Ned Soseman /
07.01.2011 12:11 PM
What's the difference between 59.94fps and 60fps?

The Transition to Digital e-mail bag recently included a message from a reader asking if there is a difference between 59.94 and 60 fields or frames per second. The reader has five HD cameras, all from a single major manufacturer, three of one model and two of another. One model specified a video output frequency of 59.94fps, while the other was specified at 60fps. The manufacturer told the reader that 59.94 and 60 are the same. Research and a number of interesting conversations with industry experts revealed information you may find surprising.

59.94 or 60?

In most cases "60" is techno-shorthand for 59.94, but not always. In fact, 59.94 is 99.9 percent of 60.

Progressive standards are usually described by their frame rates, such as 23.976, 24.000, 29.970, 30.000, 59.940 or 60.000p. Interlaced standards, on the other hand, are typically based on field rates such as 60i.

Some video editors might say that 59.94 is a drop-frame version of 60fps, and in some ways they may be correct, even though the time code itself can be drop-frame (DF) or non-drop-frame (NDF). Knowledgeable engineers know that 59.94 fields per second is a legacy of the NTSC color system, because prior to 1954, black-and-white video was locked to the 60Hz frequency of standard AC current in the United States. The 29.97Hz frame scan rate was designed to make color TV backward compatible with black-and-white TV when color was introduced. A frequency offset of 0.03Hz was introduced to make space in time for the color sub-carrier.

Today's broadcast video common denominator is SDI, either SD or HD. SMPTE 294M specifies SDI at a 29.97 frame rate and a 59.94 field rate for SD and 1080i or p. HDTV in the 1080i format, as fed to and rebroadcast by most NBC, CBS and PBS affiliates, is at 59.94 fields per second. The 720p format, as specified by SMPTE 296M and used by ABC, FOX and ESPN, has no fields. It delivers 59.94 progressive frames per second. Both 1080i and 720p mathematically match downconverted SD video.

There are many viewers still watching NTSC televisions by using ATSC converters for off-air reception, or using cable or satellite providers that still deliver standard NTSC signals. These viewers and rebroadcasters represent the lowest common denominator of the video world in the U.S., which remains NTSC. Therefore, everything broadcast over the air must be easily convertible to NTSC standards.

3:2 or 2:3

Some newer equipment, such as sync generators, production switchers and high-end EFP cameras, offer a choice between 59.94 or 60.00. Why? The primary reason is film.

If a video or film camera shoots a true 24.00fps, its playback must be slowed by 0.1 percent (1/1000) to simulate the effect of changing the reference from 60Hz to 59.94Hz before or during the pulldown process. This reduces the pulldown speed to what is commonly referred to as 23.98fps, which if you happen to be following along with a calculator is actually 23.976fps. HD producers wanting the "film look" will shoot in 24p, which is typically 23.98fps, so no speed adjustment is necessary before adding telecine-style pulldown to create what is usually called 30fps video.

These 23.98 progressive frames per second of video can be fed raw to HD or computer monitors, or interlaced to 59.94 fields per second and played on nearly any monitor. Video at 29.97fps pulled from a 23.98 source has some duplicate fields. Raw 23.98 frame per second video has no fields.

Video captured in the 23.98 format can be edited in the 23.98fps mode with time code in Final Cut Pro or similar editors with virtually no further processing. So called 24p HD video is often edited and sometimes displayed in this fashion. True 24fps film or video can also be converted using the pulldown technique so it can be edited, distributed and displayed in the standard 29.97 format, so long as its 24 frame speed is reduced to 23.98 before ingest and editing.

Pulling film

The term "pulldown" comes from physically pulling film one frame at a time through the gate of a telecine projector. Originally, film was pulled at speeds significantly slower than real time for the best possible frame-to-frame registration.

When the 99.9 percent rate of 24fps film (23.976fps) is divided by the ATSC rate of 29.97fps, the result is 4/5. The difference in frame rates can be corrected by repeating every fourth frame of film, but this method creates significant audio issues, among other problems. A more sophisticated technique is the 3:2 pulldown process. With pulldown, film shot at true 24fps is slowed to 99.9 percent speed, which allows exactly 10 fields (five frames) of 59.94Hz interlaced video to be created from every four frames of film.

To make this possible, once the film speed is at 23.976fps, the telecine process outputs the first frame of film across three interlaced video fields, the next film frame over two video fields, the third film frame over three video fields, and the fourth over two video fields. Some facilities may use the 2:3 pulldown process, which is similar to 3:2 except that the first frame is two fields, the second is across three fields, the third is two fields and the fourth is three fields. In either case, the result is a frame rate that matches standard NTSC, SDI and ATSC frame rates. Some TV displays and DVD players with HDMI or DVI outputs will recognize pulldown encoding and reconvert the signal to true progressive at the 23.976 rate for display.

From a purist's perspective, it might make sense to some to shoot and edit in true 24fps for direct-to-film transfer, or 30.00fps or 60.00fps for eye-grabbing progressive scan presentations that will never be broadcast, distributed in SD or displayed on a NTSC-based TV. While true 24.00fps, 30.00fps and 60.00fps is possible with some equipment, most broadcast facilities are maintaining the 29.97/59.94 SMPTE standard for compatibility with cable systems, satellite receivers and others who still convert HD to SD for transmission and distribution.

A crucial difference, sometimes

The difference between 59.94fps and 60fps depends on who you ask. Many video professionals may first say they are one in the same. However, not everyone who designs or installs video equipment necessarily agrees. Some engineers installing a new sync generator, production switcher or camera system might discover that this tiny difference can become the source of some unfamiliar and tricky-to-identify problems. A true 30.00fps or 60.00fps signal may or may not interface seamlessly in a SDI-based system, depending on if, or how, certain equipment process the signals. If you work in a SDI-based facility and as you dig through the set-up menus you find a choice between 60.00 and 59.94, 59.94 is most likely your right choice. Engineering is, after all, a science not an art.

Some film makers say film as we know it is disappearing. Not because the celluloid media is going away, but because more so-called 24fps film is being shot at 23.98 fps to eliminate the need for telecine-style processing for editing and video-based distribution. Not all film purists will agree that this end justifies the means, but the trend is growing.



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