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Old 02 April 2006, 06:56   #1
redblade
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Unhappy Pal Amiga is 256

Hi.

I was just wondering why Pal Amiga is 256 lines high.

Pal = 50Hz
NTSC = 60Hz

But I thought 200Lines/50HZ = 4.

4*60HZ = 240Lines which is available on a PC Monitor, So I was wondering where the extra 16 lines come from.

Can some one please explain it to me, thank you
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Old 02 April 2006, 11:20   #2
Jerry
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Actually Pal has 288/576 lines visible (625 are being transmitted iirc). The 200/256 lines limit is a bit arbitrary, I think.
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Old 02 April 2006, 11:51   #3
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Quite interesting...

The maximum resolutions are:

PAL is 720x576 @50Hz interlaced.
NTSC is 720x480 @60Hz interlaced.

These both have the same bandwidth i.e. (720x576x50 = 720x480x60)

Same goes for non-interlaced...

PAL is 720x288@50
NTSC is 720x240@60

Same bandwidth i.e. 720x288x50 = 720x240x60

I thought there might be a relationship between the Amiga resolutions

HOWEVER

640x256@50 /= 640x200@60

BUT it is close and they are nice round number

I honestly dont know.

BUT THERE IS AN ANSWER... Why?

Because the Atari ST uses the same resolutions so it's not hardware specific.

Last edited by alexh; 02 April 2006 at 12:04.
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Old 02 April 2006, 15:19   #4
gizmomelb
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I'm guessing it's to do with engineering costs and it was cheaper to make the video hardware in the early computers/consoles 8 bit instead of 16 bit.

8 bits has a maximum of 256 possible combinations then that is the maximum vertical resolution for the PAL video display.

As NTSC has a lower vertical resolution than PAL (480 lines compared to 576. So 576-480=96 interleaved lines = 48 non interleaved lines). We end up with a maximum of 256 lines MINUS the resolution difference (48 lines) which is 208 lines of vertical resolution. This was possibly dropped down to 200 lines of displayed video on NTSC computers and consoles as it would be virtually unnoticable and allow the CPU more time for calculations (instead of having to display video).


The whole PAL and NTSC vertical resolution issue is due to legacy support of older TVs. The initital TVs in the 50s couldn't manage much more resolution than 720x480 (NTSC) and 720x576(PAL) due to the speed of the valves and other components. The high cost of TV sets (until recently) has meant that the TV broadcast networks have stayed with low resolution broadcasts as a standard, so that they could reach the widest possible audience (after all who was going to go buy a new TV set so you the could watch one network, but possibly not another network's broadcast?).

Last edited by gizmomelb; 02 April 2006 at 15:24.
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Old 03 April 2006, 00:30   #5
redblade
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hmm so why would overscan be 704 pixels instead of 720 pixels.

I guess those extra 16 pixels would of taken up expensive bytes of RAM back then, because RAM was definately expensive in the early days.

Thanks for you replies so far
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Old 03 April 2006, 11:42   #6
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Quote:
Originally Posted by gizmomelb
8 bits has a maximum of 256 possible combinations then that is the maximum vertical resolution for the PAL video display.
The video hardware is not limited to 256 lines non interlaced, it can do more with overscan. The registers are not 8-bit.

And why if it where an 8-bit limitation is NTSC 320x200? Why not 320x240?

There IS a genuine technical reason for these sizes and I will find out.

Quote:
Originally Posted by redblade
hmm so why would overscan be 704 pixels instead of 720 pixels.
I doubt you can see the pixels anywhere past 704 pixels on a TV. Most tubes do not display the pixels right on the edges. Perhaps they did an experiment and found that 704 was the widest any TV could display?

Or more likely that with 512k its the largest screen you can display and still leave enough room for Workbench?
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Old 05 April 2006, 17:04   #7
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the main video display hardware on the Amiga (the COPPER) was 16bit, but all of it's instructions were 8 bit.


page 18 of the Commodore Amiga Hardware Reference Manual:

The vertical beam position can be resolved to one line, with a maximum resolution of 255. There are actually 262 NTSC (312) possible vertical positions. Some minor complications can occur if you want something to happen within these last six or seven scan lines. Because there are only 8 bits for vertical resolution (allowing 256 different positions), one of the simplest ways to handle this is shown below.

