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I have tried 4K@30Hz using HDMI on five different TVs (not monitors): Seiki 39" US, Seiki 39" UK (US and UK modes are different), Hisense 42K680, Samsung UE40HU6900DX, LG 40UB800 (the latter three are in Italy and seem available across europe). In all cases the additional display lag wrt my retina MBpro screen (measured with the small HTML/JS at the end) was huge, in the 130-230ms range, i.e. 4-7 frames. As a comparison, even the low end 1080p TVs i tried (at 60Hz) only gave 0-16ms additional lag, i.e. one frame. The problem is not 30Hz vs 60Hz, but a video pipeline which is too deep, and cannot be shortened in any of the models i tried (even the models that have a "gaming" mode disable that control when running at 4k).
Note that even at 30Hz one frame is only 33ms so anything more than 66ms indicates excessive pipeline depth.
Based on my measurements, I think that at the moment the chipset that do 4k (upscaler etc.) are still buggy/immature in terms of features, and probably we need to wait the next generationof silicon to get 4K TVs that can be used as monitors. I have become used to the 130ms lag but it is not pleasant. Note that the Acer Chromebook C720 ($199) can drive the screen at 4K through the HDMI port. I had no problem with FreeBSD using high end nvidia cards (GT640 and GT750), whereas lower model seem unable to use pixel clocks above 165 MHz (you need about 290 MHz to run at 4k). <!DOCTYPE html><html><head><script>
Object.prototype.d = function(l) {
return (this + Math.pow(10,l)).toString().substring(1, l+1) }
function g(x) { return document.getElementById(x); }
function clock() {
var t=new Date();
g('txt').innerHTML=t.getSeconds().d(2)+'.'+t.getMilliseconds().d(3);
setTimeout(clock,1);
}
</script></head>
<body onload="clock()">
Set PC to mirror screens, take a snapshot with a camera, compare times<br/>
<div id="txt" style="font-size: 120px;"></div>
</body></html> |
https://twitter.com/id_aa_carmack/status/193480622533120001