viewable pixels on a good CRT with flat screen, right corners TV. (and any LCD of course)
Because, its CRTC was intended to be used with standard TV screen resolutions... And more!


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Offline PhilZeVibe

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Re: How many monitor lines does a real CPC show?
« Reply #9 on: 21:01, 02 October 13 »
The emulator CPCE displays all the 288 lines IIRC.
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Offline Devilmarkus

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Re: How many monitor lines does a real CPC show?
« Reply #10 on: 21:13, 02 October 13 »
Good.

Testing with CPCe then:


True: It's display is 576 pixels height.

But with 560 pixels I don't miss any pixels from this screen. (Here, the upper 16 lines are blank, and this makes sense, as the real first pixel can appear first on 1st MA register value.)
« Last Edit: 21:15, 02 October 13 by Devilmarkus »
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Offline PhilZeVibe

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Re: How many monitor lines does a real CPC show?
« Reply #11 on: 00:20, 03 October 13 »
Interestingly, the Philips CD-i highest resolution video is 768 x 560.

But the PAL SD MPEG-2 resolution is 720 x 576. ???
Source: http://documentation.apple.com/en/compressor/usermanual/index.html#chapter=18%26section=1
 
« Last Edit: 00:32, 03 October 13 by phi2x »
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Offline Devilmarkus

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Re: How many monitor lines does a real CPC show?
« Reply #12 on: 00:27, 03 October 13 »
Perhaps you forget 8 lines on top for teletext? (They're hidden) This would make sense...

With very good VHS video recorders you have been able even to record the teletext... So on some TV's you have been able to read the old teletext which had been recorded on tape :D

Ok, forget this... Teletext is stored in the blanking interval. A regular TV has 625 lines where 576 lines are used for visual content.
« Last Edit: 00:45, 03 October 13 by Devilmarkus »
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Offline TotO

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Re: How many monitor lines does a real CPC show?
« Reply #13 on: 00:49, 03 October 13 »
But the PAL SD MPEG-2 resolution is 720 x 576. ???
Only for Digital PAL, not Analog PAL.
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Offline ralferoo

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Re: How many monitor lines does a real CPC show?
« Reply #14 on: 15:02, 03 October 13 »
I think your emulator must be buggy. 288 isn't a special number at all, just how many typically fill the visible region of a normal CPC monitor.

The actual screen height is 312 lines, and depending on your CRTC configuration, some will be blank during the VSYNC period. This is typically 12 lines, although you can change it with a CRTC register.

Every line that isn't in the VYSNC period can carry pixel data and if you use a real monitor and adjust the VHOLD knob, you'll be able to see this is true. The VSYNC will show up as a solid black bar between areas of the border colour.

Offline MacDeath

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Re: How many monitor lines does a real CPC show?
« Reply #15 on: 03:25, 07 October 13 »
The issue doesn't come from what the CPC can display but how much it is coded to.

If existing programs may seems to not go beyond "272" its because it wasn't implemented.
Soo so so many CPC games actually displayed only 256x192... it's not like CPC can't display more, it's because spectrum can't really.
Like you know, a 384 mode1 wide scree with an extra character line of 8 extra pixels rows would be like... 768 octets/bytes ?

Many demos can't afford this expensive amount of wasted Memory resources. :laugh:

And most actual CRT monitors can't really display those.
Like, you know, the "safe display zone"...




but yeah, all those were already told. ;D
« Last Edit: 03:36, 07 October 13 by MacDeath »

Offline Gryzor

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Re: How many monitor lines does a real CPC show?
« Reply #16 on: 22:02, 23 October 13 »
Quote from: ralferoo
Every line that isn't in the VYSNC period can carry pixel data and if you use a real monitor and adjust the VHOLD knob, you'll be able to see this is true

Interesting. I wonder if this could be used for gameplay mechanics. Perhaps carrying a "spoiler" line with hints, for instance...

Offline Lone

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Re: How many monitor lines does a real CPC show?
« Reply #17 on: 15:28, 10 December 13 »


Hello,

I'm reopening this subject, because it's a subject i'm currently working about...
I'm writing my own emulator, and I made some research and experiment about display...


I write a custom monitor emulation, that can display everything issued from a PAL  display (i.e. line frequency of 15625 hz ).
The display is supposed to show everything (including HSync, VSync and others special area (as back porch[nb]Read the really interresting http://martin.hinner.info/vga/pal.html page for further informations[/nb] signal ).

Here's the result :


[attachimg=1]


The red line on the left is just 576 pixels high.


You can also notice some garbage up and down, and 8 black lines at the end of the screen (vsync signal). The total of these lines (288 of correct datas, 8 of garbage, 8 of  vsync, 8 of garbage ) is the 312 lines that are expected.




It would be interresting to display this game on a monitor that can show everything (like LCD ?) , to check the garbage things.






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Re: How many monitor lines does a real CPC show?
« Reply #18 on: 15:54, 10 December 13 »

Great work! :)

Flat screen Sony CRT TV can display the full 288 lines from a CPC. But not more. (the same for LCD screens)
May be, some video broadcasting monitor may allow to display 304 lines to see garbages, but not more.
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Re: How many monitor lines does a real CPC show?
« Reply #19 on: 17:48, 10 December 13 »
The red line on the left is just 576 pixels high.
You can also notice some garbage up and down, and 8 black lines at the end of the screen (vsync signal). The total of these lines (288 of correct datas, 8 of garbage, 8 of  vsync, 8 of garbage ) is the 312 lines that are expected.


Excellent work. Yes it's 288 lines (18 GEs in high, everyone has 16 scan lines / pixel in Y, so 18 * 16 = 288). Other emulators have to catch up with your excellent emulation.  :)
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Offline Executioner

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Re: How many monitor lines does a real CPC show?
« Reply #20 on: 01:03, 11 December 13 »
The display of WinAPE was based on exactly what my own CTM644 can actually display, including the last pixels in the curved bit at the bottom. If anyone has an actual CTM or GT that can display all 288 lines, even if it's just partial display of the last lines in the curved bits at the top and bottom, please post a picture of it in action.

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Re: How many monitor lines does a real CPC show?
« Reply #21 on: 01:20, 11 December 13 »
The display of WinAPE was based on exactly what my own CTM644 can actually display, including the last pixels in the curved bit at the bottom. If anyone has an actual CTM or GT that can display all 288 lines, even if it's just partial display of the last lines in the curved bits at the top and bottom, please post a picture of it in action.
The CPC emulation is not related to the CTM display.
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Offline TFM

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Re: How many monitor lines does a real CPC show?
« Reply #22 on: 03:56, 11 December 13 »
The display of WinAPE was based on exactly what my own CTM644 can actually display, including the last pixels in the curved bit at the bottom. If anyone has an actual CTM or GT that can display all 288 lines, even if it's just partial display of the last lines in the curved bits at the top and bottom, please post a picture of it in action.


Well, I can't provide a pic now. However I remember that the Plus color monitor shows all 288 lines and a couple more at the lower end


Of course this may be slightly different from monitor to monitor.


Better to provide a couple of scan lines too much instead of a couple too less imho.

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Offline redbox

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Re: How many monitor lines does a real CPC show?
« Reply #23 on: 09:35, 11 December 13 »
The ability to see all of the scan lines would be very useful for programming/debugging purposes.

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Re: How many monitor lines does a real CPC show?
« Reply #24 on: 01:28, 17 December 13 »
The ability to see all of the scan lines would be very useful for programming/debugging purposes.

It could be that the user is allowed to set the number of scan lines displayed and vertical offset so it best emulates their own monitor and for debugging purposes allow all scan lines to be displayed.