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anjawithaj

noticeable latency with blackmagic quad capture card

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Hi all,

I think I have now read every forum post with the words "latency" "capture" and "blackmagic" and tried the suggestions but we are still having about a half second latency from our live feed which is not great. I'm hoping someone can help.

We have 7 live cameras routed through a Kramer SDI Matrix and then to our Display computer which has a first-generation Blackmagic Decklink Quad capture card. When we check the signal on the Display computer using the Blackmagic Media Express software, we have minimal latency from every camera (maybe a few frames). As soon as we route it through Watchout, we have noticeable delay between the live and projected imagery. We have tried turning off de-interlacing and unchecked the frame blending option for the live feed, with no effect. We changed the WO settings to downsample to 720p and it temporarily reduced the latency to something a little more reasonable but was still significantly more delay than we'd prefer. 

The delay seems to build when we have more cameras activated and de-activated. If we shut down and re-open the Watchout program, the latency is significantly reduced.

Why is Watchout adding so much latency? What can we do to alleviate this? 

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Hi, WATCHOUT doesn't really add any latency. However, latency might come from the interface between WATCHOUT and the hardware.

An application can either use BlackMagic's proprietary framework/API Decklink or DirectShow. WATCHOUT only supports DirectShow so when you are comparing software then make sure that you are comparing using the same framework. WATCHOUT is pulling the frames from the DirectShow capture filter as fast as possible. The BlackMagic DirectShow capture filter is installed when installing the software package from BlackMagic so it might be good to test different versions of their software.

Another good test is to install the 32-bit version of Media Player Classic Home Cinema because it uses pretty much the identical DirectShow filter graph as WATCHOUT.

What normally adds latency is re-sampling of any kind like color format re-sampling, color space re-sampling, framerate re-sampling or resolution re-sampling. Especially if the hardware doesn't have these capabilities then the conversion takes place in software. WATCHOUT is capturing using the YUV 4:2:2 8-bit (YUV2) format in the resolution and refresh rate that is specified in WATCHOUT for the live video media.

Getting the frame from DirectShow and showing it on the screen takes usually two display frames due to double buffering. So if you are running the displays at 60 Hz then WATCHOUT will add around 33 ms. All other latency comes from the DirectShow capture filter and capture hardware. Most hi-end hardware adds around one frame when not re-sampling etc.. So in most cases it should take 3-4 frames from capture to display. Some hardware have low latency modes which makes it possible to capture in less than one frame time (example: Datapath livestream) which results in a 2-3 frames capture to display latency.

Blackmagic's DirectShow implementation isn't the greatest and you need to make sure that no re sampling is taking place for best results. Test Media Player Classic with the same settings as in WATCHOUT and see if you get the same results there. Make sure to select the YUV2 color format when comparing.

Best Regards,

Miro 

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