VideoHelp Forum
+ Reply to Thread
Results 1 to 6 of 6
Thread
  1. I've got some DV footage (recorded with a Sony DVCAM) of a pastor giving a sermon, but the audio level fed to the camera from the house mixer during the event was way too low; fortunately, I have a separate recording of the audio that was recorded from the monitor mixer into a netbook using Audacity at a much better level. I want to replace the audio recorded by the camera with the external recording. I'm using Premiere Pro CS4 with the NTSC DV Standard 48kHz profile, and thought it would be fairly straightforward: add the DV footage to the timeline, add the external WAV recording as a second audio track, and shift the external audio track back and forth until the external audio and camera audio play in sync with no echo; then mute/delete the camera audio and export, done.

    The problem I'm having is that while I can get the external audio to sync with the video/camera audio at the beginning, the external audio drifts out of sync as time goes on... the entire recording is somewhere around 40 minutes long, and by the end they are around 1 or 2 seconds apart (I don't remember which way it drifts, whether the external audio is ahead or behind at the end).

    I've considered the possibility that the camera and netbook recorded at slightly different speeds, but I find that really hard to believe. I've had to do something very similar in a live environment before, where a recording (taken with the same camera as my current footage and fed through a Sony DVDirect recorder) had the audio input on the camera turned way up to the point of clipping and distortion at recording time, but a separate audio-only recording (of the same audio signal from the same output on the sound board) recorded on a computer using Cool Edit Pro was fine. When it came time to playback the recording to a live audience, I played the DVD and muted the DVD audio on the sound board, and played the separate non-distorted audio recording from the computer, using play/pause on the computer to get the audio to sync with the video, and then let them play together in real-time... and they stayed in sync the entire way through (again, about 35-40 minutes).

    So, I'm wondering what's going on here. At first I thought it might have something to do with the sample rate of the audio recorded to DV by the camera being 48kHz, while the netbook recording was done at 44.1kHz... but I resampled the external audio recording to 48kHz in GoldWave (preserving length) and still have the same problem. I also wonder whether it has anything to do with the timecode display I'm using in Premiere Pro... whether choosing drop-frame or non-drop-frame timecode display would have any effect? Or is there some background conversion or resampling that Premiere is doing to my external audio?

    Any insight, pointers, suggestions, would be greatly appreciated!
    Quote Quote  
  2. Member
    Join Date
    Jan 2007
    Location
    Republic of Texas
    Search Comp PM
    A few questions...

    1. Was the DV footage recorded in SP speed or LP speed?
    2. Were both the camcorder and audio recorder running continuously, without pause, through the entire event?
    3. Are you absolutely certain that you successfully converted the 44.1kHz audio to 48kHz? And were both 16-bit sampling?
    4. When you capture the DV footage to your computer, were there any dropped frames reported?
    Quote Quote  
  3. Member Cornucopia's Avatar
    Join Date
    Oct 2001
    Location
    Deep in the Heart of Texas
    Search PM
    My guess is the DV footage was recorded at pulldown rates. IOW, instead of recording at 48,000Hz, it's recording at (30000 / 1001 / 30 * 48,000) or 47,952Hz. This is common enough. Problem might be if it doesn't tell you that - it still says it's 48kHz or something.

    Likely, your Netbook/Audacity audio is straight 48k. If you apply a pulldown to it, that is: change the base samplerate without changing the data, it should read ok in the timeline and stay in sync the whole way through. (Of course, it might need to go in the other direction (pullup vs. pulldown), depending on base assumptions...)

    If you don't have ProTools, a freeware PC app that can do this is R8brain.

    I'm making an educated guess here, because of the delay factor that you said matches up almost precisely...

    Scott
    Quote Quote  
  4. @filmboss80:
    1. SP mode.
    2. I was running the camera, and the only time it was stopped was to switch tapes partway through the event (~1h45m event), but that occurred prior to the beginning of the portion I'm working on, and it ran continuously through that portion. I'm 99% certain that the audio recording on the netbook ran continuously through the entire event; I'll confirm with my colleague that did the audio recording.
    3. I'm 99.9% certain I converted the external audio sample rate from 44.1 to 48kHz; the file properties/info in both Goldwave and Premiere reported the changed values. As for sampling bits, that I'm not sure of... I want to say the external recording was 44.1kHz and 16-bit, which I converted to 48kHz/16-bit. The Premiere workflow is 48kHz/24-bit, I believe.
    4. No dropped frames reported when I captured from the camera to the computer.
    @Cornucopia:

    That sounds like a perfectly feasible possibility, although I'm losing you slightly when you talk about the DV footage (i.e. video) being recorded in terms of Hz (as opposed to frames/s); I'm assuming it's related to the ratio between 29.97fps vs. straight 30fps? Would knowing the model # of the camera I used help determine whether that particular camera often does what you're suggesting? Regardless, I'll give R8brain a shot and see what happens.

    Thanks to both of you for your help!
    Quote Quote  
  5. Member Cornucopia's Avatar
    Join Date
    Oct 2001
    Location
    Deep in the Heart of Texas
    Search PM
    Could have been something strange in your SRC (sample rate conversion), however I think not (unless your assumptions were incorrect to begin with). Normal DV is (or ought to be) 48kHz. It doesn't usually come as 24bit, though.

    I said 44.1kHz or 44100Hz, or 48kHz or 48000Hz, when in actuality it's counting samples/second, not "cycles" per second, but it's the same thing.
    Audio counts Samples per second, whereas Video counts Frames (or Fields) per second.

    In the case I was talking about though, when a pro audio recording expects there to be a pulldown necessary, it matches the Percentage pulldown in order to keep the durations identical between Audio and Video (to maintain sync). So if you're shooting 24p on 60i, it's not REALLY 24p, it's 24*1000/1001 (or 23.976...Fps), because the 60i isn't REALLY 60i, it's 60 * 1000/1001 (or 59.94...fps).
    So, with audio you have to have the same 1000/1001 change. N'est-ce pas?
    Now, with regular DV framerates (50i or 59.94i), the audio SHOULD be true 48kHz, but something could have happened (maybe not with the recording, but with the way Premiere, etc. interpreted it)...

    Heck! Try the rate change - if it doesn't work, you're not out much time. If it works, you're good!

    Scott

    edit: this kind of stuff usually ONLY occurs when you bounce back and forth between working as FILM OUTPUT vs. working as VIDEO OUTPUT.
    Quote Quote  
  6. Member edDV's Avatar
    Join Date
    Mar 2004
    Location
    Northern California, USA
    Search Comp PM
    In my experience this drift is typical for non-genlocked* camcorders and/or separate audio recorders. The "echo" (drift) gets worse as time goes by. The drift happens even for two similar camcorders running off their internal oscillators.

    A separate issue is the 3.6 second difference in elapsed time over an hour for a camera running 29.97 fps and a stop watch. Typical audio recorders lack a precise clock reference so speed can vary enough to add to seconds over an hour.


    * pro camcorders/audio recorders are run off a common sync generator connected either with cable or in recent years wirelessly.
    Recommends: Kiva.org - Loans that change lives.
    http://www.kiva.org/about
    Quote Quote  



Similar Threads

Visit our sponsor! Try DVDFab and backup Blu-rays!