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Color quirks of the IIGS 640 mode (GSOS Desktop) [message #373640] Sun, 16 September 2018 16:48 Go to next message
STYNX is currently offline  STYNX
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I was wondering if anyone has noticed that dithered colors can be different depending on the column they "start" at? You can see this effect in the GSOS desktop for the standard drive icon. On a CRT there is a light-gray line visible in the icon on the same row as the red dots.

The B-W pattern is simply shifted by 1/640 pixel to W-B (even/odd column) and it results in a visible difference in color (intensity). Are the even/odd pixel columns in 640-mode not identical in width or is there some other trick involved?

I have noticed this effect on color on monochrome monitors. The effect is very visible on an Apple white phosphor monitor. I will try to get a measure the signal by using an oscilloscope.

-Jonas
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #373678 is a reply to message #373640] Mon, 17 September 2018 12:55 Go to previous messageGo to next message
David Schmenk is currently offline  David Schmenk
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On Sunday, 16 September 2018 13:48:36 UTC-7, STYNX wrote:
> I was wondering if anyone has noticed that dithered colors can be different depending on the column they "start" at? You can see this effect in the GSOS desktop for the standard drive icon. On a CRT there is a light-gray line visible in the icon on the same row as the red dots.
>
> The B-W pattern is simply shifted by 1/640 pixel to W-B (even/odd column) and it results in a visible difference in color (intensity). Are the even/odd pixel columns in 640-mode not identical in width or is there some other trick involved?
>
> I have noticed this effect on color on monochrome monitors. The effect is very visible on an Apple white phosphor monitor. I will try to get a measure the signal by using an oscilloscope.
>
> -Jonas

Are you looking on an RGB or composite monitor? On an RGB monitor, you may be getting aliasing against the color mask on the CRT tube. On a composite monitor you will see the effect of the NTSC color cycle aliasing against the resolution of the SHR mode. Here I show how to take advantage of this to create more colors on a color composite monitor in SHR mode: https://github..com/dschmenk/SHR-NTSC

Dave...
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #373691 is a reply to message #373678] Mon, 17 September 2018 15:45 Go to previous messageGo to next message
STYNX is currently offline  STYNX
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On Monday, 17 September 2018 18:55:43 UTC+2, David Schmenk wrote:
> Are you looking on an RGB or composite monitor? On an RGB monitor, you may be getting aliasing against the color mask on the CRT tube. On a composite monitor you will see the effect of the NTSC color cycle aliasing against the resolution of the SHR mode. Here I show how to take advantage of this to create more colors on a color composite monitor in SHR mode: https://github.com/dschmenk/SHR-NTSC
>
> Dave...

The effect is visible in monochrome mode on an Apple II white phosphor as well. It is not the result of the grating of the color mask. I have seen this much more prominent at a meeting on the weekend. The used display was a white phosphor apple II CRT. The difference is more visible with higher contrast settings.

Barely visible on a Sony BVM CRT with normal settings: https://www.flickr.com/photos/88521483@N03/43661825792/in/da teposted-public/

Same CRT with high contrast and low brightness setting: https://www.flickr.com/photos/88521483@N03/29938711278/in/da teposted-public

Apple IIgs CRT with normal settings: https://www.flickr.com/photos/88521483@N03/43818125381/in/da teposted-public/

Apple IIgs CRT with high contrast and low brightness setting: https://www.flickr.com/photos/88521483@N03/43101533994/in/da teposted-public/

High bandwidth CRT: https://www.flickr.com/photos/88521483@N03/41900406080/in/da teposted-public/

I hope I have linked the right pictures ;-)

-Jonas
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #373952 is a reply to message #373691] Sun, 23 September 2018 11:44 Go to previous messageGo to next message
Anonymous
Karma:
Originally posted by: kegs

