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Strange Audio frame Rate Issue

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Max Prum
Strange Audio frame Rate Issue
on Jul 2, 2014 at 2:28:43 pm

So I am importing externally recorded audio into Avid MC 7.04 into a 1080p/23.976 project. When I hit import it does not give me the option as it often does, to select the frame rate at which to read the timecode. When I check the frame right by choosing the "FPS" column in the bin it says 23.98 which is what it should be. If I right click to "Get Info" in the source monitor it claims to be 23.98 too. I opened all the .wav files in WaveAgent and they also claim to be 23.98. BUT when I autoseqeunce the audio the sequence is 29.97 and when I match frame and count off the frames in the source monitor the timecode is also 29.97. When I apply AUX TC and try to MultiGroup it says "No Timecode found for clip(my audio clip name)". I have to move forward so I am just grouping manually by inpoint and the audio does not appear to drift. But this is confusing to me and I am worried that this could cause an issue down the line. Has anyone seen this before? Is there a setting somewhere that is causing my imported audio to be read at 29.97 even though the AVID acknowledges it is 23.98?

Max


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Michael Phillips
Re: Strange Audio frame Rate Issue
on Jul 2, 2014 at 5:08:49 pm

Are you in an NTSC 23.976p project type?

Michael


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Max Prum
Re: Strange Audio frame Rate Issue
on Jul 3, 2014 at 3:17:44 pm

Yeah i think that was the problem, I was ingesting footage 14:1 in a 23.976 NTSC format and then switching up to 1080p/23.976 for editing. The audio I ingested at 23.976 had this problem but when I ingested at the higher resolution project type it came in normally. Thats wierd because they are both supposed to be 24 FPS? but thats it. Thanks.


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Michael Phillips
Re: Strange Audio frame Rate Issue
on Jul 3, 2014 at 5:12:04 pm

You content may be 23.976, but the NTSC 23.976 and HD 23.976 are not the same thing when it comes to timecode management. There is no 24fps timecode or format defined by SMPTE for standard definition formats. 24 and 23.976 are "fabricated"formats in the NTSC signal with forms of pullodwn; 2:3:2:3 and 2:3:3:2, etc. In 1992, Avid derived a method to which native 24fps editing could be done from NTSC and PAL source within SMPTE constraints. Tape machines and the like only ran at 30NDF, 30DF or at 25fps. So capture and layback was designed to work native 23.976/24 but within PAL and NTSC - thus the timecode counting issue.

When HD was defined and implemented, SMPTE spec includes a 24fps timecode. There is no 23.976 timecode, nor is there a 24fps DF timecode. Just as there are no 50fps and 60fps SMPTE defined timecodes.

In the world of file based, a lot changes, many parts of MC need to be updated to better serve all this, but it works if your are cognizant of what is happening. If your audio was recorded 24fps TC, then I would bring them into a 23.976p project to start.


Michael


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Robert Ober
Re: Strange Audio frame Rate Issue
on Jul 4, 2014 at 3:14:18 pm

Hello Mr. Phillips,

First, thank you for the knowledge you add to this forum.

Second, do you have a detailed article or course on this framerate nightmare? If not, is there anything you recommend? Vendors and others make this even more difficult than it could be by saying 24 when they mean 23.976 and other such inaccuracies. And what is 60i? Is that 59.97?

In MC7 is a 24p Film project really 24 not 23.976? But then it plays to an external monitor as 23.976 psf?

Third, I signed up for and am looking forward to your next Moviola Webinar.

Thanks Much,
Robert A. Ober


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Job ter Burg
Re: Strange Audio frame Rate Issue
on Jul 4, 2014 at 3:28:14 pm

MC supports both 1080p23.976 and 1080p24.000 projects. They are two different framerates. In HD, 1080p24.000 projects play out as 1080/24P or 1080/24PsF (depends on your hardware options and settings). 1080p23.976 plays out as 1080/23.976p or 1080/23.976PsF (depends on your hardware options and settings).

