[FFmpeg-user] Why is it possible to set a timecode with 24:00:00:00 and beyond?

[Dave Rice]

> On Jan 13, 2016, at 9:25 AM, Bouke / VideoToolShed <bouke at videotoolshed.com> wrote:
>> On Jan 13, 2016, at 14:17, Robert Krüger <krueger at lesspain.de> wrote:
>> 
>> On Wed, Jan 13, 2016 at 12:53 PM, Christoph Gerstbauer <
>> christophgerstbauer at gmail.com> wrote:
>> 
>>> FFmpeg allows me to set a timecode of 24:00:00:00 and beyond.
>>> 
>>> e.g "-timecode 24:00:00:00"
>>> 
>>> When I make an MXF D10 for example, the value of 24:00:00:00 is still in
>>> the file.
>>> Most but not all programs ignore this and display 00:00:00:00 correctly.
>>> 
>>> Here are some sample extractions of the timecode offset value in different
>>> mxf d10 files generated by ffmpeg:
>>> 
>>> PAL 25fps:
>>> 
>>> -timecode 12:34:56:11  -> in file: 11477b hex -> 1132411 dec
>>> 
>>> -timecode 10:00:00:00 -> in file: 0dbba0 hex -> 900000 dec
>>> 
>>> -timecode 20:00:00:00 -> in file: 1b7740 hex -> 1800000 dec
>>> 
>>> -timecode 23:59:59:24 -> in file: 20f57f hex -> 2159999 dec
>>> 
>>> -timecode 24:00:00:00 -> in file: 20f580 hex -> 2160000 dec -> 24h
>>> 
>>> -timecode 24:00:00:01 -> in file: 20f581 hex -> 2160001 dec
>>> 
>>> -timecode 25:00:00:00 -> in file: 225510 hex -> 2250000 dec -> 25h
>>> 
>>> 
>>> NTSC 30fps
>>> 
>>> -timecode 23:59:59;29 -> in file: 2782df hex -> 2589407 dec
>>> 
>>> -timecode 24:00:00;00 -> in file: 2782e0 hex -> 2589408 dec
>>> 
>>> 
>>> Why is this possible to set a timecode after 23:59:59:XX?
>>> Is there a usecase?
>>> 
>> 
>> I wasn't able to find any official spec that says what the limit for the
>> hour part is but since it in the general case does not define a time of day
>> but an offset, why limit it to 25 hours and not use the full 100? On a
>> side-note, I have not come across a real-world use case for a timecode of
>> that magnitude either.
>> 
> Some forms of TC just lack the space to store higher numbers IIRC (LTC / VITC, perhaps also in MpegII frame headers)
> But there is logic in not going higher, as it is ‘time’.
> For a real world use case, in the old days of tape, a reelname most of the time corresponded with the hour.
> So a production with more than 24 tapes needed better administration in reel names that had to be written on the tape and box.

FWIW QuickTime timecode flags includes a "24 hour max" flag to clarify when it is and is not enabled in a timecode track, but >24 hour values are certainly allowed in the QuickTime spec. https://developer.apple.com/library/mac/documentation/QuickTime/QTFF/QTFFChap3/qtff3.html <https://developer.apple.com/library/mac/documentation/QuickTime/QTFF/QTFFChap3/qtff3.html>
Dave Rice