I am experiencing display interference on a dell 24" flat panel monitor.I find that if I adjust the pixel clock settings up or down in the monitor's on-screen menus, the interference goes away for a while.

The monitor is attached to a Macbook Pro using a mini display to VGA adapter. I have found that in a different house, I get the interference problem less so it might be related to electricity supply or possibly even ethernet powerline (total guess).

What does the pixel clock setting actually do and does this behaviour point to a likely cause of the interference?


The pixel clock adjusts how wide the input pixels are. VGA is an analog input, there are no clear-cut boundaries between pixels and the monitor must guess.

If the clock setting gets misadjusted, your display gets blurry and that’s probably why it no longer causes interference.

To adjust the clock and phase settings properly, view a pixel checkerboard on your screen and use the “Auto” option in your monitor menus.

Also, have you considered connecting your screen via DVI?

  • re DVI, yes. I see snow / interference with that connection too. – codecowboy Nov 7 '13 at 8:47
  • @codecowboy DVI and doesn't suffer from those sorts of artifacts, so your monitor is probably boned. – ta.speot.is Nov 7 '13 at 8:50
  • I can see interference when displaying large patterns on my monitor too. It is more likely cheaply made than broken. DVI won’t help problems with your monitor, but it will help with interference from your graphics card. – kinokijuf Nov 7 '13 at 8:58
  • Interference can also be a result of not using the panel's native resolution. The image has to be scaled which can introduce artifacts. – Brian Nov 13 '13 at 19:48
  • @ta.speot.is Where'd you get the idea that DVI has no pixel clock? La Wik, my italics: "Three of the links represent the RGB components – red, green, and blue – of the video signal for a total of 24 bits per pixel. The fourth link carries the pixel clock. The binary data is encoded using 8b10b encoding. DVI does not use packetization, but rather transmits the pixel data as if it were a rasterized analog video signal." – Aaron Miller Nov 14 '13 at 5:46

I am no expert in this, but from what I could find out, the pixel clock (or dot clock) is the speed at which the pixels are transmitted such that a full frame of pixels fits within one refresh cycle. For example, a VGA's pixel clock set to 25 MHz (corresponding to 25 million pixels per second) is just enough to display a resolution of 640x480 at 60 Hz (note the active display is only a part of the frame). Most higher resolution video can therefore use a wide range of dot clocks, well above 25 MHz, with the current range allowing enough bandwidth to easily exceed 1600x1200 at a 100 Hz and beyond.

The best way to figure this out is to search for the video standard you're trying to do, or look at the datasheet for the screen you are trying to drive and set the correct pixel clock as per your needs.


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