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User Bits
- The last remaining thirty-two (32) bits are “extra” and are available to encode "user" information as desired.
This could be additional information such as the date, a production number, an experiment number, etc. Like the time code,
the 32 User Bits are usually sub-divided into 8 groups of 4 bits each and can have the numeric values of 0-9 and in addition,
the hexadecimal values of A-F.
Time Code Frame Rates
- SMPTE time code can be generated at different frame rates in order to accommodate the
variety of video and film frame rates in use today. This match of time code and image frame rates is necessary in order to
be able to assign a specific time code number to each individual image frame. Matched frame rates insure that a time code
frame number does not “straddle” more than one frame, or more than one frame does not straddle more than one time code
number.
Some of the more common frame rates for SMPTE time code are as follows:
30 FPS -
Compatible with US black and white (B/W) SD analog television frame rate. This time code frame rate is also
compatible with 60 FPS HD frame rate applications.
29.97 FPS -
Disregarding time code for a moment, this "slightly less than 30 FPS" frame rate resulted from the need to
slow down the 30 FPS B/W frame rate of TV in the 1950's by 0.1% in order to make color TV more "compatible" with and
watchable on B/W TV sets already in use. The 0.1% slowdown results in a frame rate of 29.97 FPS, which is simply 30 x
0.1% = 0.030 FPS, and 30 FPS (-) 0.030 FPS = 29.97 FPS.
29.97 FPS NDF -
NDF = Non-drop frame time code. Although 29.97 FPS time code is compatible with the NTSC analog
SD color television frame rate, its actual time value lags that of "real time" by that 0.1% value. This time code frame rate is
also compatible with 59.94 FPS HD frame rate applications.
29.97 FPS DF -
DF = Drop Frame time code. Drop frame time code is compatible with the US NTSC SD analog color
television frame rate of 29.97 FPS and also maintains a nominal “real time” time value by skipping frame numbers 00 and
01 at the start of every minute except on the tens of minutes. It's just the numbers 00 and 01 of the time code that are
skipped, no actual frames are skipped or dropped. This time code frame rate is also compatible with 59.94 FPS HD frame
rate applications.
There is more about this in later paragraphs.
25 FPS -
Compatible with European PAL SD analog television frame rate. Also called "EBU" time code. This time code
frame rate is also compatible with 50 FPS HD frame rate applications.
24 FPS
-Compatible with standard film frame rate of 24 FPS. This time code frame rate is also compatible with 48 FPS HD
frame rate applications.
23.976 FPS
- This is the standard 24 FPS film frame rate slowed down by the same 0.1% percentage that the B/W TV
video frame rate was slowed to when NTSC color television was adopted in the 1950's. For 24 FPS this amount is 0.024
FPS and 24 FPS (-) 0.024 FPS = 23.976 FPS. Used in various video and film recording, editing, and transfer applications.
Time Code for HD Frame Rates -
Although there are many different frame rates in use for HD video, the majority are all
multiples of the earlier frame rates used for SD television, in order to be backwards compatible. This backwards
compatibility has also allowed the earlier SD SMPTE time code frame rates to also be used for HD video. For example,
29.97 FPS time code can be used with 59.94 FPS video when allowances are made for understanding that each frame
number of the time code applies to two frames of the associated HD video, in much the same way that each frame of time
code for SD video applies to two fields of video..
Time Code Time and Real Time
- “Real time” is the passage of time as measured by a clock. Although SMPTE time
code has a clock type time format, its time value may or may not match that of real time. This means that even though it
may look like the “seconds” of the time code are changing once a second, they may be changing at a slower or faster rate.
The "real time" time value of SMPTE time code running at 24 FPS, 25 FPS, and 30 FPS matches that of real time. The
time value of time code running at 29.97 FPS DF SMPTE time code pretty much matches that of real time.
The "real time" time value of 23.976 FPS and 29.97 FPS NDF SMPTE time code runs slower and does not match that of
real time, proceeding to get further and further off as time passes.