In article <acba8f10-a1b2-422e-a1dc-799f84401cb9@googlegroups.com>,
STYNX <Jonas.Groenhagen@gmx.de> wrote:
> On Monday, 17 September 2018 18:55:43 UTC+2, David Schmenk wrote:
>> Are you looking on an RGB or composite monitor? On an RGB monitor, you
> may be getting aliasing against the color mask on the CRT tube. On a
> composite monitor you will see the effect of the NTSC color cycle
> aliasing against the resolution of the SHR mode. Here I show how to take
> advantage of this to create more colors on a color composite monitor in
> SHR mode: https://github.com/dschmenk/SHR-NTSC
>>
>> Dave...
>
> The effect is visible in monochrome mode on an Apple II white phosphor
> as well. It is not the result of the grating of the color mask. I have
> seen this much more prominent at a meeting on the weekend. The used
> display was a white phosphor apple II CRT. The difference is more
> visible with higher contrast settings.
>
> Barely visible on a Sony BVM CRT with normal settings:
> https://www.flickr.com/photos/88521483@N03/43661825792/in/da teposted-public/
>
> Same CRT with high contrast and low brightness setting:
> https://www.flickr.com/photos/88521483@N03/29938711278/in/da teposted-public
>
> Apple IIgs CRT with normal settings:
> https://www.flickr.com/photos/88521483@N03/43818125381/in/da teposted-public/
>

I think this is just in the icon for the hard drive. While the GSOS desktop
is being shown, look at memory from e1/2000 onwards. On line 32, offset 0x87
at e1/3487, you can see the main disk icon has the byte pattern: 0x33,0x33,
etc. The pixels are 2-bits wide, so this is the pixels 0,3,0,3,0,3, etc.
where 3=white and 0=black. This alternating white and black will look grey.

The very next line, which has a 0x44 pixel (red) in it has the bytes at
0x3527: 0xcc,0xcc, etc. These pixels are also 3,0,3,0,3,0, etc., but shifted
one full pixel. This pixel shift is what I think you are seeing.

I don't know why the icon was designed this way, but the line with the red
dot has a different dither pattern than the other lines of the icon.

Kent
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #373954 is a reply to message #373952] Sun, 23 September 2018 11:53 Go to previous messageGo to next message
Christopher G. Mason is currently offline  Christopher G. Mason
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On 9/23/2018 11:44 AM, Kent Dickey wrote:
> I think this is just in the icon for the hard drive. While the GSOS desktop
> is being shown, look at memory from e1/2000 onwards. On line 32, offset 0x87
> at e1/3487, you can see the main disk icon has the byte pattern: 0x33,0x33,
> etc. The pixels are 2-bits wide, so this is the pixels 0,3,0,3,0,3, etc.
> where 3=white and 0=black. This alternating white and black will look grey.
>
> The very next line, which has a 0x44 pixel (red) in it has the bytes at
> 0x3527: 0xcc,0xcc, etc. These pixels are also 3,0,3,0,3,0, etc., but shifted
> one full pixel. This pixel shift is what I think you are seeing.
>
> I don't know why the icon was designed this way, but the line with the red
> dot has a different dither pattern than the other lines of the icon.
>
> Kent
>

That is indeed the case looking at direct video captures of the IIgs
desktop with a RGB frame grabber card. They shifted that row because the
palette containing red (for the indicator light) is in that row.
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #373961 is a reply to message #373954] Sun, 23 September 2018 13:37 Go to previous messageGo to next message
STYNX is currently offline  STYNX
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On Sunday, 23 September 2018 17:53:56 UTC+2, Christopher G. Mason wrote:
> That is indeed the case looking at direct video captures of the IIgs
> desktop with a RGB frame grabber card. They shifted that row because the
> palette containing red (for the indicator light) is in that row.

I mentioned that in my first question: why is the line with the alternate B-W pattern a brighter 'gray'? You can not see this brighter line in the icon with a frame grabber or a TFT but with a CRT it is clearly visible. if you cannot see the difference on your CRT, try to reduce the brightness and increase contrast to the max.

