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Re: Locking Audio in 525
- To: telecine at sun.alegria.com
- Subject: Re: Locking Audio in 525
- From: KA2IQB at aol.com
- Date: Wed, 16 Jul 1997 17:46:22 -0400 (EDT)
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- Resent-Date: Wed, 16 Jul 1997 14:46:57 -0700 (PDT)
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<< The problem we are finding is with matrial shot at 24 FPS with audio and
timecode on DAT, DA-88 etc. >>
You didn't mention what the problem is (eg. inability to get into sync,
drift, etc.), so it's hard to come up with any specific recommendations.
<< When our URSA or MKIII are run in 525 mode they don't run at exactly 24
but 23.976 instead. This of course means that the video is now slow with
respect to the audio. >>
One of the "charms" of the NTSC system is that the frame rate is 29.97 frames
per second, not an integral number (like 30). This was established about 45
years ago when it was discovered that with 30 fps and 525 lines, beating
would occur between the existing sound and proposed chroma subcarriers in
certain monochrome TV sets. In order to be able to claim true compatability
with existing receivers, the NTSC folks had to change the line rate, which in
turn puts frame rate where it is. So 24 fps film must be slowed down to
23.97 fps to match, and of course the audio has to slowed down likewise in
order to remain in sync with the picture. Since this is only a 0.1%
slow-down, the pitch shift is practically inaudible and pitch correction is
seldom bothered with.
<< What are the different ways of now locking the Audio to the Video. >>
There are many ways of doing this. Some of the prevalent ones are:
A time code system operated from either the telecine gate tach or shaft
encoder. This will generate time code at 29.97 or 23.97 when the machine is
in 525 mode. If the audio time code recorded on the tape is an integral
number, say 24 or 30 fps, all you need is an audio recorder (typically 1/4"
or DAT) that can chase the time code and the sound should remain in sync with
the picture. Note that that the picture and the sound will promptly drift
apart if the time code on the audio tape is already at 29.97!
Many digital audio workstations and disk recorders are excellent for chasing
time code (when used with a suitable time code generator at the telecine).
Another approach is to make a sprocketed mag dub of the audio, using a
suitable reference to play back the original audio at the original speed.
Then a dubber interlocked to the telecine shaft encoder can be used to play
it back in sync with the picture at any speed within reason. This method is
particularly handy if the original audio has unsuitable time code (or none at
all) and it is undesirable to re-stripe it.
Of course, the sound can always be handled in a post-syncing suite as well.
Hope this helps.
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