PC Build - Capture Card Lanes

Hi everyone,
I am trying to build a new workstation PC at the moment including a Blackmagic Design Capture Card.
I am thinking of the Decklink Quad (https://www.blackmagicdesign.com/de/products/decklink/techspecs/W-DLK-36). Blackmagic says, it uses 8 PCIe Lanes for capturing. My mainboard only supports 4 lanes on the second PCIe Slot and I am using my graphics card in slot 1.

What would happen if I use the capture card with only 4 lanes?
Does it work at all? Am I limited in my resolution?

1 Like

I have not looked at the spec’s but from what I remember about PCIe is each lane contains power. So, I doubt it would work at all with out the available power from the other pins (lane).

Doing some back of the envelope calculations:

HDMI 2.0 (for 4k 60fps) max bitrate is 14.4Gbit/sec (1.8Gbyte)
PCI-E 3.0 lane bandwidth is 984MB (0.984GByte)

Therefore with 4 simultaneous 4k 60 inputs, max bandwidth is 7.2Gbytes/second, which is 7,3 lanes, so meaning you need at least 8 lanes to use the card at max

So that’s the math - the most common result in the event of limited bandwidth is that either a) it limits resolution b) it limits max inputs or c) it doesn’t work at all

In regards to Tod’s point electrically, yes you require a slot that is x8 or x16 in size even if it is only x4 data, otherwise it definitely won’t work

My suspicion is most likely answer a: you will be unable to use all inputs at max resolution (though whether it is an error, or a crash, I couldn’t tell you) as this is the usual behaviour for similar products, including I/O USB devices

1 Like

Thanks @wing3d
That sounds reasonable. I am already switching out my mainboard for another one which supports 8 lanes on the second PCIe Slot.