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6 - Appendix
SMPTE Background
74 Creative Professional
time! (A one hour program would actually be 59 minutes and 56.4 seconds long.) Drop
frame was designed to correct this time difference. In 30 Drop Frame, every minute
except 00-10-20-30-40-50 have the first two frames, 00 and 01, “dropped”, hence the
name drop-frame.
Why use SMPTE?
SMPTE sync, although well over 30 years old, has the advantage of being able to be
recorded as an audio track. This allows it to be used with virtually any kind of recording
equipment from tape recorders to computer-based digital audio recorders. You can even
buy phonograph records with a SMPTE stripe!
SMPTE was designed in the days when tape dropout was a common occurrence and so it
was designed to convey “absolute” location information. Since each frame of SMPTE
code provides its own unique identification, it provides the ability for a receiving device
to recover from data dropout. In addition, edits can be performed in the middle of a
song with just a few seconds of pre-roll before the punch-in point. SMPTE is also
standardized, which means that code generated on different makes of equipment will
be compatible with each other. SMPTE also has fairly good resolution, especially at the
subframe level. You’ll be happy to know that the Sync Daughter Card resolves to the
subframe level. The chart below shows subframe accuracy at the three frame rates.
Striping SMPTE
Printing SMPTE to a track is called striping (as in stripe). SMPTE time code is recorded
on an unused audio track of another recorder, then played back into the Sync Daughter
Card. The Sync Daughter Card passes the location information on to the host computer
as MTC quarter-frame data to be used by an application such as an audio recorder or
sequencer.
SMPTE is usually recorded at about -3 VU on semi-pro gear, -10 VU on professional gear
and 0 VU on video gear. Experiment to find the optimum levels. When printing to a
time code track of a video deck, be careful. The time code playback head locations on
video decks are not standardized and can cause gross timing errors. Time code which is
striped on an audio track will always be in sync with the picture. SMPTE code is tradi-
tionally recorded on the right channel of a video recorder.
Avoiding SMPTE problems
Problems in reading SMPTE time code can often be related to poor quality code on the
tape. Poor quality code can be caused by a number of problems, the most common
being dirty or misaligned heads, amplifier clipping, or too many generations of audio
dubbing. Other problems can be caused by running the SMPTE signal through signal
processing devices such as Limiters, Reverbs, Harmonizers, etc. (Don’t laugh, it has been
done!) In fact, many video decks have built in AGCs (Automatic Gain Controls) which
will ruin the SMPTE signal if the input level is too high. Always check playback to insure
that the time code is usable. In general, no signal processing should be used on the
SMPTE signal. SMPTE code is delicate and should be treated as such.
SMPTE Subframe Resolution
Frames-per-second Resolution
24 fps .521 mS
25 fps .500 mS
30 fps .417 mS