Fields and Frames

Television was designed to mimic the movie screen, which flashes images on a screen.  TV is unable to flash the images because it would take a tremendous amount of power to light up all the phosphor coating at once.  So instead, it was decided that it would be much more economical to light up a tiny dot, and scan that dot continuously across the screen.

The phosphor helps out greatly because not only does it light up - but it retains the light for a short period  .  .  .  long enough for the beam to scan across all other areas of the screen before returning to that one spot and refreshing it with power.

This was an ingenious concept, and allowed an entire frame to be painted onto the screen.  Unfortunately, at 30 frames per sec, the viewer noticed a lot of flicker.  It turns out that any series of images that are presented at a rate lower then about 50 to 60 times per second, cause flicker.  OK no problem - they could simply speed it up and scan 60 frames onto the screen every second.  Well that didn't work out either, because it required too much bandwidth.

Then the next ingenious idea came about.  Why not separate a frame into two sub-frames, where each subframe only had half of the scan lines ??  It worked !!  And by alternating the lines, first all odd, then all even - the subframes melded together nicely.  The design was ready for prime time - 60 subframes each second, to make 30 frames each second - and the flicker was gone.  All they had to do was get rid of that clunky term, subframe.  They decided to call them fields.  

Field Order

One of the subtler and frustrating aspects of working with mixed sources, largely because poor documentation makes it hard to determine the field order used by different formats. The two fields are often called "upper and lower", "field1 and 2", "odd and even".  The term, "dominant field" refers to the first field.

The only time field order is important is when using video software, and hardware such as capture cards.  The field order for capture cards (used to get video into your PC) and other hardware varies, depending on the manufacturer of the equipment.  It is often not even stated in the user's guide !!  Yet it is critical to know, since it is a setting within the capture software utility.

The first line of the frame may be displayed before the second or the second may be displayed before the first. Guessing the wrong field order results in video that stutters when there is any motion because the content is being displayed out of order.

Fortunately for Television - the field order is not important, since the two fields simply alternate.  What is important is to scan the even lines beginning at the top left of the acreen, and scan the odd lines starting from the top middle of the acreen.

Note that the second field begins halfway across the screen - that way it will not simply scan over the exact same positions as the field 1 scan lines.  This ingenious method causes the field 2 scan lines to be traced exactly between the field 1 lines !!  Since the fields alternate, even-odd-even-odd, it is cyclical and the order is not important.  However, the Television receiver must be able to distinguish between the even and odd lines within the received signal.