Acorn Archimedes

[hubersn]

Maybe the "not 24 MHz" is the root of the "not syncable on 15 kHz monitors"? Could you try 24 MHz, MonitorType 0 or 1, and e.g. Mode 12, if it then outputs a proper PAL TV compatible output?

Maybe the bigger issue is 15 kHz needs composite sync.

Not for me - I connected the MIST to all of my 15 kHz capable monitors, and all of them are happy with seperate syncs (the NEC 3D even needs it, it displays the screen too high if composite sync is used on real Archies) on real Archies, but don't work with MIST.

BTW, sync output is configurable on real Archimedes with configure sync 0 for seperate sync (or hold down t at bootup) and configure sync 1 for composite sync (or hold down r at bootup). However, ISTR there were also some jumpers involved for the old models (A3000 and before).

Have fun
hubersn

[hubersn]

...
Those 400 lines@70 Hz is just what your monitor reports, it is not the truth about the real video mode, which is almost certainly 320x256 or 640x256.
...

These are not TV-standard resolution either...

What are (PAL) TV standard resolutions in your opinion?

In my view, everything that a typical RGB-input CRT can sync to is "PAL TV standard", which is usually 15625 Hz line frequency and either interlaced or progressive signal at 25 or 50 Hz. Be that an Amstrad CPC with 160x200@50Hz or an Archimedes with 640x256@50Hz or a Sega MegaDrive with 320x240@50Hz.

hubersn

[slingshot]

Configurable sync type output is not implemented in the core, however even a *Configure MonitorType 0 doesn't have an effect on the video mode. Maybe the EDID data is hardcoded somewhere, just don't know where. How the Archie reads it?

But here's a core with 24MHz VIDC clock, try it if you want. It's much worse on VGA. Disabling scandoubler (pressing the middle button) even will switch to composite sync now.

[DrOG]

What are (PAL) TV standard resolutions in your opinion?

In my view, everything that a typical RGB-input CRT can sync to is "PAL TV standard", which is usually 15625 Hz line frequency and either interlaced or progressive signal at 25 or 50 Hz. Be that an Amstrad CPC with 160x200@50Hz or an Archimedes with 640x256@50Hz or a Sega MegaDrive with 320x240@50Hz.

hubersn

It doesn't matter what my opinion is! The only important thing is what the standards prescribe:

1. In case of PAL system it should be 625 total lines, which means 576i or 288p visible lines at 50Hz.

2. NTSC standard means 525 total lines, in practice it's 480i/240p at 60Hz.

There's no real horizontal resolution, instead there's a bandwidth, which means in practice picture quality. Around 3MHz it's acceptable, 5MHz is near old studio quality.

[hubersn]

It doesn't matter what my opinion is! The only important thing is what the standards prescribe:

1. In case of PAL system it should be 625 total lines, which means 576i or 288p visible lines at 50Hz.

2. NTSC standard means 525 total lines, in practice it's 480i/240p at 60Hz.

There's no real horizontal resolution, instead there's a bandwidth, which means in practice picture quality. Around 3MHz it's acceptable, 5MHz is near old studio quality.

So, with that as our working definition, why do you think the standard TV modes on RISC OS like 640x256@50Hz do not fit the PAL TV standard? The A3000 e.g. even had a BAS output, i.e. monochrome PAL compatible composite signal. Works fine on any PAL TV.

hubersn

[desUBIKado]

Configurable sync type output is not implemented in the core, however even a *Configure MonitorType 0 doesn't have an effect on the video mode. Maybe the EDID data is hardcoded somewhere, just don't know where. How the Archie reads it?

But here's a core with 24MHz VIDC clock, try it if you want. It's much worse on VGA. Disabling scandoubler (pressing the middle button) even will switch to composite sync now.

Works! We have already RGB output on Archimedes core.

Tip: You must repeatedly press the 0 key on the numeric keypad at boot time for the monitor to go into TV mode

And as I said previously:

1. when I try to load a floppy from OSD, "Archie" isn't default folder. Which is it?
2. A reset option would be welcome.

[hubersn]

But here's a core with 24MHz VIDC clock, try it if you want. It's much worse on VGA. Disabling scandoubler (pressing the middle button) even will switch to composite sync now.

