Apple CRT Display Frequency boost?

[Wololo]

Hello,

I Just picked up a 17inch apple CRT (VGA - Diamondtron) monitor and was a little confused by the Hz specs.

These are the factory settings:
"640 x 480 60–85
800 x 600 60–85
832 x 624 75
1024 x 768 60–85
1152 x 870 75
1280 x 1024 75
1600 x 1200 60"

But it says that:
"Your monitor works with video circuitry that operates within these ranges:
horizontal scan rate of 30–85 kilohertz
vertical refresh rate of 48–160 hertz"

Ideally i'd like to run it around 90-100hz vertical at a decently high resolution. Would this be possible?

Thanks

[Chief Blur Buster]

The way your specs displays a range, means your display is a "multisync" CRT. The good news is because it quoted a "Vertical Refresh Rate" range, and the range goes all the way to 160.... It pretty much means you can just skip the 'standard factory settings' -- and create custom resolutions (within specs) to your heart's content as long as they're inside the range.

Custom refresh rates. Your tube display is capable of any custom refresh rate from 48 Hertz through 160 Hertz. The only catch is that the higher the refresh rate (vertical refresh rate), the lower the vertical resolution you can get.

Vertical Refresh Rate is the same thing as traditional refresh rate.
Hoirizontal scan rate is number of pixel rows per second that the display can do.
The blanking interval allowance is usually approximately 5%, so add 5% to Horizontal Scan Rate needed for a Vertical Refresh Rate. Older modes like 480p are 525 scan lines, which is more than 5%, so we'll go with the more conservative number

[taking out a calculator]

The most important number for custom resolution mathematics:
85KHz horizontal scan rate = 85000 pixel rows per second (scanlines per second)

That defines your possible vertical resolution for a given refresh rate target.

640x480 - that's about 525 scan lines per refresh cycle. So 85000 / 525 = You can do up to 160Hz at 640x480

1024x768 -- that's about 805 scan lines per refresh cycle. So 85000 / 805 = You can do up to 105Hz at 1024x768

1920x1080 -- that's about 1125 scan lines per refresh cycle. So 85000 / 1125 = You can do up to 75Hz at 1920x1080

Horizontal resolution doesn't have an effect, as long as you maintain horizontal blanking interval percentage. You can easily do 3840x1080 as easily as 1920x1080 with exactly the same horizontal scan rate and vertical refresh rate. However, things do get very fuzzy at high refresh rates.

Now, working backwards from 90 Hertz, let's see how many scanlines per refresh cycle you can get...
85000 Hz horiz / 90 Hz vertical = 944 pixel rows per refresh cycle.
Allowing roughly 5% for blanking interval, that's about 900 visible scanlines.
Widescreen 16:9 - You can easily do 1600x900 at 90 Hz, with a Vertical Total of 944 in Custom Resolution
Standard 4:3 - You can easily do 1200x900 at 90 Hz, with a Vertical Total of 944 in Custom Resolution

If you wanted to bump that up to 100 Hz,
85000 Hz horiz / 100 Hz vertical = 850 pixel rows per refresh cycle.
Allowing roughly 5% for blanking interval, that's about 810 visible scan lines.
Widescreen 16:9 - You can easily do 1440x810 at 100 Hz, with a Vertical Total of about 850 in Custom Resolution
Standard 4:3 - You can easily do 1080x810 at 100 Hz, with a Vertical Total of about 850 in Custom Resolution

To Create Custom Resolution, you can use a Custom Resolution Utility (CRU) either:
-- NVIDIA Control Panel
-- AMD Catalyst Control Center
-- ToastyX CRU

In the CRU tools, there will be lots of stuff like "VESA GTF" or "Reduced Blanking" which is the automatic math calculations. Use GTF, when it comes to a CRT. However, you will probably have to select "Manual" and slightly reduce the blanking interval size to get more refresh rate. (VBI sizes can go as small as less than 1% on an LCD, but you'll usually need at least 2-3% absolute minimum for a CRT). Too-short VBI sizes will usually make the picture fairly squat, but that's fine for widescreen modes, unless the top/bottom of picture starts distorting. Then you have to use a bigger VBI size to undo the distortion. If you want widescreen (e.g. watching Netflix movies) -- you can do 16:9 on a 4:3 tube by reducing vertical picture height, and a short VBI actually also helps you do that too. You can create multiple custom resolution modes, there's no native resolution for a CRT.

[Wololo]

Thank-you for your in depth reply. You've answered all my questions.i'll be sure to direct people to your answer if I come across anyone who needs help with crt monitors.

[takes out calculator]

[Chief Blur Buster]

You are welcome!

If you are using a digital graphics card, don't forget to get a very good adaptor that maximizes bandwidth -- otherwise you won't be able to create your highest spec modes. And picture could be fuzzier with the worst adaptor. Some places like HardForum.com's Display section are great for Adaptor info. Google through the "Sony FW900 thread" of theirs for more great info on high-end CRTs too.

But if that massive 400 page thread (several thousand posts) makes your head spin, just ask your favourite CRT questions here, experienced CRT tweakers are around here to help too.

Cheers!