Creative 0404 Why use SMPTE?, Striping SMPTE, Avoiding SMPTE problems, SMPTE Subframe Resolution

6 - Appendix

SMPTE Background

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 00-10-20-30-40-50have 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.

SMPTE Subframe Resolution

Frames-per-second	Resolution
24 fps	.521 mS
25 fps	.500 mS
30 fps	.417 mS

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.

74 Creative Professional