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VHS/ Sync question

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Chris Cea
VHS/ Sync question
on Jun 11, 2011 at 4:34:23 pm

HI all,

I was looking to find out if a sync generator was necessary when capturing from VHS using a Decklink HD Extreme 3D card in a MAC PRO.

Here's my situation...

I am capturing S video out of a SONY SVO 5800 (which has an internal TBC) into FCP 7 at the moment and it appears that longer captures (60 minutes or so) show some audio drift. Would a sync generator do anything for me in this situation? Something that is important to mention here is that all of these tapes have numerous control track breaks in them. I am attempting to use the "capture now" feature in FCP to digitize the entire tape in one pass.

I have read many of the posts here on the cow recommending running the VHS off to DV or Beta Sp and then digitizing that, which would have uninterrupted TC but I was looking to avoid that step since I have over 50 VHS tapes to capture.

Just an FYI too.... I did post this a few weeks back on the Black Magic forum but got little response there so I thought I would give this forum a shot. Especially since I know MR. Z monitors this one.


Thanks for your time everyone
Chris


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Andrew Rendell
Re: VHS/ Sync question
on Jun 14, 2011 at 9:43:15 pm

As no one else has pitched in yet, I'll say that I'm not absolutely certain about your particular kit but as a general point, if the tape and the computer aren't locked together somehow there is quite likely to be some drift over time which will show up as a loss of sync, as you seem to be getting. Using a good quality reference signal for both the VHS and Blackmagic card is very likely to improve the situation...


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Chris Cea
Re: VHS/ Sync question
on Jun 14, 2011 at 10:33:27 pm

Hi Andrew, I appreciate you taking time out to answer my question. As far as sync generators go... any particular model or make you are partial to? I know I have read positive things about Horita. What about BlackMagic? Are their convertors any good?

Thanks again
Chris


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Andrew Rendell
Re: VHS/ Sync question
on Jun 16, 2011 at 1:00:49 pm

I don't really have any preferences atm, but either of those should do the job.


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Richard Crowley
Re: VHS/ Sync question
on Jun 17, 2011 at 2:37:15 pm

I would FIRST try capturing for an hour from some OTHER source to establish WHERE the problem is. Since the audio and video are literally, physically locked together on the tape, I can't see how that could be the problem, or how using an external sync generator would have any affect at all on this problem.

Do you have any other long-form capture experience with this Decklink card? There have been several infamous examples of computer interfaces that had problems with long-form audio/video sync. The problem was the design of the interface card. The audio and video sampling are, by necessity, done by separate circuits, and their rates are not locked to each other. Not a particular problem for short clips, but too much drift for long-form.


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Andrew Rendell
Re: VHS/ Sync question
on Jun 17, 2011 at 4:25:33 pm

My thinking is this: the deck and the computer both think that they're running at precisely the right speed, but in the real world they're likely to be very slightly different, so the computer will think the video is slightly wrong and occasionally the video circuit will drop a frame to make it what it thinks of as being right. The audio encoding part of the card probably won't, so on the digitised file the audio may drift out of sync. I have seen that happen on an Avid (4 or 5 frames over an hour IIRC) although it was a few years ago (and with the old Meridien hardware).

Now, if the deck and the capture card are both genlocked to a stable b&b source they have to be running at precisely the same speed, so that particular problem shouldn't occur, and as Chris has a deck that can do that I think it's a sensible thing to try.

You're right that it could be the card and the proper engineering approach would be to do some tests to establish exactly where the problem is occurring. Perhaps I am slightly sloppy in my approach of "this quick fix has worked for me before so lets try it again"... we'll found out if Chris wires up the genlocks and it makes no difference.


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Chris Cea
Re: VHS/ Sync question
on Jun 17, 2011 at 9:41:08 pm

Many thanks on the suggestions Richard and Andrew. I still haven't picked up the sync generator yet since this is actually a personal project and not a paid client job.

Richard, I don't have another deck to capture from unfortunately. I do have a DVD player I can capture from. My question to you is... how will that help me to determine if this is an internal issue with the Black Magic card? Unless this is an issue with the VHS deck which I doubt... I'm guessing I will have this same issue from another source (in this case DVD). What's your take on that if you don't mind me asking?

Andrew, what you are saying (about the 4 or 5 frames off after about an hour) is right on point. That is exactly what I am experiencing. Short captures look great, long captures show a drift. As you also pointed out the deck I have does have the capability of taking a reference signal.

In the meantime I'm probably just buy the sync generator anyway b/c if Mr. Zelin comes on here and sees me struggling with a $300 purchase... I'm in big trouble :)

Thanks again guys for taking the time to help me out.

Chris


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Bob Zelin
Re: VHS/ Sync question
on Jun 19, 2011 at 3:53:42 pm

Richard had the correct answer for you. Forget your VHS machine for now. Record another source - like a Beta VTR, Digi Beta VTR, etc.
Your audio drift may be your edit system, setting not setup correctly, etc. I don't know what MAC you have, but if you have your card in the wrong slot, or if you have an older 1,1 Mac Pro and have your lanes set to x1 for the BMD card, you can get this problem (it happens with the AJA Kona card as well).

Once you determine that your system is ok, and then see that your problem is directly related to the VHS machine, you need a FRAME SYNCHRONIZER to lock up your horrible VHS machine to your blackmagic (or AJA or Matrox) input. These products do not have Frame synchronizers to lock up bad sync signals. (but I can't afford to buy or rent a frame synchronizer).

Once you do that, you can certainly buy a Blackmagic Sync Generator ($295 I believe) or go to ebay and find a used Horita BSG-50 (not BG50), and stick the black signals into both the VTR Refernce input, and the Blackmagic Reference Input, to get both units to lock.

But I would do NONE OF THIS, until I followed Richards advice, and TRY ANOTHER SOURCE VTR like a Beta VTR, because audio drift can be a lot of things in an NLE system besides no black reference.

Bob Zelin



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Chris Cea
Re: VHS/ Sync question
on Jun 20, 2011 at 6:45:51 pm

Thanks for all the info Bob, As Always you, very helpful.

Just out curiosity... what slot should my Decklink HD Extreme card be in? I had this Mac Pro (2.66 12 core with 24 gigs of RAM) built by a VAR so I hope they didn't make a mistake. It looks as if the card is installed in slot 2. It takes up slot 3 as well b/c of the HDMI ports. Slot 1 is taken up by the ATI Radeon 5870 video card.

in the meantime I will follow your (and Richards) advice and work on getting another deck in here to see if I am still experiencing audio drift.

Many thanks once again
Chris


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Andrew Rendell
Re: VHS/ Sync question
on Jun 21, 2011 at 9:44:10 am

Richard and Bob reveal my secret past as editor at a big broadcaster then at a serious facility before becoming freelance, where all editing systems are installed/accepted by engineers before I got my hands on them. Which basically means that I tend to take it for granted that the editing system works perfectly, which is a dangerous assumption to make...

On the other hand, I did get the ball rolling ( http://xkcd.com/386/ comes to mind).

:-)


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