-Jonas
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #373964 is a reply to message #373961] Sun, 23 September 2018 14:45 Go to previous messageGo to next message
STYNX is currently offline  STYNX
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On Sunday, 23 September 2018 19:37:24 UTC+2, STYNX wrote:
> On Sunday, 23 September 2018 17:53:56 UTC+2, Christopher G. Mason wrote:
>> That is indeed the case looking at direct video captures of the IIgs
>> desktop with a RGB frame grabber card. They shifted that row because the
>> palette containing red (for the indicator light) is in that row.

here are some close ups with a high bandwidth monitor: https://www.flickr.com/photos/88521483@N03/albums/7215769843 6594262/with/43957961445/

I think it could be that the white of the shifted pattern is a bit brighter OR the pixel is 'longer'. If the pixel width is not the same for even and odd pixel-columns in 640-mode it would explain the brightness difference. Even if the difference is just a few percent, it would be noticeable.

-Jonas
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #374059 is a reply to message #373964] Tue, 25 September 2018 23:48 Go to previous messageGo to next message
Christopher G. Mason is currently offline  Christopher G. Mason
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On 9/23/2018 2:45 PM, STYNX wrote:
> here are some close ups with a high bandwidth monitor: https://www.flickr.com/photos/88521483@N03/albums/7215769843 6594262/with/43957961445/
>
> I think it could be that the white of the shifted pattern is a bit brighter OR the pixel is 'longer'. If the pixel width is not the same for even and odd pixel-columns in 640-mode it would explain the brightness difference. Even if the difference is just a few percent, it would be noticeable.
>
> -Jonas
>

Looks like monitor artifacts from the shadow mask/aperture grill more
then anything.
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #374066 is a reply to message #374059] Wed, 26 September 2018 05:11 Go to previous messageGo to next message
STYNX is currently offline  STYNX
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On Wednesday, 26 September 2018 05:48:30 UTC+2, Christopher G. Mason wrote:
> On 9/23/2018 2:45 PM, STYNX wrote:
>> here are some close ups with a high bandwidth monitor: https://www.flickr.com/photos/88521483@N03/albums/7215769843 6594262/with/43957961445/
>>
>> I think it could be that the white of the shifted pattern is a bit brighter OR the pixel is 'longer'. If the pixel width is not the same for even and odd pixel-columns in 640-mode it would explain the brightness difference. Even if the difference is just a few percent, it would be noticeable.
>>
>> -Jonas
>>
>
> Looks like monitor artifacts from the shadow mask/aperture grill more
> then anything.

The brighter line is consistent on 3 different Monitors with completely different shadow mask/aperture grill. It even shows up on the monochrome composite signal. Monochrome monitors don't have a shadow mask/aperture grill.

-Jonas
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #374068 is a reply to message #374059] Wed, 26 September 2018 05:53 Go to previous messageGo to next message
STYNX is currently offline  STYNX
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On Wednesday, 26 September 2018 05:48:30 UTC+2, Christopher G. Mason wrote:
> Looks like monitor artifacts from the shadow mask/aperture grill more
> then anything.

here are new pictures from the Sanyo MD5912CX 80col monitor:
https://www.flickr.com/photos/88521483@N03/albums/7215769843 6594262

The Sanyo MD5912CX has no shadow mask/aperture grill since it is a monochrome monitor!

-Jonas
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #374069 is a reply to message #374068] Wed, 26 September 2018 06:29 Go to previous messageGo to next message
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On Wednesday, 26 September 2018 11:53:57 UTC+2, STYNX wrote:
> On Wednesday, 26 September 2018 05:48:30 UTC+2, Christopher G. Mason wrote:
>> Looks like monitor artifacts from the shadow mask/aperture grill more
>> then anything.
https://www.flickr.com/photos/88521483@N03/44013352645/in/al bum-72157698436594262/

The pixels of the mouse pointer are of different intensity based on even and odd row...
Monitor: Sanyo MD5912CX with IIgs BW NTSC

