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As I'm sure you know, motion pictures are comprised of a series of still images played in succession. When still images are displayed at a fast enough speed, just like a flip book, they create the illusion of motion. Along with advances in technology and the advent of television came the need to synchronize multiple picture and sound recordings, as well as the equipment used to record and play them. So-called SMPTE timecode solved this need. Timecode was developed by the Society of Motion Picture and Television Engineers in the late 1960s. Timecode assigns a specific numeric value to each frame or still image of the motion picture, which consists of a two-digit hour, two-digit minute, two-digit second, and two digits that represent each individual frame.
Take a look at this clip from the short film ELI which we will be working with in this course. The Timecode value has been overlaid, or burnt in, on each frame so that every single frame of the clip has its own unique Timecode value. You can see that the Timecode value counts upward, like a digital clock, as the story unfolds and the picture moves forward in time. There are a few different ways to write timecode, if you will, but they all convey the same timecode information. Linear Time Code, also called Longitudinal Time Code or LTC, is an audible electronic signal that can be striped onto an analog or digital recording tape to allow the recording to sync with other elements.
VITC, or Vertical Interval Time Code, is timecode that is embedded in visual media using bars of black and white. VITC has to be recorded along with the video content while LTC can be striped to the media in advance; however, VITC can be read while the machine is stopped while LTC cannot. We will see what's called burnt-in timecode on our picture file throughout this course. This allows us to visually see the timecode on the picture as we play the session. There are still several more types of timecode, including MIDI timecode.
Information on these can be found online should you want to look into them further. Film uses 24 frames per second and is based on the measurement of the physical length of film in feet. NTSC Television uses 30 frames per second and 29.97 frames per second. This came about because North America uses electric power with an alternating current of 60 cycles per second, also known as 60 hertz. Because electronics in North America run on a 60-hertz power cycle, electricity provides a natural reference of 60 cycles per second.
Each frame of video has two fields--an upper field and a lower field--and since each field is synchronized to a single cycle of 1/60th of a second, the standard of 30 frames per second was the natural choice. In many other parts of the world, electrical power is based on a cycle of 50 hertz, which led to the frame rate of 25 frames per second. This is the frame rate of PAL, another major broadcast standard. With the introduction of color television in the 1950s, the standard 60 hertz refresh rate of black-and- white television had to be slowed down slightly, to 59.94 hertz.
This change was necessary to accommodate the additional color information in each refresh cycle. As a result, a modified frame rate was born. The slow-down from 60 hertz to 59.94 hertz resulted in the new frame rate of 29.97 frames per second. Remember, each video frame has two fields: an upper and lower field. This frame rate was later formalized by SMPTE with the advent of timecode. Since most television programming starts and ends on a strict 30- or 60-minute schedule, drop-frame timecodes were introduced to make the hours and minutes in 29.97 timecode more closely match actual clock time.
If you're wondering why timecode didn't match clock time, imagine that if you slowed a clock down so that it only had 59.94 seconds in a minute. Then a minute would actually take a tiny bit longer. That tiny bit is exactly 0.06 seconds, so skipping the count of two frames-- 00 and 01--in every second except on every even tenth second, The Society of Motion Picture and Television Engineers arrived at the drop-frame standard that compensated for the missing 0.06 seconds and allowed timecode to synchronize with clock time.
While there are still other variants of timecode in use today, hopefully this helped to clarify the origin and practical application of timecode. If you want to know more about timecodes and broadcast standards, there are vast resources available online. Just type "NTSC," "PAL," or "timecode" into your favorite search engine.
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