‘600hz sub fielding’

A member the public has written to one of my editors asking, in short, what is ‘600Hz sub fielding’? The editor passed it on to me, and my initial thought was: ‘I have no idea’. Fortunately our correspondent quoted some material he’d found on the Web. So I did a bit of googling around myself and found that this seems to be a Panasonic technology for its plasma displays, and was preceded by a 480 hertz version, which Panasonic named its ‘480 Hz sub-field drive’. This version is described here. It seems likely that the only difference between the two is that the 600 hertz version does more of the same.

Here’s what it says:

A standard video signal is actually a series of still images, flashed on screen so quickly that we believe we are watching a moving image. The typical frame rate used in North America is 60 frames per second (60Hz) meaning that a TV would display 60 individual still images every second. Sub-field drive is the method used to flash the individual image elements (dots) on a plasma panel. For each frame displayed on the TV the Sub-field drive flashes the dots 8 times or more, meaning that the dots are flashing 480 times per second (480Hz) or more. (Example: 60 frames per second x 8 sub-fields = 480 flashes per second).

Underneath that is a wonderfully ambiguous graphic. It shows six picture frames. Underneath this row of frames it says ‘Each Original Frame has Over 8 Sub-fields. 480 Hz Sub-Field Drive (information is changed at each dot 480 times per second, or more)’. Underneath each of the frames is a set of eight rectangles, each with a letter. Under the first frame the latter is ‘A’ in each of those eight rectangles. Under the second it is ‘B’ and so on.

So the caption for the graphics implies that the ‘information is changed at each dot’ eight times per frame or more. But the little rectangles imply that the information stays the same. So what are we to make of this?

First, ‘field’ in this context would mean video field. An interlaced video frame consists of two fields. In reality, in the US system, most of the time the picture consists not of 60 frames per second, but either 24 or 30 frames per second. But, absent Blu-ray, this is delivered in the form of 60 fields per second. A field is half a frame. One of the fields consists of the pixels in every second row of the frame, while the other field consists of the rest of them. More about that

So ‘sub-field’ would mean ‘below the level of the field’. Given that a field is largely a time-based object, a sub-field only occupies part of the time taken by a whole field. In the case of a 480 hertz sub-field, that is one eighth of the time (60/480 = one eighth).

Now, what follows is my guess work. I shall ask Panasonic to cast an eye over it and let you know in due course if they agree.

Unlike LCD TVs, plasma TVs work by stimulating some gas, which emits ultraviolet light, which in turn excites some coloured phosphor, which then glows. Compare with CRT TVs: these shoot electrons down a vacuum tube, which excites some coloured phosphor, which then glows.

CRT TVs from the early part of this decade had a problem: they were called on to handle different signal frequencies, and their method of operation didn’t lend itself readily to the task. Here’s what happens when you excite some coloured phosphor, whether with electrons or with UV light. Its light output ramps up very rapidly, and once the stimulation ceases, it fades away, somewhat more slowly.

With CRT TVs, each phosphor sub-pixel (a red, green or blue dot) was hit just once by a quick spurt of electrons. The electron beam would move on that the sub-pixel would have to wait for the next frame, one twenty-fifth of a second later (or one thirtieth in the case of US TV) before it would be refreshed. Ideally, this would have gone to a suitable output level instantaneously and maintained its brightness until the briefest of instants before the next spurt of electons came its way, switching off instantly and fully just before that moment. What in fact happened was it would ramp up to full brightness extremely quickly, and then immediately start to fade.

The precise rate of fading could be affected by the selection (and presumably processing and treatment) of suitable phosphors. If you had a CRT TV that refreshed itself every 25th of a second, then it would need phosphors that lasted a relatively long time. But the 100Hz TVs that appeared in the late 90s and early 2000s needed to have their phosphors die out in half that time (1/50th of a second), clearing the pixel for the next frame.

Some CRT TVs ran at 100Hz for standard definition signals, but could also handle high definition, in which case they would drop back to 50Hz. I hated HD on those TVs because it would flicker. The rapid fading of the phosphors, selected for 100Hz operation, meant that the pixels would fade noticably before each new frame.

Plasma TVs have basically avoided this problem. Until now I had assumed that it was because they would excite the phosphors in a pixel, and just keep on exciting them until shortly before the next frame was due. Now, with a bit if research, it is becoming clear what I should have realised earlier: Each pixel is refreshed a number of times during the 25th or 30th or 24th of a second occupied by any given frame.

This has a number of advantages. One is that it saves power: instead of maintaining a constant ‘on’ pixel, a pulse is provided to kick it on, and then further pulses to maintain its on state are provided as needed. Second, by using a short duration phosphor, chosen according to the pixel refresh rate, you can have a rapid turn-off of the pixel at the end of the frame display period, allowing the new frame’s pixel information to replace it with the very minimum of a ‘black’ period between the two frames. These TV engineers really are very clever.

Presumably, then, a slower than 480 hertz refresh rate used to be employed. When Panasonic went for 480 hertz, then it became something to boast about. 600 hertz makes it even more boast-worthy. But the reason would seem to be not what happens during the display of any given frame, but how quickly it can change frames when the time comes. A quick change means more precisely defined motion.

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