Latency

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David Heitbrink

  • Posts: 1
  • Joined: Wed Jan 28, 2015 6:32 pm

Latency

PostThu Jan 29, 2015 10:53 pm

I have a built a recording app around Declink Quad (and the single channel Declink that does HDMI), I am recording an driving simulation that is running on clock that is set to 240 Hz, the projectors are Gen Locked at 60Hz, and the cameras running to the Declink cards are running at 30Hz 1920x1024.

I am using DirectShow (we changed over our original capture App that used SD cameras and some else's capture card to HD + Declink capture). In my filter graph, I have a filter that renders a frame number (our 240 Hz clock tick) and some additional data on top of the captured frame. I pass the data and frame number directly to my rendering filter (its gets sent over the network from one of the computers) the problem is there seems to be about 1 second of latency by the time my rendering filter gets its frame, and it is getting it's data at 60 Hz, so the data that gets overlaid on the video is 1 second newer than the data that is getting overlaid. The rendering filter also saves out a file with the current simulator frame number + video time, so we can cross reference data later.

My question is, is there any way for me to know how much latency I am getting in from the capture to the overlay filter. I can buffer my incoming data, but I need to know how much latency I have so I know how far back in my buffer I need to go.

I also notice there are timecode outputs from the capture card, and seems like I can get the raw VANC stream from my incoming sample, so its like I can get a timecode generator and inserter hardware, but I would still need a way to get that time code before it gets feed into the capture card so I can correlate our simulator frame number with time code.

Right now I can live about 100ms of accuracy in terms of lining up our frame numbers, however in the future I will need to get down to a single frame accuracy.
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Yoshan Atukorale

Blackmagic Design

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  • Joined: Mon Feb 23, 2015 12:30 am

Re: Latency

PostMon Feb 23, 2015 12:49 am

Hi David

There is approximately 3 frames of latency on the DeckLink side (represented by the DeckLink Video Capture / Blackmagic WDM Capture filters) for non 4K DeckLink devices. So the majority of latency would be in the transform filters in your graph.

The DeckLink Video Capture / Blackmagic WDM Capture filters do not provide latency information.

However as you are getting a large latency between the capture and render filter, you could get an idea of the latency of video at different points in your graph by connecting a video renderer filter such as VMR 7/9.

On a side note if you decide to use the DeckLink API at a later point, you can measure latency on the DeckLink side (video frame completely received to application callback) by calling the GetHardwareReferenceTimestamp() method on a received frame.

Regarding timecode support, the Quad card is able to capture and render VITC / RP188 timecodes, however the DeckLink Video Render filter / Blackmagic WDM Render filter does not have a timecode input pin, so they cannot output timecodes. To output timecodes, you will need to use the DeckLink API.

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