Simply put, timecode is a numeric address recorded on a piece of professional tape that gives each frame a specific numeric address.
It is written like this:
01:30:24:05. That represents 1 hour, 30 minutes, 24 seconds, and 5 frames.
In NTSC, there are 2 types of timecode: non-drop-frame & drop-frame.
Before I go too much further into timecode, though, you have to understand a few things about how NTSC TV works:
In North America, Standard Definition TV operates in 29.97 frames per second(fps). What’s more, this can be broken down into 59.97 fields per second.
What’s the difference between frames and fields? To display an image, a TV has to draw the image onto the screen nearly 30 times in a second. Physics won’t allow the cathode ray emitter (the doo-hickey that makes TV images appear on your TV screen) to draw the entire frame that quickly, so the designers cheated a little. Each NTSC frame is comprised of 525 scanlines. First the odd lines are drawn on the screen, side to side, top to bottom. 1,3,5,7,9 up to line 262.5. Then the cathode ray emitter gets reset to the top again and starts the even scanlines. 2,4,6,8,10 up to 262.5. The odd field’s scanlines added to the even field’s equal 525 scanlines. Viola! Interlaced video!
Why 29.97fps? Because in the early days of broadcast TV, things ran at 30 fps – but only in black and white. When color TV was rolled out, the method by which they had to introduce color into the broadcast signal would have fried most TV’s in North America. So rather than force the US consumer to throw away a perfectly good TV, they came up with a work-around. Tweak the frame just slightly so as not to adversely effect the B/W TV’s already out there. Hence the frame rate was adjusted down by .03 frames per second (or, more specificaly, .06 fields). Makes math a little difficult to deal with though, since now there are 1798.2 frames in a minute instead of 1800.
Now…back to timecode…
All of that is important to understand why we even need such a thing as non-drop and drop-frame timecode in the first place. Timecode gives you an address to refer to when dealing with bits and pieces of the footage you have on a given tape. It can also be used for synchronization between video and audio (and other) gear.
If I tell you a shot starts at
01:30:24:05. You can cue the tape to that point on the tape and there it is. Accurate cueing becomes very important…no…absolutely essential when you are editing shots together or logging footage for an editor to use in an edit suite.
So what’s all this about drop-frame and non-drop-frame timecode? (refferred from now as DF & NDF TC from now on) Let me start by making one thing perfectly clear: THIS DOES NOT MEAN THAT ANY FRAMES OF CONTENT ARE DROPPED OR OTHERWISE LOST! I have had more than one conversation about DF & NDF TC in which it became clear that the other person thought that frames of the material were being dropped out to make this work. No! No! No!
An important thing to keep in mind is that the best kind of timecode (i.e. the most useable) is time code that continuously ascends on successive frames. If you start with TC
00:00:00:00 the next frame should be
00:00:00:03 and so on. In NTSC NDF, frames are
:29, seconds are
:59, minutes are
:59 and hours are
:23 – this matches up with a 24-hour clock or time of day clock…almost.
If you do the math you can see that simply counting 29.97 frames per second from
23:59:59:29 for 24 hours will be off by a significant number of frames at the end of that period. After 24 hours pass on a wall clock, your NDF timecode clock will be at
24:01:26:14, that’s 2,592 frames off – or
00:01:26:15 in HH:MM:SS:FR’s.
To reduce the numbers down to something more manageable, try looking at just an hour of real time versus an hour of NDF timecode – there’s a 108 frame difference – 3 seconds and 18 frames. Somehow you need to lose 108 frames from the count so that
01:00:00:00 hits at the same time on tape as your wall clock hits one hour.
The very simple definition of drop frame is: Drop 2 frames from the count every minute except the tenth mintues (:10, :20, :30, :40, :50) except :00 which drops 2 frames as well. (Notice I said “drop 2 frames from the count?”)
If you jog through, frame-by-frame, at a regular minute turnover on a piece of DF timecoded media, you will see the count go from
00:02:00:02 on the very next frame. No content has been dropped, just frames. Do that 54 times an hour and you get 108 frames dropped out of the count so that at the end of a real-time hour a DF timecoded hour come out equal.
So, after all that the question always gets asked: So what? Who cares about all this precision with timecode and time of day? Broadcasters. Remember they live and die by the clock. They sell time. So everything must be precise. Drop frame code is how they acheive that.
Now of course all these new HD formats are going to change things a little, but drop-frame versus non-drop-frame issues are going to be with us for quite some time to come.