Author Topic: 60Hz CPC  (Read 7624 times)

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Offline arnoldemu

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60Hz CPC
« on: 12:09, 19 January 10 »
Were any 60Hz CPC's ever made?

There is a link on the CPC6128 PCB that can be made/broken to define 50Hz or 60Hz and the OS modifies the CRTC values when it recognises this.

In SOFT968 it mentions this is for the American CPCs. Were any American CPCs actually made?
If they were, did they need a special monitor?

The CPC would be outputting horizontal and vertical syncs compatible with 60Hz TV, but R,G,B.

The CPC monitor is effectively a TV anyway, so did American CPC monitors have a NTSC TV inside them and therefore must have had the appropiate IC in them to generate NTSC image?

*or* did it end up being a PAL CPC in America, generating a 60Hz signal but with 220V for power?

If it was 60Hz.. then potentially CPC images were different because of the NTSC artifacts?

Lots of questions.. anybody have any answers?
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Offline ukmarkh

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Re: 60Hz CPC
« Reply #1 on: 13:31, 19 January 10 »
The CPC 6128 was launched in America, the head office was in LA... all the CPC 6128 stock sold out, but Amstrad only used the CPC 6128 to test the water, and prefered to sell the PCW instead.

As the 6128's were bundled with monitors, I imagine they would have been the same as over here.

Offline Bryce

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Re: 60Hz CPC
« Reply #2 on: 14:46, 19 January 10 »
Hi Arnoldemu,
            the CPC monitors are much simpler than even the simplest TVs. The monitors are neither PAL nor NTSC. The green monitor used the PC1031 chip and the colour monitor used a PC1378, these are known as horizontal deflector coil drivers and they produce the signal the screens coil needs to scan the beam. They have a simple RC (Resistor and Capacitor pair) input to adjust the scan frequency (this is what you adjust when you turn the V-Hold adjustment below the screen) and can sync the screen to match the frequency of the input signal. Electrically the US monitors would have been identical to European models, other than the Power supply transformer which would need to have had a 110VAC input. For this reason, the transformer is on a seperate PCB and can be selected in production depending on what country it's being delivered to.

Bryce.

Offline Gryzor

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Re: 60Hz CPC
« Reply #3 on: 13:58, 21 January 10 »
But... weren't those monitors built around TV tubes? At least that's what history says...

Offline Bryce

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Re: 60Hz CPC
« Reply #4 on: 14:32, 21 January 10 »
Yes, it's a TV tube just like any other CRT device, but the circuitry is completely different. A old TV recieves a UHF or VHF signal that needs to be split into RGB / Hsync / Vsync / sound etc. by some complicated circuitry, all this is absent in the CPC monitors because they just aren't required. PAL / NTSC / SECAM are methods of mixing all these RGB and other signals into a single analogue wave form so that they can be transmitted through the air or down a coax cable. The CPC produces RGB and a composite sync signal (V and Hsync in one) that are being fed more or less directly to the tube (the Sync signal is being decoded by the chips I mentioned above). So no UHF/VHF signal ever gets created or decoded, meaning no PAL/NTSC coding/decoding or circuitry is required, so the terms don't really apply when speaking about CPC monitors.

Bryce.

Offline arnoldemu

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Re: 60Hz CPC
« Reply #5 on: 18:35, 21 January 10 »
Yes, it's a TV tube just like any other CRT device, but the circuitry is completely different. A old TV recieves a UHF or VHF signal that needs to be split into RGB / Hsync / Vsync / sound etc. by some complicated circuitry, all this is absent in the CPC monitors because they just aren't required. PAL / NTSC / SECAM are methods of mixing all these RGB and other signals into a single analogue wave form so that they can be transmitted through the air or down a coax cable. The CPC produces RGB and a composite sync signal (V and Hsync in one) that are being fed more or less directly to the tube (the Sync signal is being decoded by the chips I mentioned above). So no UHF/VHF signal ever gets created or decoded, meaning no PAL/NTSC coding/decoding or circuitry is required, so the terms don't really apply when speaking about CPC monitors.