-Jonas
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #374078 is a reply to message #374069] Wed, 26 September 2018 14:46 Go to previous messageGo to next message
David Schmenk is currently offline  David Schmenk
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On Wednesday, 26 September 2018 03:29:45 UTC-7, STYNX wrote:
> On Wednesday, 26 September 2018 11:53:57 UTC+2, STYNX wrote:
>> On Wednesday, 26 September 2018 05:48:30 UTC+2, Christopher G. Mason wrote:
>>> Looks like monitor artifacts from the shadow mask/aperture grill more
>>> then anything.
> https://www.flickr.com/photos/88521483@N03/44013352645/in/al bum-72157698436594262/
>
> The pixels of the mouse pointer are of different intensity based on even and odd row...
> Monitor: Sanyo MD5912CX with IIgs BW NTSC
>
> -Jonas

The VGC outputs 640 SHR pixels in the same duration as the 560 Apple II modes. I don't know which source the VGC is clocked off, but perhaps there is an internal division that requires a slightly asymmetrical pixel output?
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #374105 is a reply to message #374078] Thu, 27 September 2018 08:53 Go to previous messageGo to next message
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On Wednesday, 26 September 2018 20:46:17 UTC+2, David Schmenk wrote:

> The VGC outputs 640 SHR pixels in the same duration as the 560 Apple II modes. I don't know which source the VGC is clocked off, but perhaps there is an internal division that requires a slightly asymmetrical pixel output?

I am thinking that might be the case. The 320-mode does mot have this effect. The frequency devision seems to be symmetrical in 320-mode but maybe a further division can only be archived asymmetrical?

I find I very interesting that there are a lot of people who deny any effect at all and think that this is an effect stemming from the used monitor. Im still baffled why no-one has mentioned this before? I would say that the ratio might even be as high as 60/40 if comparing the pixels on a monochrome display... maybe this is another reason why the 640 mode patterns are blended on a CRT under certain conditions?

https://www.flickr.com/photos/88521483@N03/44013352645/in/da teposted-public/

-Jonas
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #374179 is a reply to message #374078] Sat, 29 September 2018 20:23 Go to previous messageGo to next message
Anonymous
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Originally posted by: Doug Dingus

> VGC outputs 640 SHR pixels in the same duration as the 560 Apple II modes.

That is very likely the explanation. Old Apple video os NTSC color cycle aligned. The GS is not.

Most displays sample at 13.5Mhz and depend on the pixel clock being well aligned with the color clocks. Had the GS just come with a wider screen, these artifacts would not be present.

The extra pixels should have come with a smaller border.
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #374196 is a reply to message #374179] Sun, 30 September 2018 11:42 Go to previous messageGo to next message
Charlie is currently offline  Charlie
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On 9/29/2018 8:23 PM, Doug Dingus wrote:
>> VGC outputs 640 SHR pixels in the same duration as the 560 Apple II modes.
>
> That is very likely the explanation. Old Apple video os NTSC color cycle aligned. The GS is not.
>
> Most displays sample at 13.5Mhz and depend on the pixel clock being well aligned with the color clocks. Had the GS just come with a wider screen, these artifacts would not be present.
>
> The extra pixels should have come with a smaller border.
>

For what it is worth, the effect Jonas describes is clearly visible on
my 19" monitor in both widescreen and 4:3 modes. That's IIGS RGB
through a SCART cable connection.

http://noboot.com/charlie/CS_MENU/CS_GS_SCART.html

Charlie
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #374270 is a reply to message #374179] Tue, 02 October 2018 12:58 Go to previous messageGo to next message
STYNX is currently offline  STYNX
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On Sunday, 30 September 2018 02:23:09 UTC+2, Doug Dingus wrote:
>> VGC outputs 640 SHR pixels in the same duration as the 560 Apple II modes.
>
> That is very likely the explanation. Old Apple video os NTSC color cycle aligned. The GS is not.
>
> Most displays sample at 13.5Mhz and depend on the pixel clock being well aligned with the color clocks. Had the GS just come with a wider screen, these artifacts would not be present.
>
> The extra pixels should have come with a smaller border.

