Receiver Input Lag

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ChiefBigFeather
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Receiver Input Lag

Post by ChiefBigFeather » 08 Apr 2018, 18:51

Dear BlurBusters Community,

first, I will bore you with some background information :)
Setting up my receiver with HDMI audio via my GPU has been a giant pain. As many here can probably understand, I really do not want any input lag from running my video via my receiver. So I have been trying to run it in multi monitor mode. The problem with that is, either enhanced sync (double buffer v-sync) bugs out and crashes or my GPU doesn't want to put my vram into idle so it also draws significantly more power.

So I called the Denon support to see if I could find out if that thing has any input lag. The support told me, the receiver does not have a frame buffer. I have no idea if they lied or if this is even possible. But no frame buffer, no input lag right?

Supposed my Denon AVR 1300x really has no frame buffer, maybe someone with intricate knowledge about how frame syncing and pass through works can explain how this is supposed to work. The device can display it's menu via HDMI onto the passed through desktop in the background. Is that even possible to overwrite those pixels without buffering the frame?
Would reliable fast sync/enhanced sync (true double buffer v-sync) be possible if the receiver really passes the video signal on to the display without any frame buffer? Would any sync information be passed along untempered with?
I plan on replacing the mainboard on my display with Cirthix' custom one. Do you think I will be able to pass through custom refresh rates (I cannot test that atm because my current display also only supports HDMI standard refresh rates)?

Thanks in advance for any input on that matter!

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lexlazootin
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Re: Receiver Input Lag

Post by lexlazootin » 08 Apr 2018, 20:20

(double buffer v-sync) bugs out and crashes or my GPU doesn't want to put my vram into idle so it also draws significantly more power.
That's interesting, so when using your Receiver you can't use V-Sync because it causes GPU crashes? wow. But the memory clock issue i've seen before with plugging in higher refreshrate monitors or multi monitors or something like that. It might only happen on AMD? i can't remember correctly.

"So I have been trying to run it in multi monitor mode."
Do you mean like extending your desktop to two monitors?
The support told me, the receiver does not have a frame buffer.
That's pretty extensive knowledge that only someone would know if it actually didn't have a framebuffer. Cirthix has added overlays on his screens without buffers before.

I really don't think you're going to get Cirthix boards to work with a receiver, they require such crazy bandwidth that goes beyond any specification and most things will just reject it outright.

ChiefBigFeather
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Re: Receiver Input Lag

Post by ChiefBigFeather » 08 Apr 2018, 20:35

lexlazootin wrote:
(double buffer v-sync) bugs out and crashes or my GPU doesn't want to put my vram into idle so it also draws significantly more power.
That's interesting, so when using your Receiver you can't use V-Sync because it causes GPU crashes?
V-Sync works, Enhanced Sync doesn't. Enhanced Sync is AMD's name for true double buffer V-Sync.

The vram clock is a model specific problem. Extended monitor mode comes with additional problems. My GPU (afaik not limited to mine) register HDMI devices first. So the receiver will end up being the first device by default, some programs will use that as an excuse to start on that monitor. Since it is only a dummy monitor, this is really annoying.

The myriad of problems coming with HDMI audio never cease to amaze me :(
Now I managed to brick my drivers in duplicate monitor mode.

The HDMI of my receiver is rated 2.0. If it is about pixel clock, doing 1080p@120Hz should be possible. What I don't know is: If there is no frame buffer, is the video signal independent of whatever the receiver does? Does it mean that all video and sync information is simply passed on as if there was no receiver between the cables? Can it even work that way technically?

Sparky
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Re: Receiver Input Lag

Post by Sparky » 08 Apr 2018, 21:27

ChiefBigFeather wrote:Enhanced Sync is AMD's name for true double buffer V-Sync.
Um, normal v-sync has always been double buffered. Enhanced sync is AMD's branding for what Nvidia calls "fast sync". It's equivalent to any triple buffered v-sync implementation which drops excess frames, like the typical behavior of windowed fullscreen.

It's really misleading to call it "true" anything. Best to actually describe the behavior. Triple buffering that doesn't drop frames is still triple buffering, because it still has two back buffers and one front buffer, it just prioritizes frame pacing over latency when framerate exceeds refresh rate.

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Re: Receiver Input Lag

Post by Chief Blur Buster » 08 Apr 2018, 22:14

Over the last two decades, different names have been done, but NVIDIA Fast Sync and AMD Enhanced Sync is quite similar to 3Dfx Voodoo Triple Buffering (circa 1996-1999), where framerates above refreshrates replace the waiting back buffer with a fresher buffer (for the next refresh cycle), while the front buffer is still being displayed in the current refresh cycle.

Names and terminology has been very confusing. So describing the framebuffer workflow has sometimes been recommended.
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ChiefBigFeather
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Re: Receiver Input Lag

Post by ChiefBigFeather » 09 Apr 2018, 04:21

Oh, I didn‘t know that. Sorry for posting misinformation.

It does crash in cloned monitor mode. On my system.

Do you know how those frame syncing questions work if I run the signal through the receiver?

Sparky
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Re: Receiver Input Lag

Post by Sparky » 09 Apr 2018, 04:32

If the receiver doesn't add anything to the video signal, it probably just splits off the data lines for decoding, and passes everything through, instead of re-encoding it. If there's any buffering at all, I suspect it would be limited to single packets.

If it does add something to the video signal, it gets more likely that it would buffer a full frame.

ChiefBigFeather
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Re: Receiver Input Lag

Post by ChiefBigFeather » 09 Apr 2018, 11:41

Sparky wrote:If the receiver doesn't add anything to the video signal, it probably just splits off the data lines for decoding, and passes everything through, instead of re-encoding it. If there's any buffering at all, I suspect it would be limited to single packets.

If it does add something to the video signal, it gets more likely that it would buffer a full frame.
The menu of the receiver is displayed as an overlay over the picture passed through. So it kind of adds something to the video signal. I don't know if it has to buffer the frame to do that.

Is there an easy way for a layman to test for input lag?

Are HDMI switches free of input lag? Can they convey sync data properly?

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RealNC
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Re: Receiver Input Lag

Post by RealNC » 09 Apr 2018, 17:38

ChiefBigFeather wrote:The menu of the receiver is displayed as an overlay over the picture passed through. So it kind of adds something to the video signal. I don't know if it has to buffer the frame to do that.
Monitors overlay their OSD without having to buffer the whole frame, so I think we can assume that the presence of an overlay does not imply frame buffering.
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ChiefBigFeather
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Re: Receiver Input Lag

Post by ChiefBigFeather » 09 Apr 2018, 19:14

While not perfectly accurate, I figured I can test the input lag of my receiver:

I simply connected a second monitor to my system and took photos running a stopwatch. While the accuracy is limited by frametime and the online stopwatch I used, it showed a 31 ms difference three times, and a 62 ms difference once. The difference in frame time between receiver and no receiver is not measurable by this method.
Last edited by ChiefBigFeather on 09 Apr 2018, 19:51, edited 2 times in total.

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