Video
Non-square Pixels - 2
Viewable area
Analogue TV signals contain more information than is intended to be displayed on the screen. There are extra lines (the systems have 625 and 525 lines total respectively) not all of which carry picture information, and the lines themselves start and end off the edges of the area that is designed to be viewable. TV sets traditionally mask off even more of the edges, a nowadays unnecessary idea called overscan which is another issue, but there is more information in the signal than is even specified as displayable.
The DV standard is no different, the 720 pixels that make up the image width include eight at each end that are outside the specified viewing area. This means that only the centre 704 represent the "true" TV-format part of the picture.
704 is a slight approximaton, the figure is not quite the same for both systems, and arises because the absolute specifications are based on timing, not pixel numbers. However, 704 is close, and a workable number.
Aspect ratio
Let's see the effect of all this on the frame format. A TV picture is intended to be displayed at an aspect ratio of 4:3 (ignoring wide-screen for a moment). If pixels are square, a 576-line picture is 768 pixels wide, and a 480-line picture is 640 pixels wide.
However, a DV frame is always 720 pixels wide, and is only correctly displayed when the middle 704 pixels occupy a physical width 4/3 that of the height. The result is that the pixels are rectangular - short and fat at 25 fps (576 lines) but tall and thin at 30 fps (480 lines).
The actual aspect ratios of the pixels are 12:11 at 25 fps and 10:11 at 30 fps. as illustrated in the diagram below:
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