Analog displays don't _sample_, they just display what the amplifiers send through. Most digital displays don't show the effects I have described. The SyncMaster 940MW, that Charlie mentioned is the first display that seems to have a decent enough capabilities to view IIgs RGB. I just bought one :-)

The alignment to the NTSC color burst is only relevant in NTSC color video (CVBS/composite) monitors and will only result in mismatched colors if alignment is off. Old analog monitors have a band width that is a result of the speed of the amplifiers and the installed filters. Most older monochrome 80col displays have a switch on the back to 'enable' 80 col. mode. In most cases this just deactivates a filter that is used to reduce unwanted noise in the 40col. mode (it limits the bandwidth with analog filters).

Modern LCD/TFT based displays use a sampling frequency that can be widely tuned to get the pixels of the video signal. The frequency can usually be modified manually when VGA-input is used. The sampling frequency can also be shifted by using the 'phase' setting. Since the sampling of RGB via SCART or any other 15khz analog input is usually geared towards TV, gaming consoles and VCR, the IIgs is often sampled at too low frequencies (the mentioned 13.5-14.5Mhz with minimal or no automatic correction). The pixels in the signal are not well aligned with the sampling frequency of the signal. There is (mostly) no manual correction for SCART RGB input. There might be a chance to get a good output if the monitor takes 15khz VGA.

-Jonas
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #374273 is a reply to message #374270] Tue, 02 October 2018 16:40 Go to previous messageGo to next message
Michael J. Mahon is currently offline  Michael J. Mahon
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STYNX <Jonas.Groenhagen@gmx.de> wrote:
> On Sunday, 30 September 2018 02:23:09 UTC+2, Doug Dingus wrote:
>>> VGC outputs 640 SHR pixels in the same duration as the 560 Apple II modes.
>>
>> That is very likely the explanation. Old Apple video os NTSC color
>> cycle aligned. The GS is not.
>>
>> Most displays sample at 13.5Mhz and depend on the pixel clock being well
>> aligned with the color clocks. Had the GS just come with a wider screen,
>> these artifacts would not be present.
>>
>> The extra pixels should have come with a smaller border.
>
> Analog displays don't _sample_, they just display what the amplifiers
> send through. Most digital displays don't show the effects I have
> described. The SyncMaster 940MW, that Charlie mentioned is the first
> display that seems to have a decent enough capabilities to view IIgs RGB.
> I just bought one :-)
>
> The alignment to the NTSC color burst is only relevant in NTSC color
> video (CVBS/composite) monitors and will only result in mismatched colors
> if alignment is off. Old analog monitors have a band width that is a
> result of the speed of the amplifiers and the installed filters.

The video amplifiers were seldom a limiting factor, but the need to
separate (and suppress) both the 3.58MHz chroma sidebands and the 4.5MHz FM
sound subcarrier necessitated the bandwidth limitation.

If comb filters had been more affordable, the luminance bandwidth could
have been extended from 3+MHz to 4+MHz, but the cost/benefit ratio just
wasn’t there.

> Most older monochrome 80col displays have a switch on the back to 'enable'
> 80 col. mode. In most cases this just deactivates a filter that is used
> to reduce unwanted noise in the 40col. mode (it limits the bandwidth with analog filters).

This must not have been very common. I don’t think I ever saw such a switch
in over a dozen different monitors. The only justification would have been
to deemphasize the “picket fence” in color areas, since there really is no
“noise” to suppress in 40-column modes.

> Modern LCD/TFT based displays use a sampling frequency that can be widely
> tuned to get the pixels of the video signal. The frequency can usually be
> modified manually when VGA-input is used. The sampling frequency can also
> be shifted by using the 'phase' setting. Since the sampling of RGB via
> SCART or any other 15khz analog input is usually geared towards TV,
> gaming consoles and VCR, the IIgs is often sampled at too low frequencies
> (the mentioned 13.5-14.5Mhz with minimal or no automatic correction). The
> pixels in the signal are not well aligned with the sampling frequency of
> the signal. There is (mostly) no manual correction for SCART RGB input.
> There might be a chance to get a good output if the monitor takes 15khz VGA.
>
> -Jonas
>
Yes, I’ve often wished more digital monitors had an option to “tune” the
sampling frequency.

