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Taylor: Avoiding Jitter in Composited Frame DisplayTaylor: Avoiding Jitter in Composited Frame DisplayPosted Dec 1, 2012 20:03 UTC (Sat) by epa (subscriber, #39769)In reply to: Taylor: Avoiding Jitter in Composited Frame Display by dlang Parent article: Taylor: Avoiding Jitter in Composited Frame Display
If the monitor is a CRT getting it's feed from a VGA cable, change the voltage on the cable and it changes the display instantly....but not necessarily the part of the display you want to change. If the electron gun has just passed the bit you were interested in you might have to wait up to 1/60 seconds to update that bit.
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Taylor: Avoiding Jitter in Composited Frame Display Posted Dec 1, 2012 20:39 UTC (Sat) by alankila (subscriber, #47141) [Link]
That is why it would make tremendous sense to sync the monitor refresh rate to some integer multiple of the video stream. For instance, 24 Hz video would probably look pretty good at 24 Hz * 3 = 72 Hz refresh frequency, which would make frames stay on screen for 3 full monitor frames and thus avoid another kind of jitter, that which comes for instance when 60 Hz refresh is used and frame display times must alternate between 2 and 3 frames, turning smooth movement jerky. (2 * 24 + 3 * 24 = 120, which is 2 * 60, so it works out.)
It's a pity that with transition to LCD we seem to have lost the ability to control the display vertical refresh timing, as the refresh seems to be simply stuck at 60 Hz for most of us now.
Taylor: Avoiding Jitter in Composited Frame Display Posted Dec 2, 2012 9:53 UTC (Sun) by Jonno (subscriber, #49613) [Link]
> It's a pity that with transition to LCD we seem to have lost the ability to control the display vertical refresh timing, as the refresh seems to be simply stuck at 60 Hz for most of us now.
100 Hz and/or 120 Hz sees to be coming though, first in low resolution TV sets (advertised as "High resolution full-HD with 100 Hz technology"), but some high end computer monitors already have it and I expect one or the other to become the new standard, replacing 60 Hz, in a year or two.
Which will become the winner is still up in the air though. One one hand 100 Hz is perfect for 25 fps (4 frames/frame); and not to bad for 24 fps (every sixth frame is 10 ms longer), and 30 fps (every third frame is 10 ms longer), and slightly cheaper to manufacture. On the other hand 120 Hz is perfect for both 24fps (5 frames/frame) and 30 fps (4 frames/frame), while not being too bad for 25 fps (every fifth frame is 8.33 ms shorter), and is a multiple of the old 60 Hz standard.
Taylor: Avoiding Jitter in Composited Frame Display Posted Dec 2, 2012 10:42 UTC (Sun) by farnz (guest, #17727) [Link] Note that the driver for high frame rates on the link is 3D TV - the killer is the link bandwidth to transfer a high refresh rate image. Many screens are already capable of 300 Hz or 600 Hz refresh, if only you could get image data to the panel that quickly. Standard TV-style 1080p60 has a pixel clock of 148.5 MHz - 3D TV uses tricks similar to old-style interlace to get "120 Hz" refresh in that, by effectively transmitting two 74.25 Mhz pictures (2x1080i60 for example). If we wanted to send 300 Hz down the wire, we need a pixel clock of 742.5 MHz. Note too that we send 24 bits per pixel, for 17.82 GBit/s data to shift on a 1080p300 display. Add in the fact that "retina" type displays have 4 times the pixels, and a single 3840x2160p300 display needs you to transfer 71.28 GBit/s just for scanout. Note that 71.28 GBit/s is the lower bound there; that's just the RAM throughput needed to drive a 300 Hz retina display, assuming a static image. For comparison, a desktop IvyBridge CPU from Intel has 25.6 GByte/s peak memory bandwidth for all uses; that's 204.8 GBit/s in the best scenario. We're talking about eating 1/3rd of that for scanout, assuming the memory controller can sustain the peak. In practice, you normally see around half the peak rate as sustained data rate with reasonably optimized memory access pattern - we're now talking about eating 2/3rds of the available memory bandwidth just to scan out the image, leaving us with very little memory bandwidth left to render it. Bear in mind too that graphics is often memory-bandwidth limited; one of the things you're paying for when you buy a $600 card rather than a $50 card or integrated graphics is a very high bandwidth memory bus. Eating lots of that memory bandwidth for scanout is going to adversely affect performance of the rest of the GPU's operation.
Taylor: Avoiding Jitter in Composited Frame Display Posted Dec 2, 2012 10:56 UTC (Sun) by dlang (✭ supporter ✭, #313) [Link]
one thing to remember is that dedicated video cards tend to have higher memory bandwidth to their dedicated memory than the system does to it's main memory (they need far less of it, so it's affordable)
the display framebuffer only needs to be a few 10s of MB of ram. So the 3d TV units may display at 120 or 240 Hz, but they receive their input at a much lower rate, just storing two copies of each frame to alternate between.
Taylor: Avoiding Jitter in Composited Frame Display Posted Dec 3, 2012 6:27 UTC (Mon) by Jonno (subscriber, #49613) [Link]
> Note that the driver for high frame rates on the link is 3D TV
Well, that might be *one* driver, but most non-3D TVs sold here for €200 or more operates at 100 Hz, and supports 1920x1080 input at 100 Hz over HDMI (1.3+). Probably 100Hz was picked because it is the lowest multiple of the PAL frame rate that don't suck when displaying NTSC content...
