Members zakadams Posted January 12, 2010 Members Share Posted January 12, 2010 Hi guys, I'm hoping that you can give me some ideas for this problem I'm having: At our church we do a video recording of our service and its my job to edit it down. We send audio and video to final cut pro to capture, but in case there is an audio issue, we also record audio to a flash recorder. Both audio streams are from the same 48K source on our digital board. Well, this week the audio didn't get recorded with the video for some reason, but I have the audio backup. My problem is that they aren't in sync. The audio in the video project is set at 48k and so is the mp3 recording on the flash recorder. If I line it up in post, by the end of the 30 minute message, the audio is about a second ahead of the video. At first I thought it had something to do with the sample rate, but it seems like the change over 30 minutes would be much greater if the sample rate was off. Someone told me that the nature of mp3s was that their "timing can drift" but I've never run across that before. Any suggestions? Link to comment Share on other sites More sharing options...
Members scarecrowbob Posted January 12, 2010 Members Share Posted January 12, 2010 The sample rates are probably okay, but since they runn off different clocks they might not line up, in exactly the same way. The math isn't too hard: 30:00:00 minutes == 1800s 1 second disparity == 1/1800 = .0005 % difference in clock speed. I'm a Premiere guy, but you can stretch video events, and I'd just stretch the video to fit the audio. Link to comment Share on other sites More sharing options...
Members zakadams Posted January 12, 2010 Author Members Share Posted January 12, 2010 Yeah, stretching the video works, I guess I just still don't understand why they are off. The way I understand it, the clock for both devices originates from the digital board. If that's set as master for both signals what's causing the difference in timing? Link to comment Share on other sites More sharing options...
Members Cheese Grits Posted January 12, 2010 Members Share Posted January 12, 2010 Do you actually have a separate digital cable running from the source to the capture device to carry the clock signal. On my Firepod, for example, I would have to run an S/PDIF cable from the clock to the Firepod to have them synced to the same signal. The S/PDIF input also allows the FIREPOD to receive and transmit word clock. Word clock is the synchronizing signal that indicates the sampling frequency or rate of sample words over a digital audio interface. From the manual, in case my explanation was unclear. Link to comment Share on other sites More sharing options...
Members Ethan Winer Posted January 12, 2010 Members Share Posted January 12, 2010 If I line it up in post, by the end of the 30 minute message, the audio is about a second ahead of the video. Bob is correct that the problem is due to clock drift. The fix I suggest is much simpler than trying to time-stretch, which won't necessarily work since clocks can speed up and slow down over time. Better to just cut the audio every five minutes or so at a natural pause, and slide the audio left or right by one or two frames as needed. If you have scene transitions, those are perfect places to re-sync. --Ethan Link to comment Share on other sites More sharing options...
Members IsildursBane Posted January 12, 2010 Members Share Posted January 12, 2010 Bob is correct that the problem is due to clock drift. The fix I suggest is much simpler than trying to time-stretch, which won't necessarily work since clocks can speed up and slow down over time. Better to just cut the audio every five minutes or so at a natural pause, and slide the audio left or right by one or two frames as needed. If you have scene transitions, those are perfect places to re-sync.--Ethan Yep. That's the best and easiest way. I've got a couple direct-to-disk recorders that occasionally drop frames at file breaks and this is what I do. -Dan. Link to comment Share on other sites More sharing options...
Members zakadams Posted January 13, 2010 Author Members Share Posted January 13, 2010 Do you actually have a separate digital cable running from the source to the capture device to carry the clock signal. On my Firepod, for example, I would have to run an S/PDIF cable from the clock to the Firepod to have them synced to the same signal.From the manual, in case my explanation was unclear. My understanding of digital (correct me if I'm wrong) is that the mixing board sends word clock through the AES connection to the recorder. There are 2 AES outs going to 2 separate recording systems but the same master clock from the digital board, so I don't understand what is causing the differences. Link to comment Share on other sites More sharing options...