--
-michael - NadaNet 3.1 and AppleCrate II: http://michaeljmahon.com
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #374297 is a reply to message #374273] Wed, 03 October 2018 15:10 Go to previous messageGo to next message
STYNX is currently offline  STYNX
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On Tuesday, 2 October 2018 22:40:53 UTC+2, Michael J. Mahon wrote:
> ...
> The video amplifiers were seldom a limiting factor, but the need to
> separate (and suppress) both the 3.58MHz chroma sidebands and the 4.5MHz FM
> sound subcarrier necessitated the bandwidth limitation.
> ...
Im Peking more about RGB monitors. I have experienced a lot of 'bleeding' on monochrome and color monitors if the drivers for the cathodes were getting near their max output. Just reduce brightness ... a lot... and increase contrast a bit until you can barely see the image. you can get very crisp visualization on the screen that way. The drivers, amplifiers and filters are just a part of the degradation of the signal. The phosphor itself will be responsible for a lot of the blur and blending. BUT I think the circuit is very responsible as I have seen very crisp images on a high-bandwidth analog monitor that was designed for 15-50khz, despite very high brightness. But im no expert and there is a lot I don't know.

> ...
> This must not have been very common. I don’t think I ever saw such a switch
> in over a dozen different monitors. The only justification would have been
> to deemphasize the “picket fence” in color areas, since there really is no
> “noise” to suppress in 40-column modes.
> ...

I have seen digital hf noise (like periodic snow when the ram was accessed or when the serial line was used) on cheap 80 monitors that were just remodeled televisions. The better 80col monitors had often a 80/40 switch at either the back or in the front under a latch together with the controls. In 80col mode you could see more of the snow than in 40col mode. I had suspected the these switches were meant for noise reduction. The 80/40 monitors that I have seen were often from Philips as they seem to have produced a _lot_ of monochrome monitors for the euro-market. (I had around 7 different 80col monochrome monitors from Philips and just 1 from Sanyo).

-Jonas
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #374298 is a reply to message #374297] Wed, 03 October 2018 18:06 Go to previous messageGo to next message
Michael J. Mahon is currently offline  Michael J. Mahon
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STYNX <Jonas.Groenhagen@gmx.de> wrote:
> On Tuesday, 2 October 2018 22:40:53 UTC+2, Michael J. Mahon wrote:
>> ...
>> The video amplifiers were seldom a limiting factor, but the need to
>> separate (and suppress) both the 3.58MHz chroma sidebands and the 4.5MHz FM
>> sound subcarrier necessitated the bandwidth limitation.
>> ...
> Im Peking more about RGB monitors. I have experienced a lot of 'bleeding'
> on monochrome and color monitors if the drivers for the cathodes were
> getting near their max output. Just reduce brightness ... a lot... and
> increase contrast a bit until you can barely see the image. you can get
> very crisp visualization on the screen that way. The drivers, amplifiers
> and filters are just a part of the degradation of the signal. The
> phosphor itself will be responsible for a lot of the blur and blending.
> BUT I think the circuit is very responsible as I have seen very crisp
> images on a high-bandwidth analog monitor that was designed for 15-50khz,
> despite very high brightness. But im no expert and there is a lot I don't know.

Most video amplifiers will widen an input pulse if overdriven. My advice
for adjusting brightness and contrast is to first adjust the brightness
just a little below where blacks are invisible (no scan lines), then adjust
the contrast until the highlights are just bright enough to be
satisfactory. This both ensures that the video amplifiers are operating in
their linear regions and that the CRT is not being overdriven.

Since the cross section of the (focused) electron beam is roughly Gaussian,
overdriving the CRT will result in an apparent widening of the beam,
comparable to defocusing.

If a large area of the screen is high brightness, another defocusing effect
can result from a drop in the high voltage supply due to excessive current
draw. This also results in the raster enlarging (since a lower acceleration
voltage makes the beam easier to deflect), resulting in the effect being
called “blooming”. Good color monitors usually have well regulated high
voltage (to maintain good convergence) and so are less prone to blooming
than monochrome monitors.

