Origin and History of Time Code

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In order to better understand the variety of SMPTE formats, let's take a look at how time code developed and eventually turned into the SMPTE format.

Time Code and Film

In the early days of film, scenes were filmed from beginning to end with one camera and in one complete take. Film scenes paralleled the way plays on stage were viewed . The only editing that would take place would be to cut all the scenes together in the correct order. The high-speed edits and rapid-fire cuts that are the norm today were simply not used in early film.

Gradually, as filmmakers began to explore the new medium, new techniques of filming caught on. Soon, scenes were filmed in multiple takes using different camera angles, in order to capture the action in new and exciting ways. These new filming techniques made the editing process much more complicated, as each scene had multiple edits throughout. Keeping track of all the pieces of film and where they went became an arduous process. Film editors needed a system that allowed them to keep notes about their editing and exchange that information with other editors. They began to write on the edge of the film, placing notes and then numbers to help the editing process. This evolved into the use of "feet and frames" as a measuring tool. Each second of film contains 24 individual frames. A foot -long piece of 35mm film has 15 frames in it. Using feet and frames with edge numbers made complicated editing much more feasible . This was the first time code, and it was used while editing film on machines similar to the one shown in Figure 3.2.

Figure 3.2. A Moviola machine that is used to edit film and magnetic audio film together using the system of feet and frames for counting.

NTSC versus PAL

With the advent of television and then videotape recording, new editing systems developed that allowed video to be edited from multiple-source tapes onto a final master tape. Manufacturers of different videotape systems developed their own versions of editing systems that used proprietary time codes that were unusable by other systems. The lack of an industry-wide standard became a big problem, as no one manufacturer's system could use another's time code information. In 1969, the Society of Motion Picture and Television Engineers developed a new time code standard that allowed all editing systems to speak the same language. However, there are still two different standards in video. The first is the NTSC video standard, which is used in America and Japan, for the most part. The second is the European video standard, called PAL (Phase Alternate Line), or EBU (European Broadcast Union).

The frame rate of television in the United States was based on the carrier frequency of electrical current coming out of the AC outlet. This was a convenient source of a stable time reference that video systems could use. Each frame of a NTSC video signal contains two fields, and each field contains half of the horizontal lines that comprise the video image one field has all the even lines and the other contains the odd lines. As the AC line frequency is 60Hz, engineers used that cycle to trigger each field of video. Black-and-white television had a resulting frame rate of 30 frames per second (60 cycles/2 fields per frame = 30 frames per second). With the advent of color TV, the frame rate had to be lowered by .1%, resulting in the 29.97 frame rate we commonly use today. Obviously, video no longer uses the 60Hz AC line frequency to trigger video fields. Now, quartz clocks that can run at other speeds are used for more accuracy and dependability .

In Europe however, the reference frequency used in electrical systems is 50hz. So in the PAL/EBU system, 25 frames per second are derived from the 50 fields per second that the AC line frequency generates.

Color versus Black and White

When color video was developed, broadcasters wanted to keep the current black-and-white transmission system intact. Nobody wanted to force consumers to buy new televisions in order to see the new color broadcasts. Engineers finally came up with a system that allowed color TV to be backwards compatible with existing black-and-white televisions. Due to the frequencies involved, a slight adjustment had to be made to the frame rate in order for the new color system to work with both color and black-and-white televisions with predictable results. The frame rate was slowed down by .1%, resulting in a rate of 29.97 frames per second. This small adjustment allowed the new color TV broadcasts to be viewed on the older black-and-white TVs. 29.97 is the current frame rate for all NTSC video systems in use today. Audio-only productions typically use the old 30 frames per second code simply because it follows the real time clock and can be easily divided into seconds for quick adjustments.

While the new frame rate solved the problem of black-and-white compatibility, it created a new problem for video editors. As the new frame rate was a little slower than 30fps, it would not display the correct time in relation to the clock on the wall after a minute or so. This drift would increase as time went on. After an hour of real time, 29.97 SMPTE reads: 00:59:56:12, a difference of over three seconds. For relatively short program material, such as commercials, this was not a problem. However, for anything longer than one minute, the time code drift is significant. This is where drop-frame time code comes to the rescue. Drop-frame SMPTE skips some frame numbers in order to achieve accurate real time display. The actual video frames themselves are not lost. The time code merely skips certain frame numbers along the way, so that it can display the current real time more precisely.

NOTE

TIME CODE CALCULATORS

A good way to understand how different types of SMPTE time code relate to one other is to use a time code calculator. Many freeware versions are available on the Web for both Mac and PC. Download one and start converting different types of SMPTE, starting with simple increments of time such as one hour. You'll quickly see how each frame rate and frame count relates to one another. Having one of these calculators around can be very handy when you're in the trenches, working.

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