Members Ethan Winer Posted January 13, 2010 Members Share Posted January 13, 2010 There are 2 AES outs going to 2 separate recording systems but the same master clock from the digital board, so I don't understand what is causing the differences. If you really are clocking both devices from the same clock, they shouldn't drift. Are you saying that both your video camera and audio recorder can receive clock sync, and both are wired appropriately and set to use external sync? --Ethan Link to comment Share on other sites More sharing options...
Moderators Lee Knight Posted January 13, 2010 Moderators Share Posted January 13, 2010 Yeah, I'd just cut the audio where you need to line it up again. Cross fade some ambience in your holes to disguise. Link to comment Share on other sites More sharing options...
Members zakadams Posted January 13, 2010 Author Members Share Posted January 13, 2010 If you really are clocking both devices from the same clock, they shouldn't drift. Are you saying that both your video camera and audio recorder can receive clock sync, and both are wired appropriately and set to use external sync?--Ethan Well, maybe not now that I think about it. The audio recorder gets its signal via AES direct from the board. The audio from the video feed is sent AES to another digital board in our video room, out AES from that into a AJA KONA video capture card to the Mac. I'm not sure now that the Kona is synced because it's an extra step past the original clock. That might be the issue? Link to comment Share on other sites More sharing options...
Members Ethan Winer Posted January 14, 2010 Members Share Posted January 14, 2010 The sync'ing needs to happen live while the audio and video recorders are recording. Doing that later while editing is too late. --Ethan Link to comment Share on other sites More sharing options...
Members zakadams Posted January 14, 2010 Author Members Share Posted January 14, 2010 The sync'ing needs to happen live while the audio and video recorders are recording. Doing that later while editing is too late.--Ethan It does all happen live, but I'm wondering if the original clock from the source is not passing through to the recorder. It's like this: Live digital board-->Audio RecorderLive digital board-->Video Room board--->Video Recorder I'm starting to think that the video room board (which is clocked to the live board) isn't passing that same clock to the recorder (which is not directly clocked to the live board) Link to comment Share on other sites More sharing options...
Members tacdryver Posted January 15, 2010 Members Share Posted January 15, 2010 Interesting thread..I put up some You tube vids, and because the audio was so horrible through my old camcorder, I decided to direct video into the laptop, audio through a Fostex MR8..then sync up later...easier said then done...when I added the audio onto Windows Movie Maker...they have option to nudge the audio left or right to sync up with the video...my first attempt at this...it seems a little off... http://www.youtube.com/watch?v=SJQTtw3-7lc then I decided to simply use my wife's Karaoke mic to record real time with video...into the laptop...sound quality came up, synced,but not what I would call perfect...this is that attempt.. http://www.youtube.com/watch?v=Zhi6fjJahj4 On the earlier vid, the problem was one little nudge left, audio was too early, one little nudge right, maybe it was right on, but later, it seems off...there is no doubt that somewhere along the way, something got a little compressed or stretched...I asked around on how to do live video with music quality audio...and while there are dif methods...the best seems to be to run audio through the mixer for quality control, then mixer out to the video cams audio port..a good cam, which I don't have will have good audio inputs with audio level meters that you can adjust...once set...when you record, video will be recording at the same time as audio.. So while I am going ultra cheap, 15 year old Hi8, barely ok video quality, Karaoke mic, barely acceptable audio quality...kids with Dad's vid camera can put out a better product because of that audio in capability and much higher vid resolutions......the more I learn I really think to do this right is get an HD cam with audio inputs, a small mixer...done... From the camera back to the computer for editing there should be separate audio/video tracks...then you can bump the volume, etc for taste... But for right now...it's about making the best of cheap gear...which is kinda fun..as I am )a complete noob at the video thing, it turns out you have to know something about live sound as opposed to just recording people talking...it's fun trying to learn how to get this right.. Link to comment Share on other sites More sharing options...
Members Rikv Posted January 28, 2010 Members Share Posted January 28, 2010 if stretching the complete audio track doesn't work, you can try to cut the footage in little pieces, sync those pieces, when done place everything next to one another. Link to comment Share on other sites More sharing options...
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