Audio sync issues with DATV Express and VMix

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Audio sync issues with DATV Express and VMix

Post by G8LES » Fri Jun 07, 2019 10:45 am


I know that the DATV Express and Minitioune are both USB2 but they are plugged into USB3 sockets on my PC and I am getting very good quality comments in full HD on Oscar 100, does it make a difference, I don’t know. The capture devices are Black Magic Design Decklink cards which may make the main difference and the BMD Desktop video app allows upscaling of SD wide footage to HD, and the PC is Dual 6 core XEONs so has plenty of power, although with H265 encoding and MT decoding it is at 76% capacity on performance.

Now to the main topic.

With the DATV Express on the PC it depends what symbol rate and codec as to the required Lip Sync delay needed, H265 is the longest delay at 1 M/S and it is about 803 mSec set in VMix. This delay increases as the symbol rate lowers. It frustratingly varies between transmitter on/off as well, without changing the settings.

I tried the free OB studio and DATV Express can’t see it as an input so that’s off the menu.

VMix was best but if you need more than 4 inputs such as videos, titles, test cards, and it is easy to get to 12 or 15 sources this means upgrading to the full HD version rather than HD Basic which takes you from 4 to a 1000 inputs max, I guess that’s enough! It gives one external output only.

Unfortunately DATV Express can only see VMix main audio out, not the external audio which has a delay twiddle, and the output audio is normally set to none so that you only hear the Tx audio via Minitioune via the sound card not the VMix output as well which would be a nuisance with the original and the delayed audio.

Setting the Audio delay in VMix.

Unfortunately too the global input delay available in the VMix settings menu only applies to new inputs added, and only when they are added, but you need to determine the delay needed with a test Tx signal being received for each transmission setting with say the camera input. I asked VMix that it applied retrospectively but they don’t seem in any way keen to change it.

In VMix each input has its own audio delay twiddle which is the best place to try the delay setting with a hands clap on camera, as doing it that way, VMix doesn’t need a re-start to change the delay and it instantly applies. As I said earlier we need a global setting to easily adjust it. As the global input audio delay is a one-time apply when creating a new input, now we are left with changing all the inputs’ audio delay settings, and this is not a sensible solution.

Now I have just tried Virtual Audio Cable 4.6 (VAC). This is a software audio router just like a patch panel. VMix can see it as a valid output routing and so can DATV Express see it as a valid audio input. That’s good.

OK so now does this VAC app help? If there was an audio delay app that sat on VAC that would work. I found one but it diverts the sound card output so that isn’t much help. If you know of one let me know. A lot are designed to sync an FM Radio with a TV broadcast with the radio being plugged into the sound card line input.

Voxengo does a VST audio delay but that is a plugin not a standalone app, what a pity.

Unfortunately VMix External out has an audio delay built in but DATV Express can only link to VMix main audio out and VMix External does not allow audio routing. The 4K version of VMix has two External outputs, DATV Express can see External 2 video but the audio still comes from VMix Master.

The simplest solution is that DATV Express links to VMix External audio which has a variable audio delay, if it can be “seen” as a VMix output, that’s for the clever coding folks to know how to do that.

Or the Standalone audio delay.

Ahah Audacity will do it by setting the Latency to minus 4,500 to 6000 msec and interface with Virtual Audio Cable 1 out from VMix as the Audacity input source and VAC cable 2 for Audacity output to DATV Express live audio in.


You can also link your camera Mic to the current output by using the triggers. Let’s say we have a test card up. Create an audio input for the camera Mic or PC Mike. On the Test card set the triggers in the input settings to call in the audio input with transition in and turn it off on transition out. Or if you don't want it automatic just click the audio input, Audio button to green to get it to overlay audio.


Mike G8LES

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