NextPreviousHomeChromakey video render question

jeffo | 10 years ago | 4 comments | 2,153 views

I just finished a project in which I used the Bluff green screen effect to create a key to use in Vegas Pro 10. It's a one minute TV commercial with a spokesman.

In order to get a sharper image, I imported the original footage at a resolution of 1200x900. Bluff produced a very nice key from the green screen footage - (even though the footage was underexposed). I rendered it out as an uncompressed 1200x900, avi movie with audio.

The movie I rendered from Bluff lost audio sync, even after multiple tries. In matching it to the original, I was able to get it to work by adjusting the velocity of the Bluff clip in Vegas to 98.5%.

Here is the Bluff greenscreen key footage:

And here is what I was able to render out by doing a crop in Vegas, speeding up the Bluff clip for the key and filling the interior with the original footage:

My question is to inquire as to whether there is a way to keep the audio sync on imported video footage when I am rendering out a green screen key.



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very interesting your channel on youtube, good jobs.
I think that sometimes depending on the format of video that we put in BluffTitler have one outcome or another.
would be interesting to know that video format and audio introduced in BluffTitler.
you could make several different audio tests.
maybe find a way to avoid altering the playback speed

maitegras, 10 years ago

A video file produced by BluffTitler has 2 streams:
-a video stream
-an audio stream

To synchronize video and audio, the audio is set as the master stream and the video as the slave. This means that the software that is playing the video is instructed to make the video follow the audio. The audio stream works best as the master stream because audio can't be delayed without producing undesirable awful sounds. Delaying video is less noticible.

Of course it's up to the player software what to do with this info. Video takes more bandwidth, especially uncompressed, so it's possible that your computer has problems making the video follow the audio. If the player is your NLE this however should have no effect on the exported video: due to performance problems the audio is out of sync in your NLE, but the exported video will play perfect in, for example, WMP.

michiel, 10 years ago

Oh, and it's technically impossible that the greenscreen effect (or any other fx) is making the audio out of sync. Removing this effect will produce the exact same result.

michiel, 10 years ago

Thanks for the responses.

I have not had time to do extensive testing, but I hope to in the next few days. The info you have given is very helpful.

It seems likely that the large file size of the video imported into Bluff may be causing it. The audio stream exported by Bluff matched the original to the frame. I did tests with HD and SD imports (avi files), and had similar results each time.

The original video into Bluff was always an uncompressed avi. I could certainly be wrong, but I am doubtful that machine speed was the culprit, since I am rendering everything out, bringing the rendered file into Vegas, and then rendering all of that material into a new file. I am doing the tests on Windows 7, a new HP desktop and matching laptop.

I was really curious to see if anyone else had experienced this, or if it is unique to me.

I have had the same thing happen in render tests on both machines. This weekend when I am not under deadlines, I will test renders without including the audio channel at various import resolutions into Bluff to see if it may be an issue between the slave and master streams. If you would like me to, I'll report back my findings here.

Again, thanks for a great production tool and the information.


jeffo, 10 years ago

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