Bryce.
interesting thankyou

So no artifacts and no "loss" of colour.

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Offline Bryce

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Re: 60Hz CPC
« Reply #6 on: 21:03, 21 January 10 »
Absolutely none, the monitor is more or less like a TV with only a (custom) SCART input, the signal is never modulated or demodulated at any time, which is where the loss of quality and artifacts happen. Think of PAL / NTSC like an MP3 music codec, as soon as you compress something there's always a loss of some sort compared to the original. Or just plug your CPC into an MP1/2 / TV setup and you'll soon see how much loss of quality is involved.

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Offline Devilmarkus

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Re: 60Hz CPC
« Reply #7 on: 23:04, 22 January 10 »
In the past I played with the "60hz" bridge on my CPC.
On green monitor it looked ugly (Picture made "waves" on the screen) but on CTM it looked well with black stripes @ top & bottom.

This is what also my emulator does.

Also the emulator speeds a bit up when I set it to 60hz (like the original CPC, it also is faster with 60hz)

So maybe it's a nice feature for emu's...

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Offline Bryce

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Re: 60Hz CPC
« Reply #8 on: 13:22, 24 January 10 »
Why is the CPC faster at 60Hz? The CPU should still be running at the exact same speed? Or are there wait states in the CRTC that are then shorter?

Bryce.

Offline Devilmarkus

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Re: 60Hz CPC
« Reply #9 on: 14:47, 24 January 10 »
I think it's faster because earlier VSync.
You can check this with your CPC @ home:
It has 4 "bridges" on mainboard.
the first 3 are to set the computername.

The one to the right is for changing between 50/60hz.

Picture shows example for my CPC 464 (from 1991)

The 3rd bridge is closed because it's a "Schneider".

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Offline Bryce

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Re: 60Hz CPC
« Reply #10 on: 15:51, 24 January 10 »
Ok. I didn't think that a shorter wait for the Vsync would have such an effect on the speed of the machine, that it would be noticeable in any way to the user. But obviously the Vsync wait is a serious bottleneck in the design. I wonder how much faster the CPC would be if it didn't have to wait at all (ie: Double-Buffered Video RAM)

Bryce.

Offline nocash

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Re: 60Hz CPC
« Reply #11 on: 16:51, 24 January 10 »
What are you talking about there? The machine is, of course, not getting faster with 50 or 60 Hz.

Saying that 60Hz is faster is Gamers-Language, it's true only in so far that most computer/console games with animated graphics are synchronized with vsync. For millions of reasons: To get a constant speed (no matter how many sprites on screen), to get flicker free drawing (in case of single frame buffer), to get smooth scrolling etc. The overall effect is that the "computer looks faster" at 60Hz - at least in most cases. If it just missed a vsync irq, because 60Hz was too fast for the game, then some games might wait for the next vsync, and effectively slowdown to 30Hz.

From the programmers point of view it's vice-versa: 60Hz acts slower, because you need to "waste" more time on vsync irq handling, leaving less time to your actual program code. Whereas, on the CPC, the BIOS is doing some tasks (like keyboard scanning) constantly at 50Hz (even if the frame rate is 60Hz). So, on the CPC, the slowdown in 60Hz mode may be less dramatic as on other computers.

NB. the CPC allows to use the whole 64K as VRAM, so it can have four frame buffers. But even with two or more buffers, games should be ideally still synchronized with vsync for getting smooth animations.

For the official hardware: I'd have expected that CPCs in "60Hz-countries" were jumpered to 60Hz, especially if amstrad did (or did intend) to sell MP1/MP2-style television modulators for that countries.

And the "black bars on top/bottom" that's of course only occuring if you use a 60Hz computer on a 50Hz monitor. On a 60Hz monitor (or multisync TV/Monitor) the picture would be stretched vertically, resulting in lower vertical resolution, different aspect ratio, and no bars.

