SMPTE Timecode - Drop Frame Timecode

Drop Frame Timecode

Drop frame timecode dates to a compromise invented when color NTSC video was invented. The NTSC designers wanted to retain compatibility with existing monochrome TVs. To minimise subcarrier visibility on a monochrome receiver it was necessary to make the color subcarrier an odd multiple of half the line scan frequency, the multiple originally chosen was 495. With a 30 Hz frame rate the line scan frequency is (30 x 525) = 15750 Hz. so the subcarrier frequency then became (495/2 x 15750) = 3.898125 MHz. This was the subcarrier frequency originally chosen, but tests showed that on some monochrome receivers an interference pattern caused by the beat between the color subcarrier and the 4.5 MHz sound intercarrier could be seen. The visibility of this pattern could be greatly reduced by lowering the subcarrier frequency multiple to 455 (thus increasing the beat frequency from approx 600 kHz to approx 920 kHz) and by making the beat frequency also equal to an odd multiple of half the line scan frequency. This later change could have been achieved by raising the sound intercarrier by 0.1% to 4.5045 MHz, but the designers, concerned that this might cause problems with some existing receivers, decided instead to reduce the color subcarrier frequency, and thus both the line scan frequency and the frame rate, by 0.1% instead. Thus the NTSC color subcarrier ended up as 3.57954545 MHz (actually 315/88 MHz), the line scan frequency as 15734.27 Hz and the frame rate 29.97 Hz (exactly 30/1.001 Hz).

This meant that an "hour of timecode" at a nominal frame rate of 29.97 frame/s was longer than an hour of wall-clock time by 3.59 seconds, leading to an error of almost a minute and a half over a day, as the timecode was calculated in a manner that assumed the frame rate was exactly 30 frame/s.

To correct this, drop frame SMPTE timecode was invented. In spite of what the name implies, no video frames are dropped (skipped) using drop-frame timecode. What's actually being dropped are some of the timecode "labels". In order to make an hour of timecode match an hour on the clock, drop-frame timecode drops frame numbers 0 and 1 of the first second of every minute, except when the number of minutes is divisible by ten (i.e. when minutes mod 10 equals zero). This achieves an "easy-to-track" drop frame rate of 18 frames each ten minutes (18,000 frames @ 30frame/s) and almost perfectly compensates for the difference in rate, leaving a residual timing error of roughly 86.4 milliseconds per day, an error of only 1.0 ppm.

That is, drop frame TC drops 2 frames every minute, except every tenth minute, achieving 30×0.999 = 29.97 frame/s. The error is the difference between 0.999 and 1/1.001 = 0.999000999000999….

For example, the sequence when frames are dropped:
01:08:59:28
01:08:59:29
01:09:00:02
01:09:00:03

For each tenth minute
01:09:59:28
01:09:59:29
01:10:00:00
01:10:00:01

While non-drop time code is displayed with colons separating the digit pairs—"HH:MM:SS:FF"—drop frame is usually represented with a semi-colon (;) or period (.) as the divider between all the digit pairs—"HH;MM;SS;FF", "HH.MM.SS.FF"—or just between the seconds and frames—"HH:MM:SS;FF" or "HH:MM:SS.FF". The period is usually used on VTRs and other devices that don't have the ability to display a semi-colon.

Drop frame timecode is typically abbreviated as DF and non-drop as NDF.