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I need a bit of help on BWF recorders

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Bouke Vahl
I need a bit of help on BWF recorders
on Jul 17, 2009 at 6:31:17 pm

Since i'm not a sound guy, but involved in anything that has to do with timecode, i need a bit of help.

What exactly does the 'preroll' setting found on BWF recorders do?

(I know for sure it's on a Sonosax and Edirol)

It seems to mess up the LTC output vs the BWF TC, but i have NO idea what is going on.

Can someone enlighten me on the subject?

Thanks in advance,


Bouke

http://www.videotoolshed.com/
smart tools for video pro's


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Ty Ford
Re: I need a bit of help on BWF recorders
on Jul 18, 2009 at 4:49:43 am

Hello Bouke and welcome to the Cow Audio Forum

Yes, pre-roll messes with timecode. Pre-roll is basically a ram bucket. When you hit record at 12:00 and the ram bucket is large enough to handle six seconds, the audio machine thinks it went into record at 11:54, six seconds BEFORE you hit the record button. When it time stamps the file, the timing is off by the amount of the pre-roll.

Regards,

Ty Ford

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide






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Bouke Vahl
Re: I need a bit of help on BWF recorders
on Jul 19, 2009 at 11:15:05 am

Ty,
Thanks for the explanation.
If i understand it well, this should NOT be a problem :-)
As the file actually starts recording before the operater hits the record button, and the proper start time is set, it 'should' be in sync with the LTC output...

My goal is to sync the BWF files with the LTC output recorded on the cameras audio track. (As for example when using still cameras.)

What happens (at least with the Edirol), is that the timestamp is NOT in sync with the LTC output...

But at least i now understand what 'preroll' means on these recorders. (Although i think it's a wrong term to use for 'pre recording'.

Another question, how come lot of BWF recorders always start recording on a whole second? Worst case this means the recorder goes into record about a whole second after hitting the rec button....

But first, i'm off for a week enjoying some time off!

Later,



Bouke

http://www.videotoolshed.com/
smart tools for video pro's


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Ty Ford
Re: I need a bit of help on BWF recorders
on Jul 19, 2009 at 12:26:26 pm

Bouke,

While I have no direct experience with the issue, I have heard that in some situations in which a recorder is connected to a camera for timecode, pre-record does create problems. I think the problem was "fixed" by setting the audio recorder in a different mode. I just don't recall the specifics. The guy at the newsgroup rec.arts.movies.production.sound would know.

As to the start time, I'm guessing it has to do with whether you are using time of day code or not. In TOD, versus rec run or free run, you are locked to the clock in the recorder. rec run, in which the audio recorder is synced to the smpte output of the camera. Then, the audio recorder is taking the timecode from the camera. (I'm guessing maybe the pre-record may mess with that). Unless the camera can run at TOD, each file starts with 0:00:00.

Regards,

Ty Ford

PS: enjoy your vacation



Want better production audio?: Ty Ford's Audio Bootcamp Field Guide






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John Fishback
Re: I need a bit of help on BWF recorders
on Jul 19, 2009 at 3:37:13 pm

I've never used an external audio recorder on a video shoot where we were syncing timecode. However, I've done it many times for multicamera shoots. In that case, we feed reference video to all cameras and/or decks as well as a timecode signal. That way timecode is always in sync. Sometimes the timecode is TOD and for cameras that have to roam and not be cabled, we'd sync with cables attached and then disconnect the cables. Sync would hold at least for a couple of hours, usually longer. Depending on the audio recorder you're using (Does it have ref or word sync in? Does it have timecode in?) you could use a similar workflow to achieve sync. You might have to find a menu setting which allows ext sync and type of timecode.

John

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Greg Curda
Re: I need a bit of help on BWF recorders
on Aug 2, 2009 at 3:51:26 am

Hi Bourke,

BWF is only a file format, and should have nothing to do with the recording process. That should be a function of each individual recorder.

We all are assuming that camera and recorder are TC locked, either via hard wire or jamming. When you return from your lollygagging (hehe!)...could you detail your workflow for us? Perhaps then we could see where the sync problem is.


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Bouke Vahl
Re: I need a bit of help on BWF recorders
on Aug 2, 2009 at 8:42:28 am

Greg,

Don't worry, i know plenty about the BWF format.
And, it's not 'my' workflow. I've written a program that can decode LTC and turn that into a QT TC track (see the rsync 'n link thread a bit up).
People who use that take the LTC from the BWF recorder and put that on an audio channel of the cam.

Now it seems that when using preroll on the BWF recorder things are not in sync.
But i never actually saw the files, i just got questions from customers so i cannot put my finger on it what exactly happens where.
My guess is that the files with 'pre recording' get stamped the time the record button is hit, not compensated for the extra recording time.
But this is pure speculation...



Bouke

http://www.videotoolshed.com/
smart tools for video pro's


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Greg Curda
Re: I need a bit of help on BWF recorders
on Aug 2, 2009 at 1:20:45 pm

Hi Bouke,

Ah...I see...a thousand pardons.

Can I assume that the gear in question would be generally considered "pro-sumer"? If that's true, this is an area that I am not expert in, but would love get a handle on. In the past, if I were confronted with cameras that were not TC capable, I would simply get another camera. I know this approach could be considered "bourgeois", but I did have the luxury of budget.

