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Why transatlantic ping is faster than sending a pixel to the screen


"I can send the IP packet to Europe faster than I can send a pixel to the screen. How f’d up is that?" John Carmack asked in his Tweeter. His tweet caused a broad resonance in the community, so Carmack explained that to measure the lag on Sony HMZ-T1 analog display he used a program that changes the contents of the buffer by pressing a button on the controller, and a screen with the video camera 240 fps. Then he counted the number of frames between pressing the button and changing pixel.

The operational frequency of controller is 250 Hz, and here the lag cannot be measured.

For example, Carmack took the old CRT monitor with a vertical sweep frequency of 170 Hz. Then in optimal conditions it displayed 2 frames between pressing the button and changing pixel, namely there is the lag about 8 milliseconds.

The situation is much worse conserning modern LCD displays. The same Sony HMZ-T1 updates a picture in 18 frames, namely there is the lag about 70 + milliseconds. The LCD displays show worse results than CRTs due to the specific character of technology. Perhaps the newest OLED displays can be compared with the worst of the CRTs. According to the tests of Carmack, Emagin z800 showed the best result among non-CRTs, which roughly corresponds to the CRT monitor with a display refresh rate of 60Hz.

One of the problems with Sony HMZ display is explained by the low-quality drivers that instead of optimization the performance they try to put everything into the buffer, because it's easier.

Transatlantic ping can be measured by a command ping, which shows the travel time of the package to a remote server and back. It must be divided by two.

John Carmack hopes that the display manufacturers will draw a conclusion from that, and they will fix all the bugs and release much faster displays.

On topic:
Exploring Input Lag Inside and Out, AnandTech
Skull 3 may 2012, 17:00
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