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Hi Guys,
There is a good reason why NTSC uses 29.97 fps instead of 30 fps. Originally, B&W NTSC video ran at 30 fps. When they developed the scheme for adding color to NTSC video, while still retaining compatibility with existing B&W TVs, they found it was necessary to change the frame rate to 29.97 fps because of the way the color subcarrier information is added to the B&W video signal. Of course, there is a lot more detail to it all then what I've said above... If you want to know all the details, try http://www.ee.washington.edu/consele.../ntsc/95x4.htm for starters...
If you try to capture at 30 fps, when your source is only 29.97 fps, you should see one dropped frame every 33 seconds.
Most VCRs and camcorders really do put out a signal that is very close to 29.97 fps. The playback mechanisms are electronically controlled so as to keep it very close, despite the fact that tapes stretch or shrink depending on temperature, etc. Many VCRs and camcorders even have built in Time Base Correctors (TBC) which keeps the timing of the signals even more precise.
When you drop a frame during capture, it will cause your audio to become one frame out of sync with your video - unless you are using AVI-IO, which will compensate for the dropped frame to keep the audio in sync.
Having your audio one frame out of sync with your video will be very hard to notice. But, if you drop many frames over time, the loss of sync will become more noticeable.
Audio sync problems also come about when you have video clips with varying frame rates (29.7, 30.1, etc.) like you get from the PC-VCR Remote captures. When you try to edit a bunch of these clips together to create one new .avi file with a constant frame rate, you will find that the audio will get out of sync in the final .avi. This is another reason to use AVI-IO, which will capture each clip with exactly 29.97 fps frame rate.
David, your 6 dropped frames over a 4 minute video would only cause about 1/5 of a second of lost time (6/30 = 1/5). I doubt that this would be very noticeable. How are you getting the audio that you are using to replace the original audio? This is probably where your problem is coming from.
Rick
There is a good reason why NTSC uses 29.97 fps instead of 30 fps. Originally, B&W NTSC video ran at 30 fps. When they developed the scheme for adding color to NTSC video, while still retaining compatibility with existing B&W TVs, they found it was necessary to change the frame rate to 29.97 fps because of the way the color subcarrier information is added to the B&W video signal. Of course, there is a lot more detail to it all then what I've said above... If you want to know all the details, try http://www.ee.washington.edu/consele.../ntsc/95x4.htm for starters...
If you try to capture at 30 fps, when your source is only 29.97 fps, you should see one dropped frame every 33 seconds.
Most VCRs and camcorders really do put out a signal that is very close to 29.97 fps. The playback mechanisms are electronically controlled so as to keep it very close, despite the fact that tapes stretch or shrink depending on temperature, etc. Many VCRs and camcorders even have built in Time Base Correctors (TBC) which keeps the timing of the signals even more precise.
When you drop a frame during capture, it will cause your audio to become one frame out of sync with your video - unless you are using AVI-IO, which will compensate for the dropped frame to keep the audio in sync.
Having your audio one frame out of sync with your video will be very hard to notice. But, if you drop many frames over time, the loss of sync will become more noticeable.
Audio sync problems also come about when you have video clips with varying frame rates (29.7, 30.1, etc.) like you get from the PC-VCR Remote captures. When you try to edit a bunch of these clips together to create one new .avi file with a constant frame rate, you will find that the audio will get out of sync in the final .avi. This is another reason to use AVI-IO, which will capture each clip with exactly 29.97 fps frame rate.
David, your 6 dropped frames over a 4 minute video would only cause about 1/5 of a second of lost time (6/30 = 1/5). I doubt that this would be very noticeable. How are you getting the audio that you are using to replace the original audio? This is probably where your problem is coming from.
Rick
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