15
Time Code and Time of Day
- Although it ultimately depends on the accuracy of the time code generator, when running at
one of the integer frame rates or 24, 25, or 30 FPS, SMPTE time code can be set equal to and will maintain accurate time-
of-day time.
Drop Frame Time Code
- The exception to time code time matching real time is the time code used with the NTSC video
system. In this system the frame rate as represented by the frame numbers is 30 FPS. However, the frame numbers are
counted up by a time base that is running just slightly slower than 30 FPS, running at only 29.97 FPS. So, after counting for
one second of real time, the frame number isn't at frame 30 yet, almost, but not quite. It takes just another 0.03 seconds
more of real time for the next 29.97 FPS tick to roll the frame count over to the 30th frame. So this SMPTE clock is
running slower than real time, even though it has an hours/minutes/seconds display format.
As time goes by, the amount of real time error continuously increases until the SMPTE time code time value eventually
lags that of real time by about 3 1/2 seconds an hour. To compensate for this error, the normal frame number counting
sequence is altered slightly during generation of the time code.
In 30 FPS time code the frame number count starts at frame “00”, advances on up to frame number “29”, then wraps around
to frame 00 and starts over; 00,01,02………….28,29,00,01,02. Each time the frame count wraps around to frame 00 the
seconds change to the next second, then eventually the minutes and hours change in typical clock fashion. However, after
counting for one minute the time code time value has fallen behind real time by about two frames worth of time, about 66
thousands of a second (66ms).
The method chosen to correct this two frames a minute lag in real time was simply to start the frame count at 02 instead of
00 at the start of each new minute. Then continue counting as normal. This is called “drop-frame” time code, although no
frames of anything are actually dropped.
So, with drop frame time code, at the start of each minute the frame count wraps from 29-to-02 instead of 29-to-00,
skipping the numbers 00 and 01; …….27,28,29,02,03……. The result is that the SMPTE time code time gradually falls
behind real time for a minutes worth of time, then jumps ahead when the next new minute starts, then gradually falls behind
again. Although there is a continuously varying short time error, the overall real time error is greatly reduced.
Actually, to fine tune the real time accuracy of drop frame time code, the once a minute drop-frame correction is not
performed whenever the minutes change occurs at the start of a new tens of minutes. At the start of each tens-of-minutes
the frame number count wraps normally, from 29-to-00, rather than from 29-to-02.
Drop frame correction of the time code is a continuous process and it is not noticeable that it is occurring when looking at a
real time clock display using 29.97DF SMPTE time code.
Non-Drop Frame Time Code -
Non-drop frame time code is time code using the 30 FPS time code numbering system that
is actually counted or advanced at the slightly slower frequency of 29.97 times-per-second, and in which drop frame
correction is not performed. This causes the time code real time value to lag behind and not match that of actual real time.
However, in this format there are no skipped frame numbers.
6
MAINTENANCE
6.1 Cleaning
1. Do not attempt to disassemble your TRG-100 to clean it.
2. Clean your TRG-100 using only a damp cloth.
3. NEVER use water or solvents such as alcohol, window cleaner, etc., to clean your TRG-100.
6.2 Service and Troubleshooting
If you suspect your TRG-100 is not operating properly, check the following:
1. Check all video and time code coaxial cables and connections for opens or shorts.
2. If using an AC power adapter different from the one supplied with the TRG-100, make sure it supplies the TRG-100 with
at least 9 volts (maximum of 12 volts) when the TRG-100 is switched on.