Editing SMPTE timecode (section)

== Drop-frame timecode ==
Drop-frame timecode originates from a compromise introduced when color NTSC video was invented. The NTSC designers wanted to retain compatibility with existing monochrome televisions. To minimize 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&nbsp;Hz frame rate the line scan frequency is (30&nbsp;× 525)&nbsp;= 15750&nbsp;Hz. So the subcarrier frequency would have been {{sfrac|495|2}}&nbsp;× 15750&nbsp;= 3.898125&nbsp;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&nbsp;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 approximately 600&nbsp;kHz to approximately 920&nbsp;kHz) and by making the beat frequency also equal to an odd multiple of half the line scan frequency. This latter change could have been achieved by raising the sound intercarrier by 0.1% to 4.5045&nbsp;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 3.579{{overline|54}}&nbsp;MHz ({{sfrac|315|88}}&nbsp;MHz), the line scan frequency as 15.{{overline|734265}}&nbsp;kHz ({{sfrac|9|572}}&nbsp;MHz) and the frame rate 29.{{overline|970029}}&nbsp;Hz ({{sfrac|30|1.001}}&nbsp;Hz).<ref>"Color Television Standards: Selected Papers and Records of the NTSC" edited by Donald Fink, McGraw-Hill, 1955</ref>

The altered frame rate meant that an hour of timecode at a nominal frame rate of 29.97&nbsp;frame/s was longer than an hour of wall-clock time by 3.6 seconds (for 29.97 non-drop timecode of 01:00:00:00 drop-frame timecode is 01:00:03;18 and for non-drop 00:59:56:12 drop-frame is 01:00:00;00), leading to an error of almost a minute and a half over a day.<ref>{{Cite web|last=Strachan|first=David|title=The Right Time|url=https://evertz.com/resources/The-Right-Time.pdf|access-date=27 August 2021}}</ref>

To correct this, drop-frame SMPTE timecode was invented. In spite of what the name implies, ''no'' video frames are dropped or skipped when using drop-frame timecode. Rather, some of the ''timecodes'' are dropped. In order to make an hour of timecode match an hour on the clock, drop-frame timecode skips frame numbers 0 and 1 of the first second of every minute, except when the number of minutes is divisible by ten.{{efn|Because editors making cuts must be aware of the difference in color subcarrier phase between even and odd frames, it is helpful to skip pairs of frame numbers.}} This causes timecode to skip 18 frames each ten minutes (18,000&nbsp;frames&nbsp;@ 30&nbsp;frame/s) and almost perfectly compensates for the difference in rate (but still accumulates 1 frame every 9 hours 15 minutes).{{efn|Drop-frame timecode drops 18 of 18,000 frame numbers, equivalent to {{sfrac|1|1000}}, achieving 30&nbsp;× 0.999&nbsp;{{=}} 29.97 frame/s. This is very slightly slower than the true NTSC frame rate of {{sfrac|30|1.001}}&nbsp;{{=}} 29.{{overline|970029}}&nbsp;frame/s. The difference is one additional NTSC frame per 1,000,000 timecode frames, a residual timing error of 1.0&nbsp;[[Parts-per notation|ppm]] or roughly 2.6&nbsp;frames (86.4&nbsp;milliseconds) per day which is considered negligible.}}<ref>{{Cite journal|date=February 2008|title=ST 12-1:2008 - SMPTE Standard - For Television — Time and Control Code|url=https://ieeexplore.ieee.org/document/7289820|archive-url=https://web.archive.org/web/20180619173001/https://ieeexplore.ieee.org/document/7289820/|url-status=dead|archive-date=June 19, 2018|journal=St 12-1:2008|pages=1–40|doi=10.5594/SMPTE.ST12-1.2008|isbn=978-1-61482-268-4 |quote=When drop-frame compensation is applied to an NTSC television time code, the total deviation accumulated after one hour is reduced to approximately 3.6 ms. The total deviation accumulated over a 24-hour period is approximately 2.6 frames (~86 ms).|url-access=subscription}}</ref>

For example, the sequence when frame counts 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 timecode is displayed with colons separating the digit pairs—"HH:MM:SS:FF"—drop-frame is usually represented with a semicolon (;) 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''.{{efn|The period is usually used on VTRs and other devices that don't have the ability to display a semicolon.}} Drop-frame timecode is typically abbreviated as DF and non-drop as NDF.