John, You described drop frame time code only for 29.97 fps.With 29.97 drop frame time code you need to drop 18 frames every ten minutes. So you drop two frames every minute except for every tenth minute. However, 29.97 is really 30/1.001, so you really needed to drop 17.982 frames every ten minutes, and in a 24 hour period you're still off of real time by 2.59 frames. Acceptable for editing, and you jam sync once per day if you have to maintain real time over several days.
With 24/1.001 fps, you have to drop 14.3856144 frames every ten minutes in order to stay current with real time. That's a much harder thing to do.
I came up with "Drop one frame every odd minute, drop two frames every even minute, except drop only one frame every odd ten minutes, except drop only one frame every odd hour."
That keeps you aligned with real time for the first 2 1/2 hours, but you are off by one frame at 3 hours 20 minutes. In a 24 hour period you are away from real time by 4.472 frames.
Russ' solution is pretty much the same thing with the one frame every odd minute and 2 frames every even minute, but he add in additional times to make up for the gradual drift in "my" simplified solution.
I think that my simplified solution would be easier for a human to remember and calculate for editing purposes, but Russ' solution stays with real time much more closely.
Sounds like a great paper to write for SMPTE, because I don't think there are any standards for drop frame time code in 23.976 fps. The Sony HDCam manual says you must use non-drop frame when shooting 23.976.
John S.----- Original Message ----- From: "John Willkie" <johnwillkie@xxxxxxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx> Sent: Friday, November 30, 2007 1:49 PM Subject: [opendtv] Re: A bit OT: 24 Frames Drop Frame Timecode
Russ; There is a specific way to do the counting, and it applies to all frame rates. Frames are numbered from 0 to (integer frame rate - 1) However, when doing drop frame, each second starts with frame 2 (you skip over the first two) EXCEPT for second 0, 10, 20, 30, 40 and 50, when you count from 0 to (integer frame rate - 1)I don't know if I completely understand what you are doing, but it seems to me that your algorithm has problems within each second, not just a long-termdrift over time. I can't remember where this is covered, but I suspect that it's covered in SMPTE-12M or one of the documents that refer to it. I did find it in an authoritative document. Also, 12M and other related documents are undergoing a rewrite, so you can get up to speed on these by becoming a SMPTE member and participating in the group/groups working on the document(s). www.smpte.org Hope this helps John Willkie www.EtherGuideSystems.com -----Mensaje original----- De: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx] En nombre de Russ Wood Enviado el: Friday, November 30, 2007 1:45 AM Para: opendtv@xxxxxxxxxxxxx Asunto: [opendtv] A bit OT: 24 Frames Drop Frame Timecode Hi from a long-time lurker from across the pond. I don't know where else to ask this question but I hope there are some experts on here with the answer. 24 fps video derives from the film world and in order to make it easily transmittable in a US TV environment this is often resampled at 23.976 fps. When 29.97 fps video is used the timecode is often drop-frame to make elapsed times closer to real time. The company I work for (Softel) makes captioning workstations among other things and we are being asked by our customers to use 23.976 drop-frame timecode. The algorithm for 29.97 drop-frame is not really applicable for 23.976 so what algorithm should we use? Is there a standard anywhere that defines this. The following algorithm gives a drift of < 1 frame per 24 hours:- 1) When the seconds digits are 59 go direct from hh:mm:59:23 to hh:mm:00:01 2) When the minute least significant digit is even go direct from hh:mm:59:23 to hh:mm:00:02 2a) Except when the minute digits are 00 or 16 or 30 or 44 (Note 15 and 45 are not even numbers) 2b) And hour is not 00 or 03 or 06 or 09 or 12 or 15 or 18 or 21 I don't know of any equipment out there that actually generates or reads 23.976 DF timecode but our customers are nevertheless asking for DF. Russ Wood - Softel Ltd. Pangbourne UK ________________________________________________________________________ This e-mail has been scanned for viruses by MessageLabs. ---------------------------------------------------------------------- You can UNSUBSCRIBE from the OpenDTV list in two ways: - Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org - By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word unsubscribe in the subject line. ---------------------------------------------------------------------- You can UNSUBSCRIBE from the OpenDTV list in two ways:- Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org- By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word unsubscribe in the subject line.
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