Of course, my dream display, a 32" 16:10 monitor with a 3840x2400 resolution at 100/120 Hz, would require a better link than DP 1.2 (Which is limited to 60 Hz at that resolution) or the not-yet-released HDMI 2.0 (which will match DP 1.2 but no more). Lets hope for DP 1.3...
Taylor: Avoiding Jitter in Composited Frame Display Posted Dec 3, 2012 11:24 UTC (Mon) by farnz (guest, #17727) [Link] Are you sure it's 1080p100 on the link, not 1080p50 and upconverted to 100 Hz internally? The two are quite different - the internal upconversion won't affect the compositor on a PC, and one of the "joys" of consumer targeted marketing is that it deliberately blurs the difference between "1080p100" and "100 Hz display". I've not seen a TV here that does 1080p100 (in the UK) - most do "100 Hz motion processing" (i.e. upconvert incoming 25/50 fps material to 100 Hz), but I went for an LCD set that does variable frame rate - 23.97/24/25/29.97/30/50/59.94/60 fps, based on what the input is doing - instead of one that upconverts to 100 Hz. Of course, the ultimate, if we could afford the link bandwidths involved, would be a set where no pixel ever subtended more than half an arc-second on the viewer's eye, and the vertical refresh rate was 300.300 kHz, which is an exact multiple of all the commonly used frame rates.
Taylor: Avoiding Jitter in Composited Frame Display Posted Dec 3, 2012 12:16 UTC (Mon) by tuna (guest, #44480) [Link]
There are monitors which can accept a 120 fps input, but I do not know of any device that accepts 100 fps input. Do you have any links to such devices?
Taylor: Avoiding Jitter in Composited Frame Display Posted Dec 3, 2012 16:29 UTC (Mon) by dgm (subscriber, #49227) [Link]
I guess most TV sets sold in Europe would accept it. Here AC frequency is 50Hz, and the TV encoding standard works on multiples of that.
Taylor: Avoiding Jitter in Composited Frame Display Posted Dec 4, 2012 14:12 UTC (Tue) by tuna (guest, #44480) [Link]
Knowing is better than guessing. Can you name any device that accepts a 100 fps input signal?
Taylor: Avoiding Jitter in Composited Frame Display Posted Dec 5, 2012 12:07 UTC (Wed) by epa (subscriber, #39769) [Link]
In the old days there were plenty of CRTs which could manage 100Hz refresh. I tried to drive mine at that refresh rate whenever possible, being sensitive to flicker.
Taylor: Avoiding Jitter in Composited Frame Display Posted Dec 5, 2012 17:14 UTC (Wed) by dlang (✭ supporter ✭, #313) [Link]
refresh works significantly different on LCD screens vs CRT screens.
on CRT screens the image is drawn at one point and fades over time, refresh rate is how many times a second this point goes over the screen. there is always some flicker and high refresh rates minimize this
on LCD screens the image is stable, not 'draw and fade', different levels of brightness are achieved by flickering the pixels at different rates, but each pixel is run at that rate continually. Refresh becomes just the rate at which the image can be changed.
Taylor: Avoiding Jitter in Composited Frame Display Posted Dec 4, 2012 10:15 UTC (Tue) by iive (guest, #59638) [Link]
“Many screens are already capable of 300 Hz or 600 Hz refresh...“
Not really.
There were Plasma displays that advertised 600Hz, showing same image 10 times. Not sure I've ever seen plasma monitors...
LCD are slow. Only expensive gaming gear can work at 120Hz in 3D shatter mode without producing artifacts known as ghosting. And usually this is at the edge of their capabilities.
The problem is that the liquid crystal have a turning time. It have to physically move in order to change from one pixel value into another. You can have higher framerates only if you have small gradual changes in the pixel values. The advertised 300-600Hz refresh is just that... advertised. The "extra" frames are "interpolations" of fading pixels.
OLED may be able to get higher refresh rates. But they are still not in mass production.
Taylor: Avoiding Jitter in Composited Frame Display Posted Dec 6, 2012 0:26 UTC (Thu) by intgr (subscriber, #39733) [Link]
> LCD are slow. Only expensive gaming gear can work at 120Hz in 3D shatter mode without producing artifacts known as ghosting
Ghosting is irrelevant here -- it's just latency. We're talking about reducing jitter, not latency.
Taylor: Avoiding Jitter in Composited Frame Display Posted Dec 2, 2012 11:11 UTC (Sun) by alankila (subscriber, #47141) [Link]
If we have 100 Hz full-field replacement rate (that is, 100 real pictures per second to both eyes), then I stop worrying about this kind of jitter because I believe that humans in general aren't truly sensitive to jitter less than about 10 ms. However, rather than raising the bandwidth cost, power usage, and wasted memory bandwidth, it might make sense to just give software a toggle that allows it to tell the display the timestamp it should be begin (or be in middle of) transition to the new frame.
If you think about LCDs as a system, they would be ideal for this: they are displaying one picture using the current frame, and the computer uploads the new frame to them at some rate, which can be decoupled from the frame switch rate as long as the data is transferred before the frame switch is requested. Normally the switch is triggered synchronously over the entire display area when the new frame is fully transferred, but nothing would prevent there being a delay after the frame has been transferred where it waits for a specific timestamp.
Taylor: Avoiding Jitter in Composited Frame Display Posted Dec 3, 2012 14:32 UTC (Mon) by ballombe (subscriber, #9523) [Link]
60Hz is the US standard, but in Europe 50Hz is more common
(AC is generally 50Hz in Europe).
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