Thanks for your efforts, it'll take a few days until I can test this.

Have fun
hubersn

[slingshot]

Works! We have already RGB output on Archimedes core.

Tip: You must repeatedly press the 0 key on the numeric keypad at boot time for the monitor to go into TV mode

And as I said previously:

1. when I try to load a floppy from OSD, "Archie" isn't default folder. Which is it?
2. A reset option would be welcome.

Indeed, but it's OK with 25 MHz, too. Only the composite sync is required for my TV. If anyone can point out how the monitor type detection works in Archie, I would modify the core to boot with type 0 if scandoubler is in disabled state.

The two questions:
- I think nothing is set in the firmware for the Archie core type.
- I would also like a reset, but the Amber CPU doesn't have a reset signal at all. Cite from opencores.org:

For example there is no reset logic, all registers are reset as part of FPGA
initialization.

The only way currently is to use MiST reset (left button).

[hubersn]

If anyone can point out how the monitor type detection works in Archie, I would modify the core to boot with type 0 if scandoubler is in disabled state.

The settings are from CMOS RAM, not from ROM (not sure where from CMOS is initialized if CMOS reset is done!). Autodetection of monitors via VGA 15pin ID pins was only added in later machines (A5000 and later), A3000 and earlier worked strictly from CMOS config.

CMOS values (240 bytes) are hardcoded inside core IIRC from looking at the sources. See here for which values do what:
http://www.riscos.com/support/developer ... html#40894

CMOS is in the clock chip, a standard Philips PCF8583 connected via I2C.

EDIT I found the old discussion in Stardot forum where I tried to get to the ground of that issue back then... https://stardot.org.uk/forums/viewtopic.php?t=15398

Have fun
hubersn

[DrOG]

Hi!

The 25kHz core's interlaced output rolls over SCART, but it's VGA-combatible in prógressive mode.

The 24kHz core has a VGA-incompatible video (640x500@57Hz) in progressive mode, games run in strange video resolution/refresh rates as well (720x400@67Hz), but pressing numpad 0 many times during boot the 576i mode is perfect over SCART and component, not only the desktup, but games also... Nice!

[slingshot]

Hi!

The 25kHz core's interlaced output rolls over SCART, but it's VGA-combatible in prógressive mode.

The 24kHz core has a VGA-incompatible video (640x500@57Hz) in progressive mode, games run in strange video resolution/refresh rates as well (720x400@67Hz), but pressing numpad 0 many times during boot the 576i mode is perfect over SCART and component, not only the desktup, but games also... Nice!

The rollover is because of no composite sync (no vsync on the hsync pin), but seems 25MHz is OK for TV then, no need for adjustable frequency. Or I can add a 24/25 MHz switch tied to the scandoubler switch, so scandoubler ON - 24 MHz with composite sync, scandoubler OFF - 25 MHz with separate sync.

[hubersn]

Or I can add a 24/25 MHz switch tied to the scandoubler switch, so scandoubler ON - 24 MHz with composite sync, scandoubler OFF - 25 MHz with separate sync.

Most faithful to the original hardware and its configuration options as well as maximizing screen compatibility in the absence of a true CMOS RAM, I would indeed propose to tie the following things to the scandoubler config:

• Scandoubler on means separate sync and monitortype 4 and 25.175 MHz VIDC clock, maximizing compatibility with VGA
• Scandoubler off means composite sync and monitortype 1 and 24 MHz VIDC clock, maximizing compatibility with Scart/15 kHz RGB screens as well as keeping it as close to the original hardware so that originally used multiscan monitors can still use the Multiscan modes (18-21) and the not-quite-VGA modes (25-28)

Thanks for your efforts so far!

Have fun
hubersn

[DanyPPC]

Tested successfully the 24mhz core pressing "0" at boot (thanks DrOG for the trick) with YPbPr cable I have a 720 x 576 50i stable image, even on games.

[hubersn]

Or I can add a 24/25 MHz switch tied to the scandoubler switch, so scandoubler ON - 24 MHz with composite sync, scandoubler OFF - 25 MHz with separate sync.

