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NTSC is defined as 30 frames transmitted each second. Each frame is made up of 525 individual scan lines. . In PAL, 25 frames are transmitted each second. Each frame is made up of 625 individual scan lines.
Early televisions varied in their displayable area because of manufacturing tolerance problems. There were also effects from the early design limitations of linear power supplies, whose DC voltage was not regulated as well as in later switching-type power supplies. This would cause the image to shrink when AC power 'browned out', as well as a process called blooming, where the image size increased slightly when a brighter overall picture was displayed. Because of this, TV producers could not be certain where the visible edges of the image would be. In order to cope with this, they defined three areas:
Title safe: An area visible by all reasonably maintained sets, where text was certain not to be cut off.
Action safe: A larger area that represented where a "perfect" set (with high precision to allow less overscanning) would cut the image off.
Overscan: The full image area to the electronic edge of the signal.
A significant number of people would still see some of the overscan area, so while nothing important to a scene could be placed there, it also had to be kept free of microphones, stage hands, and other distractions. Studio monitors and camera viewfinders can be set to show this area, so that producers and directors can make certain it is clear. When activated, this mode is called underscan.
Action safe Title safe
Vertical Horizontal Vertical Horizontal
4:3 3.5% 3.3% 5.0% 6.7%
16:9 3.5% 3.5% 5.0% 10.0%
Correspondingly overeas, its PAL:
14:9 (displayed on 16:9) 3.5% 10.0% 5.0% 15.0%
4:3 (displayed on 16:9) 3.5% 15.0% 5.0% 17.5%
Microsoft's Xbox game developer guidelines recommend using 85 percent of the screen width and height, or a title safe area of 7.5% per side.
While experts disagree on the exact percentages, they do agree it is mostly 5% - 10%.
We call this Nominal Analogue Blanking
The exact width is determined by taking the definition of the time required for an active line in PAL or NTSC, and multiplying it by the pixel clock of 13.5MHz of Digital SDTV. PAL is exactly 52?s, so it will equate to exactly 702 pixels.
SD or Standard Defination video is still used widely, and likely what you will want to aim for to reach the widest possible audience, although this is changing as HDTV emerges.
Video Format, Resolution, Pixel Aspect Ratio, Equivalent square-pixel resolution
PAL 4:3 704×576 12:11 768×576
PAL 4:3 720×576 12:11 786×576
PAL 16:9 704×576 16:11 1024×576
PAL 16:9 720×576 16:11 1048×576
NTSC 4:3 704×480 10:11 640×480
NTSC 4:3 720×480 10:11 654×480
NTSC 16:9 704×480 40:33 854×480
NTSC 16:9 720×480 40:33 872×480
29.97 vs 30 FPS
29.97 is 30 fps, just running at a different speed. The main issue to watch out here is for audio syncing issues, although for most modern devices and softwares this is not a problem. If you are using Quicktime, that is another story altogether.
When black & white TV was first introduced in the US, it played at 30 frames per second. The AC power available at every wall outlet alternates at a rate of 60 cycles per second, providing an easily available sync signal. In Europe, the AC power oscillates at 50 cycles per second, hence their adoption of 25 frame per second video rates. The conversion between times based on 25 or 30 frames per second is relatively simple. If that were the only timecode problem facing video producers this document would be unneccesary. With the development of color TV, however, the situation became more complicated. The added color component to the broadcast TV signal could sometimes interfere with the preexisting audio. Changing the audio's format would have made all existing black & white TVs incompatible. The solution was to nudge the picture rate down from 30 to 29.97 frames per second. Today, all TVs in the US, Canada, Mexico, and Japan play at this rate. This color TV format is named after the committee that defined it: the National Television System Committee, or NTSC.
A frame-counting scheme called drop-frame has been developed that will allow users to ignore the distinction between 29.97 and 30 when interpreting timecodes. The details will be discussed below, but the central idea is important:
Drop-frame timecodes are defined so as to look like 30 frame per second times, and to reflect accurately the actual time elapsed. In particular, time durations found by subtracting drop-frame timecodes will be accurate to within one or two frames over arbitrary length intervals.
The time between frames is 1/29.97 seconds, or 0.03336666... seconds, which is a bit longer than 1/30 = 0.03333... seconds. (Notice this reversal of relationship: larger frame rates mean smaller time intervals. In a conversation where one person is talking about frequency and the other about time, it is easy to get out of sync.) Another way to look at it without having to look at reciprocals is to say that 29.97 frames/s means that 2,997 full frames are presented in 100 seconds.
Mpeg-1, expressed in file formats as .mpg, is generally encoded at 1.5 Mbit/s although much higher bitrates can be used, and is generally a 4:3 aspect ration, or 640 x 480 pixels. Mpeg-2 was developed for DVD, and supports interlacing and high definition. It is used on DVDs and also for Blu_Ray DVDs.
MPEG-4 moves closer to computer graphics applications. In more complex profiles, the MPEG-4 decoder effectively becomes a rendering processor and the compressed bitstream describes three-dimensional shapes and surface texture. Mp4 achieves higher compression rations than Mpeg-2.
Despite the many years of setting standards, software and devices still give you different widths and heights, or even worse, the same width and height that represented a different region of the underlying video signal.
Know your software and the hardware you want to display it on and you will be able to achieve your intentions. Color temperature and resolution are factors that make the situation even more complex, as does pixel size and shape that you are dealing with. Aim for the middle, and go for over the top and you should do OK. Until next time............
I am MAD STUDIOS.
email me at firstname.lastname@example.org
In later articles I will cover:
1:1 pixel mapping
MPEG-4 Part 10