I did use TC recorded to audio tracks and fed to Lynx synchronizers in the 80's with great success. I have also created QTs from separate picture and track that were TC locked, albeit at the end of post.

I am still somewhat confused about "BWF recorders" and "timestamping", as all my recordings have always carried a full TC track, embedded in the .wav, or so I thought.

Operating under the general principle of, "The more I work, the less I know", I would appreciate any further enlightenment, as I need to learn more about the world of pro-sumer. Should you have extra time (haha), I would be most interested in a dialogue. Email: gcurda@gmail.com

Thanks,

Greg


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Bouke Vahl
Re: I need a bit of help on BWF recorders
on Aug 2, 2009 at 2:43:46 pm

Greg,
Let's keep the conversation here, so others can benefit.
(the Cow is high on Google...)

Timestamping in BWF is NOT the same as Timecode.
A BWF gets a 'timestamp', a number that is known as 'samples after midnight'.
This is not a 'stream', but a tiny bit of metadata added to the beginning or end of the WAV file.

So in order to calculate the Timecode, you need to know the sample rate first. Normally this is 48 Khz, but it does not HAVE to be.
(Especially going from 23.976 to 24 or vice versa, it's a known trick to alter the sample rate)

So if the Timestamp is 96000, and the sample rate is 48000, you would think the Timecode is 00:00:02:00.

But there is more to it. The playback speed comes into play.
As you know, NTSC does NOT run at 30 FPS, but at app. 29.97.
(This number is not even close enough to do decent math on BWF's,
as the numbers become quite high, but that is beyond this piece)

So a timestamp of 96.000.000.000 will give you a time of
96.000.000 / 48.000 = 2000 seconds.

In a 24 (film) project, that would correspond with a TC of 00:33:20:00
BUT, in a 23.976 (24p Video) project, the video runs slightly slower.
After 2000 seconds have passed, there are only 1998 frames displayed.
The correct timecode in a 23.976 project thus should be a bit less (2000 / 30 * 29.97) = 1998, thus two frames less making 00:33:19:21

(to do real math, don't use 29.97, but 30000 / 1001 = 29,97002997002997002997002997003

Now to further complicate things:
NOT ALL SOFTWARE WORKS THE SAME.

So always test your workflow. But in case of trouble, you now know what math to do to see where what goes wrong...


As for working with 'Prosumer' cams.
Of course, happens a lot. Putting the LTC on an audio track is your only option (except a clap and a lot of manual work)

But even with Pro cameras, there are people who prefer to use LTC on an audio track rather than having the cam locked to external TC.
There are a few reasons, mainly for tape based shootings:
(And yes, i have encountered all the trouble i'm about to mention...)

If you have to ingest, your NLE will stop capturing / re-cueing at every shot. This will take huge amounts of time extra, and if you have shot HDV and try to ingest material with TC breaks using a cheap HDV deck, the guy ingesting will go insane. (It just does not work...)

If you have not properly locked everything, the TC can slip, and the capture process can be halted as a result of that. Not a major problem, but annoying at least.

Some directors / DOP's like to have the TC hour to match the tape/disk name. (Makes logging / troubleshooting easier).
Shooting TOD of course will give you a huge amount of overlap on clips. A mistake in reel / disk / tape labeling can cause mayem in the online.

So i would suggest, do download the FCPauxTC demo from my site and start toying with it. Better to do it now than at the moment you actually need it.
Last hint, do not record the LTC too loud. Between -18 and -9 i more than high enough. (Lower probably also works)

hth,




Bouke

http://www.videotoolshed.com/
smart tools for video pro's


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Greg Curda
Re: I need a bit of help on BWF recorders
on Aug 2, 2009 at 4:20:00 pm

Bouke,

Wow! That's a lot of info. I guess you're the TC guru! I understand it to an extent, then my brain freezes...it just doesn't "feel" right to me. I guess we have American Television to thank for all this...hehe!

These are general principles that are confusing to a lot of people, me included. I think it's fantastic that there are guys like you who really understand this stuff and can help prevent the rest of us from falling into that great chasm of misunderstanding.

Am I right in thinking that the change of frame rate, and frame rate descriptions in general,are only important for sound when it comes to matching picture? While picture makes frame rate conversions, sound makes sample rate conversions? For sound, time is still time, right?

Thanks for taking the time to explain and share your knowledge.

G






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Bouke Vahl
Re: I need a bit of help on BWF recorders
on Aug 2, 2009 at 4:44:56 pm

Greg,
The lesson you should have learned:

BWF does NOT carry TC, it carries a Timestamp.
The TC readout you get in different software packages is calculated based on the sample rate and the project framerate.
(If the software is written correctly, and sadly, that's not always the case...)
Don't get confused about the math. It is only important when you run into problems. With the use of a pocked calculator you can then determine the source of the trouble. Or, if you ask help online, do include the TC numbers and TC offset, so others can do the math for you :-)
Most important:
-Do not think it is weird if you see different TC values in different software.
-Never record '24' if you're shooting '23.976'.
-Always test the entire workflow.

And, keep on learning. Almost all my knowledge about the subject comes from the World Wide Internets.
It's not that hard, it just takes a huge amount of time to ingest everything (and filter the bullshit from the valuable stuff)

later,


Bouke

http://www.videotoolshed.com/
smart tools for video pro's


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Greg Curda
Re: I need a bit of help on BWF recorders
on Aug 3, 2009 at 3:58:38 am

Thanks again, Bouke...


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