In SD, MC supports 23.976p NTSC, 24.000p NTSC and 24.000p PAL projects. Both NTSC versions will output an 29.97i SD NTSC signal (by adding 3:2 pulldown). PAL projects play out a 25i PAL signal (with 12+1 fields PAL pulldown for 24p projects).

Timecode is just a counter. NTSC supports 30 and 30DF, PAL supports 25.


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Robert Ober
Re: Strange Audio frame Rate Issue
on Jul 7, 2014 at 9:48:40 pm

NTSC supports 30 and 30DF

Isn't that an example of what I am saying? NTSC 30 is really 29.97.

Why can't we be correct?

Thanks for your expertise,
Robert


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Michael Phillips
Re: Strange Audio frame Rate Issue
on Jul 7, 2014 at 11:23:37 pm

The full matrix can be:
30DF
30NDF
29.97DF
29.97NDF

Then you add progressive versus interlace which adds further confusion started by marketing wars between Sony and Panasonic...

So you need to quantify frame rate and frame structure so 1080p/59.94 and 1080i/59.94 look the same, but one is actually FRAMES per second, and the other is INTERLACE fields per second. So the latter as more motion resolution per second, but at half the vertical resolution. It's a trade off.

Here is a book by one of the guru's in the industry. He has several that get more complex from here.


http://www.amazon.com/A-Technical-Introduction-Digital-Video/dp/047112253X/...

Or you can start with the excellent, and free Digital Factbook by Quantel:

http://www.quantel.com/repository/files/library_DigitalFactBook_20th.pdf




Michael


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Michael Phillips
Re: Strange Audio frame Rate Issue
on Jul 5, 2014 at 2:34:19 pm

As Job summed up admirably, there are distinct differences in playback frame rates between 23.976 and 24.000. This is a .1% difference between the two. So a 1 hour program at 24.000 will be (1 hour x .1%) longer when played back at 23.976. Or faster and shorted if going from 23.976 to 24.000. This is true for picture playback as audio does not use timecode as a playback speed, but as a counting method where sample rate is what determines playback speed against whatever clock is used for a given frame rate.

So... given 24fps at 48000 samples per second, any audio file that is not 48000 samples will be slowed up or slowed down. So a file recorded at 48048 in a 48000 project will play back .1% slower. If you download Sound Device's Wave Agent, you can play with sample rate to timecode and see what happens.

This same .1% is what is referred to as the "fractional" rate and is part of life in NTSC based countries (North America, Japan, etc.). What is referred to as 30fps or 60fps can be and most likely (30 x .1%) 29.97 or (60 x .1%) 59.94.

Many cameras allow you to record 59.94 or 60.000 for example so that only adds to the potential errors. For example, the Canon 5D when it first came out was true 30.000 and not 29.97 which caused sync issues when that was first used.

People tend to refer to 29.97 as 30 because humans are either efficient or lazy and it is much easier to say "thirty" than it is to say "twenty nine point nine seven".

My latest blog offers a spreadsheet that does some of the fps to sample conversion as well as the number of +/- semitones needed to maintain pitch when changing speed on audio:
http://24p.com/wordpress/?p=195


Michael


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Robert Ober
Re: Strange Audio frame Rate Issue
on Jul 7, 2014 at 10:14:20 pm
Last Edited By Robert Ober on Jul 7, 2014 at 10:33:24 pm

People tend to refer to 29.97 as 30 because humans are either efficient or lazy and it is much easier to say "thirty" than it is to say "twenty nine point nine seven".

They are not being efficient but they may be lazy. Even worse, they are being incorrect.

The one that is maybe more egregious is saying 4K for UHD.

Thanks for your explanation of the audio issues.

So 60i is 59.94 which is what i expected but is that a half res frame every 59.94 seconds?

I really think many folks could use a whitepaper on this subject. May you two fine folk could collaborate on it?

Thanks Much,
Robert A. Ober


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