Loss of AV Sync for some Captures (Decklink SDI)

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  • Posts: 1
  • Joined: Mon Apr 08, 2019 8:42 am
  • Real Name: Adrian Roe

Loss of AV Sync for some Captures (Decklink SDI)

PostThu Apr 11, 2019 1:32 pm

We have 4 independent examples where we occasionally see the audio and the video on the SDI capture from a Decklink go out of sync - sometimes by up to several seconds. We have seen this on 4 separate encoders (2 running on Linux, two on Windows (one of them GraphEdit)).

We are confident that the drift is caused by an issue in the upstream SDI (for example one case immediately followed a power outage to an upstream mixing desk) but we really need to be able to detect and deal with the issue.

We've written low level test code that just dumps the raw audio / video data and corresponding PTS directly to disk exactly as it come out of the BMD API and it is clear that the raw data is wrong (i.e you get both audio and video frames delivered with near identical PTS values that should in fact be a few seconds apart). For "problematic sources" over time we've had sources go up to about 3 seconds out...

Is there an internal audio buffer that might be queueing content when problematic sources are processed (maybe the CRC of some of the video data is bad, but the audio is still queued and output once a "good" video frame shows up, introducing lag each time that happens?) .

We have seen repros of the issue with a Blackmagic HDMI to SDI converter as input into the Decklink card, so that might be something you could use for testing...

We have tried latest drivers etc ( from 29.03.2019) but the issue is still present.

Any help very much appreciated!


  • Posts: 1
  • Joined: Thu Apr 11, 2019 5:58 pm
  • Real Name: Ryan Yao

Re: Loss of AV Sync for some Captures (Decklink SDI)

PostThu Apr 11, 2019 6:02 pm

We've seen this issue in our company's setup as well, please fix


  • Posts: 1
  • Joined: Fri Apr 12, 2019 8:06 am
  • Real Name: Thomas Preschel

Re: Loss of AV Sync for some Captures (Decklink SDI)

PostFri Apr 12, 2019 8:35 am

Our setup for reproducing this problem is:
HDMI output from satellite receiver -> Blackmagic HDMI to SDI converter -> Decklink (single) with the newest driver version, running in Windows 10.
Our customer is using a different signal source (not involving HDMI conversion) and is seeing the same problem. Other customers report no async problems with the same software and hardware.
We see this as video/audio going out of sync slowly (audio behind video), reaching about 1-2 seconds after a day.
In our tests we saw this with 576i50 input as well as 1080i50 input. The same problem was also present in previous driver versions.
The supplier of the capturing/encoding software (the original poster of this thread) is using the API for accessing the card, but I can see the problem just running video and audio capture filters in Graphedit for a few hours. Stopping the capture graph and restarting it did not reset the latency in the audio, but closing and reopening Graphedit did.

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