Flickering CRTs in peripheral vision

[davidg5678]

It's been a few months since I last used one of my classic Mac's CRTs, but I fired up several today and noticed that the screens all flicker slightly. I can only see this flickering in my peripheral vision, not when I look at the screens dead on. As I found the same effect on three different machines, I suspect that this is completely normal and strangely it took me years to notice. Does anyone else notice this issue with their Macs? Is this normal or has something gone wrong? Is there any way to reduce the flickering effect?

 

[pizzigri]

Increase refresh…. It’s the whole point of the 100hz plus crt screens. At 60hz it’s quite noticeable, but it’s much better at the Mac’s 75hz modes

 

[demik]

All CRTs screens are flickering. What differs is how your eyes is able to detect that. It's variable from person to person

I'm able to see most CRTs below 75 Hz flickering, with front vision. Also, the peripheral vision is more light sensitive, which explains why you are able to see it this way.

 

[Watzmann]

The reason is that CRTs "draw" a picture with a single electron beam, speeding line by line across the screen.

The fact that we see a coherent image and not just a moving point is due to the inertia of our vision as well as the afterglow of the screen coating.

A whole image is drawn around 50 - 100 times per second, this is the frame rate of CRTs.

When there were still a lot of CRTs I had big problems with frame rates below or equal 60 Hz - I saw a clear flickering that almost drove me crazy. My colleagues had no problems with it - that's how different individual vision is. When Windows came to offices on the original VGA screens (640x480), I always used a small tool to increase the frame rate from 60 Hz to 67 Hz - a small step for the VGA card, but one giant leap for me

With flat screens, things are completely different - the frame rate describes the frequency with which the whole image is brought "in one go" from the frame buffer to the display. Since the background lighting is practically static, without flickering, this is not perceived as flickering. Frame rates that are too low are more likely to be seen in jerky movements, e.g. of the cursor or games.

 

[davidg5678]

Thank you all for confirming what I suspected. At least this means the video circuits on the three compact macs I tested yesterday didn't spontaneously break! I wonder why I only noticed this flickering effect now, despite having used compact macs for many years? I had assumed that 60hz was way too fast for my eyes to detect. Oh well...

This gets me thinking about modifying the video circuitry on my SE/30 to refresh the CRT slightly faster than normal. I understand that increasing the vertical refresh rate means that the horizontal refresh rate also must increase slightly. I have no idea what the limits of the analog board circuitry on the SE/30 are, but I want to believe a little more speed could be coaxed out of it. If anyone knows more about what this project would entail, I would be curious to hear their thoughts.

 

[Muirium]

What these guys said. I've been playing with compact Macs lately for the first time in decades, and they all flicker quite a bit. Try not to position them in your peripheral vision, it'll drive you nuts! How people used multi-monitor setups back in the day with the weight and bulk and flicker is beyond me.

Oh, and I was there back then, and it still bothered me! Good to hear the compact Macs are 75 Hz. 60 Hz gave me headaches wherever I encountered it. Horrible! Always ran higher on my own computers.

 

[nathall]

I’ve noticed my sensitivity to flicker has gotten worse the older I get. In the late 80s and 90s I never noticed it and could not see it. Now I can perceive anything below around 70-75Hz face on, and it is irritating. My theory is it is related to the natural degradation of you eyes ability to adjust focus with age. The compacts don’t bug me yet, but my IIsi w/ Radius 24XP doing 640x480 at 67Hz sure does.