WAIT for position (0,255)
WAIT for position (0,6) [will trigger on scan line 262 (256+6)]


page 30 discusses interlaced displays with the COPPER, as well as page 34 onwards (Playfield hardware).


long way around to the PAL 256 lines, but here goes:

I'm guessing that 320x200 instead of 320x240 was chosen as the 'default' NTSC Amiga resolution due to historical and financial reasons:

Earlier computers had text or character based displays. People were already used to 40x25 text displays on their TV screens from computers such as the Commodore 64. Each character was made up of 8x8 pixels, which gave an effective full screen display resolution of 320x200 pixels. To display 40x25 text characters (total=1000) in just two colour (foreground and background) we need almost 1KB of RAM. For more foreground and backgrounds colours make it another 1KB of RAM for the colour table. These early computers generally had less than 64KB of total memory, so you generally wouldn't want to be using lots of memory just for pretty pictures (or there wouldn't be much left over for program code).

For a bitmapped 320x200 resolution, non-interlaced display (such as used by Workbench, but with just two colours) you need 8000 bytes just for screen memory.

The default bitmapped GUI 4 colour window that opens with a NEWCLI on a lo-res, non-interlaced Amiga (whether PAL or NTSC) is 320x200 pixels and takes up (minimum, without OS overhead) 16,000 bytes.

If you want more colours on screen, then double the amount of memory that needs to be reserved for the screen display for every time you double the maximum amount of colours displayable on screen.

The most common NTSC display mode for games on the Amiga probably would have been the 320x200 x 16 colours = total of 32,000 bytes to be allocated for the screen display. So 1/16th of your total memory is gone just with the screen display.

If you want it interlaced (so you get a higher vertical resolution) then double the total amount of memory again.

Pretty soon you can eat up a large chunk of the 512KB of memory you've got - just to display ONE screen (and we're still not considering the memory used to store the program code, or the graphics in the first place, unless you're loading the data in directly from disk).

So that's why I think that NTSC is generally 320x200 - text wise it is a nice size and still clear enough to read on a lo-res device such as a TV set, and memory wise it is the most economical on a system with limited memory (limited due to the expense).


Anyways - getting to the 'why 256 for PAL' question:

320x256 lines = 8,192 bytes to display a screen - a nice 'round' power of 2 number and easy for computers to handle.

COPPER video timing display is 8 bits, so maximum of 256 possible values.
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Old 05 April 2006, 17:09   #8
gizmomelb
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Quote:
Originally Posted by redblade
hmm so why would overscan be 704 pixels instead of 720 pixels.
I'm guessing the 'missing' lines are generally never displayed on NTSC or PAL TV sets as they're hidden by the shadow masks on all TVs, or it's some internal Amiga video timing thing. (see page 51 or Amiga Hardware Manual).
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Old 05 April 2006, 17:20   #9
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Quote:
Originally Posted by gizmomelb
I'm guessing that 320x200 instead of 320x240 was chosen as the 'default' NTSC Amiga resolution due to historical and financial reasons:
I dont think so... not finished looking yet. Interesting post though. For the time being it's the best proposed idea.

Quote:
320x256 lines = 8,192 bytes to display a screen - a nice 'round' power of 2 number and easy for computers to handle.
Erm, 320x256@1bitplane = 10,240 bytes (10Kbytes)

Quote:
COPPER video timing display is 8 bits, so maximum of 256 possible values.
But as you said you just combine waits to perform overscan...

Last edited by alexh; 05 April 2006 at 17:26.
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Old 05 April 2006, 17:30   #10
Toni Wilen
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Quote:
Originally Posted by gizmomelb
COPPER video timing display is 8 bits, so maximum of 256 possible values.
Only because there is not enough free bits in Copper's WAIT and SKIP-instructions.

Vertical counter VPOSR+VHPOSR is 9-bit (OCS) or 11-bit (ECS and AGA)

btw, in analog video signal there are no "real" horizontal pixels. 400 or so horizontals pixels without blurring is the best regular CRT-based televisions can do.
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