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capture "now" timecode accuracy

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joshua schwarz
capture "now" timecode accuracy
on Aug 21, 2011 at 2:01:46 pm

hi. i started a thread on the basics forum (http://forums.creativecow.net/thread/200/893897#893897) but it's not as basic as i had thought.

in fcp 7 using a blackmagic decklink card, how accurate is the timecode when digitizing tape via 9 pin control with capture "now". i trust the timecode when i digitize the using in and out points with capture "clip" because i see the deck pre-roll for capture. since i don't see the deck pre-rolling when i capture "now," doesn't that mean it's not perfectly accurate?

joshua schwarz, producer/editor | post josh productions


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Jeremy Garchow
Re: capture "now" timecode accuracy
on Aug 21, 2011 at 3:47:37 pm

There's no need for preroll as the deck is already running (preroll ensures the deck is running at full speed).

It should be accurate, but you can easily check it against the tape.


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Everest Mokaeff
Re: capture "now" timecode accuracy
on Aug 21, 2011 at 3:48:40 pm

Degree of being accurate depends on measurement scale you use. It's very accurate within the concept of capture now.

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Andrew Rendell
Re: capture "now" timecode accuracy
on Aug 21, 2011 at 5:16:42 pm

In principle, you need a preroll to sync one source with another, e.g., an edited timeline with a running deck.

When you digitise off tape the deck isn't being driven into sync with anything (the timecode for the clip is being read off the tape, it's not being made to match anything) so the only function that a preroll serves is to make sure that the playback is stable and running at the right speed, which generally takes 1 or 2 seconds (some decks can be stable in less than a second) and should happen whether the deck is being controlled or not (if it doesn't you've got a fault somewhere). Assuming that the deck is running properly there is the question of how the editing system takes in the timecode. The deck can have a timecode feed that's separate from the vision and sound and/or a timecode feed integrated with the vision signal; those feeds need to be read by a timecode reader that's a separate piece of hardware to the NLE. Decks also embed timecode in some control protocols, such as RS422 and Firewire, so most editing suites nowadays don't have separate timecode readers as they are controlling the deck by one of those methods (so you don't need one).

Now, when you take the timecode via RS422 it's coming through a different path to the pictures, so there is the possibility of the timecode arriving at a different time (so the NLE gets the relationship between the t/c and the pics wrong). To get around that, the NLE has an offset built in [details here - http://support.apple.com/kb/TA22142?viewlocale=en_US ]. That offset will depend on your hardware, but you should have an easy setup for your decklink card and once it's been set the timecode on your clips will match the tape and you shouldn't need to set it again unless you change your hardware setup in some way.

BTW there is a setting in user preferences for what happens when you digitise through a t/c break. It should be right by default (i.e., creating a new clip on each t/c break) but it's worth checking.



(hope that's more helpful than what I wrote on the basics thread - it's the weekend so I've had a bit more time to compose my thoughts)


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Bret Williams
Re: capture "now" timecode accuracy
on Aug 22, 2011 at 4:31:02 am

Beat me to it. By about 7 hours! I swear my iPhone didn't show any replies when I posted. Remarkable how similar your information was. I swear I didn't plagiarize. You too must've digitized betacam back in 2000 via firewire whilst bringing in TC through the serial adapter with a -1 TC offset. It was flawless even in FCP 1.2.1.


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joshua schwarz
Re: capture "now" timecode accuracy
on Aug 24, 2011 at 11:57:03 am

hi thanks for the detailed and thorough answers. that's exactly what i was looking for. my system is set up with proper blackburst and i have and know how to read a scope. there is in fact a setting in the blackmagic preference panel for adjusting horizontal sync. i just was not confident because i remember years of fighting with linear systems and early avids to get timecode accuracy. i know my avid is perfect but it requires me to pre-roll so i was surprised that fcp didn't. my preliminary test using visual timecode from my hvr-1500 and comparing it to what comes in over the decklink's rs-422 do indeed show that they match perfectly. only time will tell for sure. fcp does end the clip at timecode breaks but it seems to get hung up after that and i have to manually start the next clip. i can live with that for now and hopefully i will figure out how to work around the system's idiosyncrasies. thanks again for everybody's input.

joshua schwarz, producer/editor | post josh productions


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Bret Williams
Re: capture
on Aug 22, 2011 at 12:13:33 am

In theory, there is no reason for either to be off. That's what control tracks, sync, etc are for. But in practice there is sometimes a delay if you're say compressing analog video to digital, and bringing in the TC via a serial adapter. The serial adapter would probably get the TC 1 frame ahead of the video in that case. But there's a TC offset adjustment for that.

In your case the blackmagic should handle everything. And capture now would usually have more preroll than log and capture. Preroll would be equivalent to the amount of time the tape was rolling before you hit capture now. In either case, 1 second seems to be enough for modern tape decks to be up to speed.


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