[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-term
drift 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

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