With the advent of color television, North America felt the need to alter the frame-repetition frequency. We still live with the consequences of this decision.
Motion pictures made between the late twenties, the advent of sound on pictures, and the early fifties, the advent of widescreen pictures, share two main characteristics:
- Horizontal vs. vertical picture dimension ratio (aspect ratio): After experimenting with various aspect ratios the motion picture industry settled on a 1.37/1 aspect ratio called the Academy format. This aspect ratio was reduced to 1.33/1 with the advent of sound on film.
- Number of acquired pictures per second (picture frequency): The chosen picture frequency is 24 images per second. This satisfies the eye requirements with respect to recreating the illusion of movement. To satisfy a related eye requirement, critical flicker, each stationary picture of the sequence is projected twice, resulting in a “refresh rate” of 48 cycles per second. This is a compromise between the human vision system requirements and financial constraints related to film length.
When the basics of practical television were developed in the 1930s, the chosen aspect ratio was 1.33:1 (4:3) to match the contemporary film aspect ratio. While this choice resolved the picture format compatibility with the dominant film technology of the time, the chosen picture repetition rate aimed at satisfying different requirements. On both sides of the Atlantic the need was felt to relate the picture repetition frequency (refresh rate) to the power line frequency. For all sorts of historical reasons this was 50Hz in Europe and 60Hz in North America.
Transmission bandwidth constraints dictated the use of interlace scanning, and the result was 25 frames per second (50 interlaced fields per second) in Europe and 30 frames per second (60 interlaced fields per second) in North America. With the advent of color television, North America felt the need to alter the frame repetition frequency to 29.97 frames per second (59.94 interlaced fields per second). The reason was relatively simple: Given the nonlinear distortions of the television transmitters and receivers of the time (1953) it was feared that the sound carrier (4.5MHz offset from the vision carrier) might generate interference products with the chrominance subcarrier (3.579…MHz) resulting in a spurious 920kHz interference that would be quite visible. The NTSC color subcarrier has a value related to half the horizontal scanning frequency (Fsc = 455/2 Fh) and to the vertical scanning frequency and thus achieves low visibility due to a process of line-to-line phase reversal. By making the 4.5MHz intercarrier spacing a multiple of Fh/2, the 920kHz interference visibility would also benefit from the line-by-line phase reversal. Since the FCC opposed the change of the carrier spacing the only way out was to change the horizontal and vertical scanning frequencies. So we ended up with Fv = 59.94Hz instead of the original 60 Hz and with Fh = 15,734.25Hz instead of the original 15,750Hz. This had no influence whatsoever on the television receiver's synchronizing ability but played havoc with future studio operations. We still live with the consequences of this decision.
Transferring film to video
With PAL or SECAM, featuring 25 frames per second (50 interlaced fields per second), transferring film to video is usually achieved by running the film at a slightly increased speed (25/24 = 1.04166…). This shortens the duration of the projected movie slightly, which is relatively acceptable, and raises the reproduced sound pitch, which is mildly annoying.
NTSC video required a different approach. It is evident that it would be totally unacceptable to run film at 30 (or 29.97) frames per second. The solution adopted is based on the fact that 30 (television frames per second) and 24 (film frames per second) have a common denominator, namely 6. Essentially four frames projected at a speed of 24 frames per second take the same amount of time (4/24 = 1/6 sec) as five television scanning frames at 30 frames per second (5/30 = 1/6 sec). Thus if the image is scanned completely five times while four film frames are passing through the projector the two systems maintain synchronism. The relationship is maintained if one film frame is scanned with two television fields (2/60 sec), the next film frame with three fields (3/60 sec) and so on. This method is called the 2/3 pull-down. While this solution was adopted before the advent of NTSC color, with its modified scanning rates (29.97 television frames per second) it works equally well with the slightly reduced frame rates. Figure 1 illustrates the NTSC 2/3 pull-down process as follows:
- TV frame 1 contains two fields of the first (A) film frame;
- TV frame 2 contains two fields of the second (B) film frame;
- TV frame 3 contains one field each of the second (B) and the third (C) film frames;
- TV frame 4 contains one field each of the third (C) and the fourth (D) film frame; and,
- TV frame 5 contains two fields of the fourth (D) film frame.
“This film has been formatted to fit your screen”
The methods described above worked well until the early 1950s. By then there were about 15 million television receivers in use in North America. This created apathy among potential moviegoers, who preferred to stay home and watch television. The movie industry reacted by enhancing the movie watching experience visually, by using various widescreen formats as well as color, and aurally by multichannel sound. This resulted in a variety of aspect ratios requiring the widening of the screen. Table 1 lists some of the formats in existence and their characteristics.
While the variety of available formats is impressive, equally impressive is the fact that there are currently some 250 million NTSC television receivers in North America, all with a 4/3 (1.33/1) aspect ratio picture tube. It is therefore debatable whether the widescreen movie formats have made moviegoing more popular than television watching. While movie houses could adapt fairly easily, at a cost, to various film aspect ratios, television broadcasting had relatively few, and generally unsatisfactory, choices. These were:
- Avoid transmitting widescreen films.
- Use the horizontal cropping method. Figure 2 shows the manner in which a 16/9 (1.77/1) aspect ratio picture is cropped on both sides to extract a central window that fits into a 4/3 raster. In the pan-and-scan mode, the operator moves the central window in the horizontal direction to follow the main action. This is the most common approach in North America. Evidently, given the variety of formats, films cannot be projected directly on-air. Specialized production houses transfer film on videotape using skilled operators occasionally directed by a producer familiar with the original film producer's intent. By necessity, some details of the picture will be dropped so there will be a definite loss of picture information. On the other hand, the screen will be completely filled. This method is popular in North America. The film's releasing agency is usually motivated to inform the TV viewer that “This movie has been formatted to fit your screen.” Viewer beware.
- Use the letterbox method. Figure 3 shows the manner in which a 16/9 aspect ratio picture is reduced vertically and horizontally to fit inside a 4/3 aspect ratio window. The process generates black bars at the top and the bottom of the picture. The thickness of the black bar depends on the aspect ratio of the film. Letterboxing, as it is commonly called, reduces the vertical resolution because the black bars reduce the number of active scanning lines. This method is generally used in France and Germany and is shunned in the U.K.
- Use the anamorphic distortion method. Figure 4 shows the manner in which a 16/9 aspect ratio picture is squeezed horizontally to fit inside a 4/3 aspect ratio raster. This method results in anamorphically distorted shapes. In North America this method is used in the beginning and end of the movie to allow showing all the credits, many of which would be masked by the pan-and-scan process.
In the next month's article we will discuss the implications of the ATSC/DTV digital standards and the scanning format conversions usually encountered.
Michael Robin, former engineer with the Canadian Broadcasting Corporation's engineering headquarters, is an independent broadcast consultant located in Montreal, Canada. He is co-author of Digital Television Fundamentals, published by McGraw Hill.
FORMAT ASPECT RATIO ACADEMY 1.33/1 CINERAMA 2.65/1 to 3/1 CINEMASCOPE Initially 2.55/1 and later 2.4/1 SUPERPANAVISION 70 2.76/1 ULTRAPANAVISION 2.76/1 (65 OR 70 mm prints) or 2.35/1 (35 mm prints) PANAVISION Initially 2.35/1 and currently 2.4/1 VISTAVISION Approximately 1.85/1 TODD-AO 2.2/1 during filming and 2.35/1 on 70 mm print TECHNIRAMA 2.2/1 (on 70 mm prints) OR 2.35/1 (on 35 mm prints) SUPER 35 2.45/1 filmed anamorphically