Timecode problem (BWF TimeReference convert to hours:minutes:seconds:frames)

[bob-san]

Hi Guys -

I am having a Timecode problem (discrepancy).

From what I am told the TimeReference values that exist in the bwav metadata are miliseconds * sample rate (in this case 48048). If this is true, there is nothing in this Timecode data that references the frames-per-second! So the TimeReference values in bwav exported from Pro Tools is frame agnostic. Is this true?

The reason for the question is unfortunately our Video and Audio were shot in 29.97 (non-drop), but our Animation is running at 30 fps.

When importing the audio into our custom Maya toolset, the tool takes in the bwav TimeReference values and calculates the hours:minutes:seconds:frames that we are all normally used to. Since this data is frame agnostic, the timecode should match how I see it in ProTools ... but it doesn't.

For example:

01:00:00:00 in ProTools (29.97 fps)
shows up as:

01:00:03:18 in Maya (30 fps)

How is this possible?!

Here is an example TimeReference value that starts at 01:00:00:00 in ProTools:

172972800

172972800 / 48048 = 3600 seconds!

Which is:
01:00:00:00

No matter what frame rate!

.... so why there a 3.6 second discrepancy?!?!

Thanks for your help!


Cheers
- Bob-san

[joachim]

Originally Posted by bob-san

172972800 / 48048 = 3600 seconds!
.... so why there a 3.6 second discrepancy?!?!

172972800 / 48000 = 3.603,6

What was the setting of the audio recorder : sample rate 48048F (or plain 48048) and timecode frame rate to 30ND ?

Maybe check this article : http://www.trewaudio.com/articles/48048-khz/
I am not savvy enough. Maybe you get more answers in the Post-Surround-Video sub forum.

[Frank Kruse]

Originally Posted by bob-san

Hi Guys -

I am having a Timecode problem (discrepancy).

From what I am told the TimeReference values that exist in the bwav metadata are miliseconds * sample rate (in this case 48048). If this is true, there is nothing in this Timecode data that references the frames-per-second! So the TimeReference values in bwav exported from Pro Tools is frame agnostic. Is this true?

For the record: BWAV is stamped in samples after midnight not milliseconds.

F.

[bob-san]

You are correct, sorry, meant to say seconds with millisecond/decimal resolution.

Audio was recorded in Pro Tools 12.5 HD in 29.97 ND, and sync'ed in Avid with video that was also 29.97 ND.

The sample rate was recorded as 48kHz, and also delivered in the AAF as 48kHz.

I think I have figured this out though. Apparently the TimeReference values are NOT frame agnostic. Even though frame rate is not considered in the calculations here ... it is considered in the sample rate. Which is why dividing by 48048 actually works. The extra 48 there is .1%, which is the difference between 29.97 ND and 30.

One of my audio programmers wrote a command line tool for me to make this .1% change in the BWF TimeReference values, so our Maya toolset can divide by the correct sample rate of 48kHz as seen in the wav's metadata. Clearing out the problem before it even gets to them.

OF COURSE, the the audio is still playing back a bit slower at 29.97 ND, but since the audio clips are rarely longer than 30 seconds, the drift will never be noticeable between animation and audio. But just in case, now with Timecode fixed, I can opted to resample the audio, and speed it up that .1%, which is really barely noticeable, and fix the problem 100% :].

Thanks again for your help! Looks like we are good now. Hope my findings help others as well.