When the basics of television were developed in the 1930s, the chosen picture aspect ratio was 1.33:1 (4:3) to match the contemporary film aspect ratio. This choice resolved the picture format compatibility with the dominant film technology of the time. The film picture repetition rate 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.
The chosen television picture repetition rate is aimed at satisfying different requirements. On both sides of the Atlantic, the need was felt to relate the refresh rate, or picture repetition frequency, to the power-line frequency. For historical reasons, this was 50Hz in Europe and 60Hz in North America.
Film to video transfer
Figure 1. Downconversion of the 16:9 aspect ratio to 4:3 screens. Click here to see an enlarged diagram.
The 1941 NTSC television standard was the result of 10 years of experiments. It featured such novelties as 525 scanning lines interlaced into 60 fields per second (or 30 frames per second), negative amplitude modulation with vestigial lower sideband and FM audio modulation all packed into a 6MHz transmission channel. Some VHF channel allocations changes occurred at the end of the 1940s, resulting in the disappearance of Channel 1, but no other major changes were made.
In 1953, color information was added using a frequency division multiplexing of luminance and chrominance information. A slight change in vertical scanning frequency (from 60Hz to 59.94Hz) and horizontal scanning frequency (from 15,750Hz to 15,734.25Hz) was implemented, but it did not affect the perfect forward compatibility (monochrome to color) and back compatibility (to the 15 million home monochrome receivers in use).
Transferring film to video is relatively easy in PAL and SECAM because the film is run at 25 frames per second, a relatively unnoticeable increase. 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 adopted solution is the so-called 2:3 pull down. The image is scanned completely five times, while four film frames are passing through the projector.
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 the 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 and color formats as well as aurally by using multichannel sound.
This resulted in a variety of aspect ratios requiring the widening of the screen. 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. To obtain an acceptable widescreen display on a 4:3 screen, broadcasters have relatively few and generally unsatisfactory choices, as shown in Figure 1 and listed below:
Table 1. Digital television standard video formats. Vertical lines refer to the active scanning lines in the picture. Pixels refer to the number of Y samples during the active line. Aspect ratio refers to the picture aspect ratio. Picture rate refers to the number of frames or fields per second. In the values for picture rate, “p” refers to progressive scanning, and “i” refers to interlaced scanning. Both 60Hz and 59.94Hz (60x1000/1001) picture rates are allowed. Click here to see an enlarged diagram.
- The horizontal edge cropping methodThe 16:9 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, if available, the operator moves the central window in the horizontal direction to follow the main action. This is the most often used approach in North America.
- The letterbox methodThe 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 anamorphic distortion methodThe 16:9 aspect ratio picture is squeezed horizontally to fit inside a 4:3 aspect ratio raster. This method results in a 25-percent anamorphic distortion.
Table 2. Standardized video inputs. Active samples/line refers to the number of Y samples during the active line. Picture rate refers to the number of frames or fields per second. In the values for picture rate, “p” refers to progressive scanning and “i” refers to interlace scanning. Both 60Hz and 59.94Hz picture rates are allowed. Click here to see an enlarged diagram.
ATSC implications
Section 5 of A/54A Recommended Practice: Guide to the Use of the ATSC Digital Television Standard deals with the characteristics of the ATSC video systems. Table 1 (ATSC Table 5.1) lists the four basic ATSC digital video formats. Table 2 (ATSC Table 5.2), on page 18, lists the standardized possible studio digital video input formats.
The standard picture aspect ratio is 16:9. A conversion from the ATSC format to ITU-R BT.601-5 would be required during the transition period when NTSC transmitters will duplicate the 16:9 aspect ratio HDTV transmissions. It is expected that vintage (legacy) 4:3 aspect ratio NTSC television programs will occasionally be transmitted. This will require an upconversion. As shown in Figure 2, on page 18, there are three methods of 4:3 to 16:9 format conversion (upconversion). They are:
Figure 2. Upconversion of the 4:3 aspect ratio to 16:9 screens. Click here to see an enlarged diagram.
- The pillarbox modeThe original 4:3 aspect ratio picture is inserted in a 16:9 window, resulting in black side panels. The side panels occupy 25 percent of the horizontal space.
- The tilt-and-scan modeThe 4:3 aspect ratio picture is stretched in the horizontal and vertical direction to fill a 16:9 aspect ratio screen, resulting in a 25-percent loss of vertical resolution. The viewing window can be preset, or a tilt-and-scan approach can be used. Here, the operator moves the window in the vertical direction to follow the action.
- The anamorphic distortion modeThe 4:3 aspect ratio picture is stretched horizontally to fill a 16:9 aspect ratio screen, resulting in a 33-percent anamorphic distortion.
None of these methods are ideal. Experiments indicate that a 5-percent anamorphic distortion is undetectable, and a 7-percent anamorphic distortion is not objectionable.
In addition to single-pass letterbox and pillarbox effects, a concatenation of upconversions and downconversions will result in doublebox displays, as shown in Figure 3.
Figure 3. Doublebox displays resulting from multiple format conversions. Click here to see an enlarged diagram.
Legacy programs and equipment implications
TV facilities have a tremendous amount of legacy programs in analog and digital tape formats. The commonly used formats are 2in, 1in, Betacam SP, MII, D-1, D-2, D-3, D-5, DCT-700 and digital Betacam videotape recorders, providing different picture quality performance in playback. Any upconversion must, therefore, be carefully considered to generate an acceptable- DTV signal for broadcasting.
Similarly, analog NTSC production is going to continue for some time. Currently, the four major networks offer HDTV originated programs on a daily basis at peak viewing times (10 percent to 22 percent of the daily schedule). Occasionally, commercial inserts into an HDTV program have pillarbox effects. They are obviously originated in NTSC and upconverted to 16:9 HDTV. The rest of the day, the networks transmit NTSC programs upconverted to 16:9 HDTV with an assortment of letterbox, pillarbox and doublebox effects. In time, the NTSC production equipment will be replaced with HDTV equipment.
A relatively underestimated problem is the fact that CRT and flat-panel displays are affected by uneven screen display. Essentially, black panel areas will age more slowly than the active picture area, which, in time, will become darker. This should be an incentive for equipment manufacturers, broadcasters and standards bodies to agree on methods to eliminate these unwanted effects.
Michael Robin, fellow of the SMPTE and former engineer with the Canadian Broadcasting Corp.'s engineering headquarters, is an independent broadcast consultant located in Montreal. He is co-author of “Digital Television Fundamentals,” published by McGraw-Hill and translated into Chinese and Japanese.
Send questions and comments to:michael_robin@primediabusiness.com
