HiFi Writer Blog

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.

Over at Smarthouse, David Richards seems to have set the cat loose amongst the pigeons over the issue of Hitachi advertising its TVs as ‘Full HD’. Richards argues that because these TVs accept only a 1080i signal, not a 1080p one, they are not full high definition.

He assembles a wide range of opinions uniformly in support of his case.

Unfortunately, things aren’t quite so simple. I should add that I have yet to review any of the subject TVs, so I cannot offer an opinion as to their capabilities. But I can say that 1080i can indeed to ‘Full HD’.

The issue is complicated by the use of the same term to describe two — actually, three or even four — different things. First, there is the signal. HD DVD and Blu-ray players can deliver all the usual output resolutions (480i, 576i, 480p, 576p, 720p and 1080i). Almost all of them also deliver 1080p. 1080p can, in turn, be delivered in six different forms (at 24 frames per second, 25fps, 48fps, 50fps, 60fps and 72fps). Of these, all current 1080p-capable Blu-ray and HD DVD players support 50 and 60fps, and some also support 24fps. That is really a story for another day. What’s important here is what the differences are between 1080i and 1080p, at whatever resolution. So let us just talk about Blu-ray and HD DVD, and them delivering their video at 1080i60 versus 1080p60.

The problem with the Hitachi TVs seems to be that they will not accept a 1080p signal, only a 1080i one.

The other common use of the term is to describe the ‘resolution’ of the display. I have for the last few years been using Full HD or True HD to refer to a panel resolution of 1,920 by 1,080 pixels, regardless of whether or not the display is capable of accepting 1080p signals. It appears that these Hitachi TVs do indeed meet my criterion for being Full HD.

But if the TV can’t accept a 1080p signal, doesn’t that mean that its picture quality is poorer?

Not on that basis alone. If the TV has been well designed, there are in fact some slight, occasional, losses by delivering the signal in 1080i format rather than 1080p, but there is also a gain. The gain is simple: the amount of data having to be transmitted is halved. HDMI cabling quality is less of a consideration with 1080i than it is with 1080p.

Now, with a 1080p signal and a 1080p panel things are straight forward. The HD player grabs the digital data from the disc, decompresses it, and then sends it down the HDMI cable in the form of 60 frames per second. Since most HD discs are film sourced, and films are shot at 24 frames per second, there is an uneven cadence. The first frame is sent twice, the next one three times, the third twice, the fourth three times and so on. The display’s job is simply to take each of these frames, pour each into its video memory and, when full, apply this to the display driver. Ideally, each pixel of the signal is displayed in its own allocated pixel on the panel. (In practice, some displays will not permit 1:1 pixel mapping, even with 1080p signals, and should be avoided.)

Things are quite different with a 1080i signal. The HD player takes each frame and breaks it up into two fields. One field consists of 540 horizontal rows of picture pixels: every second row of the full 1080 pixel height of the picture. The next field contains the rest. These are subject to 3:2 pulldown. This is a fairly complex procedure, and to understand this I refer you to my piece Progressive scan in PAL and NTSC. Refer particularly to the section ‘NTSC — where interlacing goes wrong’. The resolution in NTSC video is different (720 by 480 pixels, rather than 1,920 by 1,080), but the interlacing process is the same.

In short, a 1080i signal takes these frames and sends them in a decidedly weird, but regular, order to the display. Now here’s the rub: the display can, if it has good quality video processing circuitry, completely reconstitute the original 1080p signal!

For example, today I am using Marantz’s fine VP-15S1 home theatre projector. A truly wonderful unit this is. It incorporates Gennum VXP video processing. When I feed it a 1080i signal from HD DVD or Blu-ray, it is quite indistinguishable from when fed a 1080p signal. Or it has been so far with the tests I’ve got.

The one significant weakness of this projector is that you cannot ‘force’ it into film mode, so it could potentially misinterpret some film source material as being HD video sourced, and therefore apply a more complicated deinterlacing algorithm, reducing picture quality. It certainly does this from time to time with PAL DVDs delivered in 576i format, so I’d be surprised if it wouldn’t do the same thing with Blu-ray and HD DVD. It’s just that I have yet to identify suitable torture tests to allow me to closely examine this point.

I also use a DVDO iScan VP50PRO video processor. Amongst its many other virtues, this can also do the same trick, and can be forced into film mode, and so reconstitute a 1080i signal to a perfect 1080p one (and even deliver it at 24, 48 or 72 hertz if your display permits).

Once again, I don’t know what kind of video processing the Hitachi TVs have, so they may not be able to do this. But I won’t know that until I try it out.

In short, the ability of a Full HD display to accept a 1080p signal is not necessarily an indicator of quality (especially if it lacks 1:1 pixel mapping), but its inability does not necessary mean that it is incapable of displaying full HD pictures. It’s just that the progressive scan business happens inside it, instead of outside it.

How competently it does this is the decisive issue.

How can you tell? Read suitable technical reviews that drill down into actual performance issues. If in doubt, purchase a 1080p display. The chances are higher that it will do a good job. After all, these days there’s not good reason why a display wouldn’t accept a 1080p signal other than penny pinching.

UPDATE (Wednesday, 14 November 2007, 11:19 am): Last night a pleasant fellow by the name of Jacob Campbell emailed me about this matter. He points out that the TV in question, the Hitachi P50X01 will indeed accept 1080p signals. So my discussion above is pretty much irrelevant to the current controversy.

When I wrote this item, I didn’t know which TV the matter related to, nor did I particularly care. I was more interested in the general 1080i/p issue. The level of confusion about this seems immense.

For example, I drew the attention of Smarthouse (the online site that has launched the accusations against Hitachi) to this comment and received a call requesting permission to republish this piece. I declined (I write for too many competing publications), but indicated that fair quoting was acceptable. A few hours later a report on my discussion appeared at Smarthouse and, as far as I could tell, quoted this entire piece, but simply broke it up into sections with a little editorial comment wrapped around it. I was also given a byline as a coauthor. I objected, and within the hour half the piece had been dropped and my name removed from the byline. I mention this in case any readers saw the report with my byline.

What was interesting about this incident was not all that stuff, but the fact that Smarthouse apparently felt it relevant to use this discussion. The TV does accept 1080p singals, including 1080p24. So my discussion adds nothing to the debate. Which begs the question of how much Smarthouse knows about this particular issue.

Had I been writing specifically about this TV, I would have learned about the TV and offered a relevant opinion. As I do in the post above.