>> ...
>> This must not have been very common. I don’t think I ever saw such a switch
>> in over a dozen different monitors. The only justification would have been
>> to deemphasize the “picket fence” in color areas, since there really is no
>> “noise” to suppress in 40-column modes.
>> ...
>
> I have seen digital hf noise (like periodic snow when the ram was
> accessed or when the serial line was used) on cheap 80 monitors that were
> just remodeled televisions. The better 80col monitors had often a 80/40
> switch at either the back or in the front under a latch together with the
> controls. In 80col mode you could see more of the snow than in 40col
> mode. I had suspected the these switches were meant for noise reduction.
> The 80/40 monitors that I have seen were often from Philips as they seem
> to have produced a _lot_ of monochrome monitors for the euro-market. (I
> had around 7 different 80col monochrome monitors from Philips and just 1 from Sanyo).

Curious. I have never seen such noise on Apple video. I would expect
low-amplitude noise to be very near the black level, which I always adjust
to be just below the monitor’s cutoff, which would render it invisible.

The only video “noise” I’ve seen is either from RAM access glitching on ][+
80-column cards or a result of a bad ground connection on a video cable.

Perhaps you were experiencing EMI from the Apple to the monitor.

If you look at the Apple video signal with an oscilloscope, it looks quite
clean.

--
-michael - NadaNet 3.1 and AppleCrate II: http://michaeljmahon.com
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #374308 is a reply to message #374298] Thu, 04 October 2018 13:43 Go to previous messageGo to next message
STYNX is currently offline  STYNX
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On Thursday, 4 October 2018 00:06:44 UTC+2, Michael J. Mahon wrote:
> ...
> If you look at the Apple video signal with an oscilloscope, it looks quite
> clean.
> ...

The Apple II series gets a lot of hate for bad video quality but it is actually very good if you use a monochrome display. The PAL Apple IIe is not as clean as the NTSC models but still a lot better than other 8-bitters of the 80s. The video circuit of some 80 col. cards is not well isolated and may catch stray signals from other cards nearby.I did not mean to imply that the A2 was generally noisy but that were some other home computers that have a noise problem on the video signal.
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #374392 is a reply to message #374270] Sun, 07 October 2018 18:43 Go to previous messageGo to next message
Anonymous
Karma:
Originally posted by: Doug Dingus

> Analog displays don't _sample_, they just display what the amplifiers send through.

Yup, forgive me for not being clear.

Analog displays do exactly what you say, and that odd timing does put luma and color info together at times not synchronized to the period of the waves composing the video signal. That will introduce artifacts.
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #374417 is a reply to message #374392] Mon, 08 October 2018 13:40 Go to previous messageGo to next message
STYNX is currently offline  STYNX
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On Monday, 8 October 2018 00:43:28 UTC+2, Doug Dingus wrote:
>> Analog displays don't _sample_, they just display what the amplifiers send through.
>
> Yup, forgive me for not being clear.
>
> Analog displays do exactly what you say, and that odd timing does put luma and color info together at times not synchronized to the period of the waves composing the video signal. That will introduce artifacts.

the NTSC was set to monochrome and has no color information. The RGB are just 3 monochrome signals that are used for the color generation by the monitor.
Here is a good quality image of monochrome pixels on a Sanyo green monitor:
https://www.flickr.com/photos/88521483@N03/44461421234/in/al bum-72157698436594262/

The brightness/width difference of the pixels is clearly visible...

-Jonas
Re: Color quirks of the IIGS 640 mode (GSOS Desktop) [message #374418 is a reply to message #374417] Mon, 08 October 2018 13:45 Go to previous message
STYNX is currently offline  STYNX
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Registered: October 2012
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On Monday, 8 October 2018 19:40:55 UTC+2, STYNX wrote:
> Here is a good quality image of monochrome pixels on a Sanyo green monitor:
> https://www.flickr.com/photos/88521483@N03/44461421234/in/al bum-72157698436594262/

different exposure:
https://www.flickr.com/photos/88521483@N03/44461591924/in/al bum-72157698436594262/

-Jonas
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