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robert.rozee
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Joined: 31/12/2012
Location: New Zealand
Posts: 2442

Posted: 12:28am 26 Aug 2020

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TassyJim said I used this as a reference:
"VESA and Industry Standards and Guidelines
for Computer Display Monitor Timing (DMT)
Version 1.0, Rev. 12
November 17, 2008"

It lists 640x480 60Hz as

Quote Adopted: n/a ** For Reference Only - Not a VESA Standard **
Resolution: 640 x 480 at 60 Hz (non-interlaced)

Either way 75Hz is a standard and any modern monitor should be able to display it.

Jim

i think i see where where the wheels started coming off the wagon...

VGA specifies:
1. a physical connector and pin assignment standard: 15-pin D-SUB, allocation of H-sync, V-sync, R, G, B, etc pins, voltage levels.
2. a set of signal specifications: dot-clock of 25.175 MHz or 28.322 MHz, resolutions up to 640x480, other timing details, etc.

collectively, this is the VGA standard as defined by IBM that has become a baseline for everything that has the 15-pin D-SUB socket on the back.

VESA (in the context under discussion) specifies a set of extended signal specifications, that:
(a) includes the IBM original specifications, while,
(b) adding on a range of supplemental modes to allow for higher resolutions and a wider range of dot-clocks.

an important part of the VESA extensions is that all equipment does NOT have to support all supplemental modes to be VESA compliant, and that the VESA standard allows for adding further modes to the standard later on.

that is, when you talk about VESA, in practice this means VESA + VGA. because VESA extends VGA, it also includes all the existing/underlying VGA specifications.

you will find that some equipment will only support some of the VESA modes, and in a cost-conscious industry unnecessary/duplucate bits will be left out. 640x480/60Hz is pretty essential to a PC as it was the fallback mode up to and including win2k.

but 640x480/75Hz uses a dot-clock of 31.5MHz, which is not in the original IBM VGA specifications. if a monitor only implements the 800x600 and 1024x768 VESA extensions, it has little reason to support this particular dot-clock rate. it is a mode that will, in practice, only ever be selected by a curious user twiddling with the monitor settings in windows.

plus, with the advent of LCD monitors, higher refresh rates became far less meaningful. essentially, LCD monitors do not suffer from flicker in the same way as CRT monitors did, and the sole reason to go for higher frame rates (such as 120Hz), is to better represent fact-moving action in sports events. there is no real reason for a monitor manufacturer to support 640x480/75Hz as it provides no enhancement over the same resolution at 60Hz.

(as an aside, the word should has been used to explain the loss of many a battle in times of war - usually by the side that came in second place)

cheers,
rob � :-)
Edited 2020-08-26 10:36 by robert.rozee

panky

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Joined: 02/10/2012
Location: Australia
Posts: 1115

Posted: 03:36am 26 Aug 2020

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Adding to Rob's well presented info above, virtually all monitors of current times support an interactive mode where on initial connection, there is a 'discussion' between the monitor and the driving source about what video modes are supported (for details, see the VESA documentation here ).

In the STM LCD-TFT Controller that is an integral part of the H743 chip used in the CMM2, we just "send" the video data - analog RGB and horizontal and vertical sync. It is up to the monitor to figure out on its own how to manipulate this data to fit on its screen and how to manage its internal clocking accordingly.

The reality is that some monitors do this better than others - certainly, all modern monitors should (there's that word again Rob �

�

) fully support the video from the CMM2 as the timing of the signals conform to the VESA standard as Peter has already pointed out.

Why then do some monitors produce jitter or miss pixels on top bottom or sides? Because they do not, to 100%, conform to the VESA standard for whatever reason.

From the CMM2's perspective, we have done our best to send you, Mr. Monitor, what we believe to be correctly timed video to the best our our (STMs LCD-TFT embedded controller) ability! If you can't display it correctly, not my (CMM2) problem.
Edited 2020-08-26 13:39 by panky

... almost all of the Maximites, the MicromMites, the MM Extremes, the ArmMites, the PicoMite and loving it!

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