Offline Bryce

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Re: 60Hz CPC
« Reply #12 on: 10:25, 25 January 10 »
I think Devilmarkus was referring to hardware "waits" rather than software. If the hardware has to wait for the Vsync, then it might make a difference to the over all CPU speed (although I thought this would be extremely minor), from a software point of view it could of course have dis-advantages as you mentioned. But I haven't looked into the hardware in enough detail to see if it really does have an effect. And as far as CPC software is concerned, I haven't a clue, cos' I've never really programmed one, other than a little Basic.

Bryce.

Offline arnoldemu

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Re: 60Hz CPC
« Reply #13 on: 12:00, 25 January 10 »
What are you talking about there? The machine is, of course, not getting faster with 50 or 60 Hz.
Yes true.

Saying that 60Hz is faster is Gamers-Language, it's true only in so far that most computer/console games with animated graphics are synchronized with vsync. For millions of reasons: To get a constant speed (no matter how many sprites on screen), to get flicker free drawing (in case of single frame buffer), to get smooth scrolling etc. The overall effect is that the "computer looks faster" at 60Hz - at least in most cases. If it just missed a vsync irq, because 60Hz was too fast for the game, then some games might wait for the next vsync, and effectively slowdown to 30Hz.
Well yes. But it depends, 60Hz can mean less frame time, but often it also means you draw less lines (e.g. especially on consoles where they draw the entire visible display, drawing a 60Hz screen means less lines overall to draw). What is more correct is that games wait for the vsync to trigger a swap of the screens. So it often means that if the game runs over a frame sometimes, instead of slowing to 25Hz it is "slowing" to 30Hz which is perceptively faster.

From the programmers point of view it's vice-versa: 60Hz acts slower, because you need to "waste" more time on vsync irq handling, leaving less time to your actual program code. Whereas, on the CPC, the BIOS is doing some tasks (like keyboard scanning) constantly at 50Hz (even if the frame rate is 60Hz). So, on the CPC, the slowdown in 60Hz mode may be less dramatic as on other computers.
I disagree here. You don't waste any more or less time on vsync. It is the same. And you're wrong about the BIOS doing some tasks always at 50Hz. In the CPC there is no difference in the gate-array interrupt on 60Hz machines and also no difference to the frame flyback interrupt. On a 50Hz machine it will be 50Hz rate, on a 60Hz machine it will be 60Hz rate. So on the CPC the perceptive speed will be the same.

NB. the CPC allows to use the whole 64K as VRAM, so it can have four frame buffers. But even with two or more buffers, games should be ideally still synchronized with vsync for getting smooth animations.
True in part. More the swapping of the screens should be synchronised with the vsync. You don't need to synchronise the drawing with the frameback. If you use triple buffering you could continue to draw on another screen while waiting to present a new one, but the cpc really doesn't have enough ram to do this.


And the "black bars on top/bottom" that's of course only occuring if you use a 60Hz computer on a 50Hz monitor. On a 60Hz monitor (or multisync TV/Monitor) the picture would be stretched vertically, resulting in lower vertical resolution, different aspect ratio, and no bars.
Really? But the CPC monitor is not a real 50Hz or 60Hz TV as Bryce has already explained?
Or do you think the board with the power on it, would have a 60Hz version of the sync and colour ICs?
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Offline Cholo

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Re: 60Hz CPC
« Reply #14 on: 23:53, 26 January 10 »
Always wondered if there was a bunch of "unplayable" US amstrad games out there.

I recall all the rumors about the ps1 50mhz pal games where badly converted from their japanese 60mhz origins. FF7 had a whole bunch of rumors about there being battles you just couldnt win because they didnt adjust the timelimits to 50mhz video framerate (or something like that).

Getting a 50/60 switch for saturn to play those jap imports games was really importent and Dreamcast and ps2 was clever enough to include the 50/60 options.

The whole "US Amstrad" bit is way before all those troubles.