Most faithful to the original hardware and its configuration options as well as maximizing screen compatibility in the absence of a true CMOS RAM, I would indeed propose to tie the following things to the scandoubler config:

• Scandoubler on means separate sync and monitortype 4 and 25.175 MHz VIDC clock, maximizing compatibility with VGA
• Scandoubler off means composite sync and monitortype 1 and 24 MHz VIDC clock, maximizing compatibility with Scart/15 kHz RGB screens as well as keeping it as close to the original hardware so that originally used multiscan monitors can still use the Multiscan modes (18-21) and the not-quite-VGA modes (25-28)

Thinking about this a bit further, you'd also want the following CMOS settings by default:

• Scandoubler on means Mode 27 and WimpMode 27
• Scandoubler off means Mode 12 and WimpMode 12

I think, by default, MonitorType 1 would lead to Mode 20 and WimpMode 20, but I'm not sure who sets what based on what and when in RISC OS...

Have fun
hubersn

[DrOG]

Tested successfully the 24mhz core pressing "0" at boot (thanks DrOG for the trick) with YPbPr cable I have a 720 x 576 50i stable image, even on games.

I took the idea from hubersn, so the credit goes to him!

@slingshot:
The idea of a 24/25 MHz switch tied to the scandoubler switch sounds great!

[DanyPPC]

Ok, thanks hubersn

[slingshot]

Btw, VIDC enhancer was implemented with 36/25/24 MHz clock. Just the software method to make it switch needs some app for RISC OS, which cannot be autostarted in the current setup. Would be good to have a sane way to use it.

[hubersn]

Btw, VIDC enhancer was implemented with 36/25/24 MHz clock. Just the software method to make it switch needs some app for RISC OS, which cannot be autostarted in the current setup. Would be good to have a sane way to use it.

RISC OS 3.1 must be able somehow to detect if it can software-switch, since it does this out-of-the-box for A5000/A4000/A3010/A3020/A4. I wil find out how this works and will get back to you.

Have fun
hubersn

[slingshot]

Great! Maybe the only thing needed is to reactivate it. But I doubt it would switch to 36MHz by itself.

[slingshot]

Btw, did you notice that in TV resolutions, some games are actually run slow? Check Cannon Fodder, the great title music is played like cat meow.

[DanyPPC]

On my TV LCD with YPbPr Cable Chuck Rock run slow, altough on VGA it runs at full speed.

[hubersn]

Great! Maybe the only thing needed is to reactivate it. But I doubt it would switch to 36MHz by itself.

OK, found some info, asked the experts.... RISC OS 2.01 onwards does not try to detect if it has multiple VIDC clocks available to switch. It just writes to a latch and leaves it to the hardware to react properly on it. All RISC OS versions starting with 2.01 have an extended screen mode list which carries information about the pixel rate needed for this mode - e.g. 36000 kHz for Mode 31 (800x600@56Hz).

Here
http://chrisacorns.computinghistory.org ... A4_TRM.pdf
on page 12 (1-8) you can see how it works - the Video control latch is mapped into address &3350048 where four bits control sync polarity and clock speed.

However, this latch is only present if IOEB is present, and I guess that the current Archie core does not have it. Not sure if you can fake it easily somehow.

Maybe there is generally a clock frequency problem here? Original Archie hardware had only that 24 MHz crystal and derived all other clocks from there, both memory and CPU being always clocked the same (8 MHz). Later hardware split that up (that is the job of IOEB) into 12 MHz for Memory and whatever for CPU and the three different frequencies for VIDC.

Hope that helps somehow...although it might confuse things further!
hubersn

[Chris23235]

I can confirm, that it works on a Commodore 1081 RGB monitor, many thanks Slingshot.

[hubersn]

On my TV LCD with YPbPr Cable Chuck Rock run slow, altough on VGA it runs at full speed.

Not sure about Chuck Rock, but early games often synced with screen refresh rate, so "slow" would be "as expected for the 50Hz refresh it was designed for" and VGA (the letterboxed modes are 70Hz IIRC) would be too fast. I dimly remember a software module that upclocked the screen refresh in the letterbox modes to 100Hz and provided a fake VSync every second screen refresh. But obviously not all screens liked the 100Hz refresh rate.

Have fun
hubersn

[DanyPPC]

In Chuck Rock case the difference is evident.
In vga mode it runs like on Amiga, but @ 50Hz it slow downs.