Hitachi, ALIS, 1080p and 1080i

In the post immediately below this one, as explained in the update at the foot of the post, I talk about 1080i and 1080p issues which are not particularly relevant to the current fuss involving Smarthouse and Hitachi.

It turns out that the relevant Hitachi TV does, in fact, accept full 1080p signals. So what is the problem?

What the issue seems to revolve around is Hitachi’s use of an ALIS panel in this TV. This is a Hitachi/Fujitsu co-development from a few years ago which simplifies the wiring in plasma panels. Instead of every pixel in the TV having its own two triggering wires, with ALIS panels each two horizontal rows of pixels have three sets of wires. Each row has one dedicated signal wire and a shared one. Consequently, the two rows of pixels cannot be triggered at the same time.

This yields some efficiency improvements, and apparenty increases the brightness of the panel against ‘conventional’ plasma technology. But it does mean that the panel is natively interlaced, not progressive.

But does this matter? As always with these things, it all depends on implementation. A truly interlaced panel will show one set of scan lines for about 20 milliseconds, and then the second set for 20 milliseconds. These are approximations, though, because plasma panels use phosphors to generate the light that you see. The electrical signal within each plasma cell is applied to some gas that generates a burst of UV light which excites the phosphors into generating visible light. This light isn’t just switched on and off instantly. It bursts fairly rapidly into life, and then fades away.

One of the important design criteria for plasma (and CRT) TVs is the ‘persistence’ of the phosphor. That is, how long does the phosphor continue to glow after it has been excited. If it dies away too rapidly before the next screen ‘refresh’ (ie. the next frame is offered for display), the whole screen will appear to flicker. If it dies away too slowly, moving pictures will be smeared. The first effect could be seen with some expensive 100 hertz CRT TVs and CRT-based RPTVs before these technologies were overtaken by panels and projectors. They would display DVDs and SDTV gloriously, with a 100 hertz refresh. But if you gave them a progressive scan signal or a HD signal (1080i), they would kick back to 50 hertz and the full screen flicker was appalling.

CRT TVs generally didn’t suffer significantly from ‘combing’ artefacts, despite the fact that fields from different frames were being kind of mixed up together. That’s because the brightness of the phosphor on one scan line would have largely died away by the time that the scan lines immediately above and below it were drawn. Remember, they were drawn some 20 milliseconds later.

But you can’t do that with a panel TV. Not if you want it to look at all watchable, anyway. What must happen with the Hitachi ALIS TVs is that half the scan lines are ‘drawn’ in a discrete instant in time, rather than painted down the screen over 20ms like CRT TVs, and then the other half are likewise drawn all together.

Given this, how can the picture look decent, without combing caused by the even field from one frame being displayed on the screen at the same time as the odd field from the next frame? On way would be to use phosphors with short persistence, so that each field has largely faded before the next is shown. But that would result in marked ‘interline flicker’. Steady horizontal objects would move up and down between two scan lines fifty times per second. This would not be nice either. Furthermore, it would darken the whole picture because half the pixels would be switched off, more or less, at any one time.

Another solution, and I suspect the one used by Hitachi, is to break with tradition. Why should fields be displayed 20 milliseconds apart from each other? Remember, the screen phosphors are kicked into life, and then they glow for a while on their own. So let’s say that the odd scan lines of the frame are fired off, and then the even ones are fired not 20ms later, but 1ms later. They would be so close together in time that the frame would be almost entirely progressive. In fact, you could say that the panel is 95% progressive, 5% interlaced. The residual combing would be so low in level to be imperceptible.

All this is supposition on my part, thinking through how one could employ the ALIS structure of plasma panel to produce a high quality picture. The timings may be different. The second field may have to fire 5ms after the first one, or maybe only 0.5ms. I don’t know and I will endeavour to find out. But I would be very surprised if it fires the full 20ms later because the picture quality problems would be pretty obvious.

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