HD1080i De-mystify HDTV 1080i ::: know why before you buy

Saturday, November 11, 2006

1080i / 1080p

Interlacing is an analog broadcast format where the display is refreshed 60 times a second with odd then even then odd rows of video. That is proper for the scanning electron gun approach of a CRT or Tube TV / display, with all its phosphor latency and blurry stuff. I will take the specific case of LCD HD displays for the moment.
Interlacing is a horizontal thing , every other line of pixels must be "laced" together to construct an image frame

... play with the flash example above, animated sliding to see what interlace really does in an image. Click "Normal" to see what is normal 1080i properly assembled into your image, - click 'See Interlace' to see what interlace processing error looks like.

LCD displays are are not interlaced display technology.
This is because LCD and such divices to not "Scan" to create a display screen, each video frame is displayed direct from memory all at the same instant. PC'c connect with DVI - simple and excellent results. HD 1080 spec is referred to as WUXGA 1920 x 1080 for this purpose.

Any HDTV that is LCD is giving you a complete image display by design and cannot "scan" display lines, instead it has 2 buffers for video display, one that you see and the other is being written/updated for the next pending image. ( high speed dual port VRAM usually , called by all kinds of names, its memory in your display ) The 1080i incoming odd-even data is assembled into the bitmap VRAM memory and shown all at once, full frames in sequence, 30 video frames every second.

So if you have a 1080 LCD display, it inherently has 1920 x 1080 pixel resolution and is in fact 1080p capable. However, not all 1080 LCD displays HD the same way, and are therefor not created equal. The Progressive display trolls are correct in one respect... how the firmware deals with incoming odd-even lines to assemble 1920 x 1080 as a complete image does matter.

Huh? firmware?

Firmware in your display is what drives the LCD display, Pixel math on a chip. When you purchase a widescreen flatpanel display, you get more than inputs and glass, you have a small amount of dedicated highspeed processor stuff in there. The quality of that processing is in the Chipset the manufacturer used, the slang for that processor chipset is "firmware", and it has to match the LCD cycle time and the memory that holds the image. It is seriously important stuff.

Even though flatpanels do not "scan lines" across the display, they do have a refresh rate, or time to change a pixel from one color to another. LCD screens have a typical refresh time of 5-10 milliseconds, and the firmware has to know that, since each pixel is a light valve that needs to open and close for the proper amount of light at the proper instant, but basically they are not independant, the entire screen refreshes at the same instant.

It seriously matters that the firmware is tuned for the display's characteristics, and therefore you want your display to be "SMART". I will go into detail in other posts but seek out DCDi / Faroudja
available in literature as Genesis Display Perfection® technology, found in Westinghouse Digital and LG LCD displays. www.genesis-microchip.com Hard working stuff - tech read = http://www.gnss.com/products/FLI5961-FLI5962_Prod_Brief_C5962-PBR-01B.pdf

what is Progressive? 1080p?

Progressive is really a term for how a single frame of video is delivered, or transported to the display device... as you can read from above, it is not how the display presents the video frame.
So the term "Progressive scan" is a horrible misnomer that applies only to CRT display technology, it should be something else, since it really applies to how the video is sent from one place to another as a stream of data that starts at the top of the video frame and progressively fills memory until it completes the frame. Progressive is the method used in sequential data delivery for almost all things digital anyhow, and has value when very high action sports that can benefit from full frame 60 times per second video is available. Unfortunately, compression used to send these full frames can seriously negate that benefit, to the point where 720p60 isnt really any better, regardless of speed or resolution. Broadcasters now know this and most everything you see is 1080i as a result.

Interlace - image over time.
Interlaced video has one problem that takes place in high action and rapid scene movements.
That is the 1/60th of a second that each odd or even row is sent is actaully 1/60th of a second of new image. Basically it is not 540 lines odd then 540 lines eeven rows sent of the same image, but instead it is 540 rows of image at 1/6th second in time, and even rows at the following 1/6oth of a second, so whatever is moving or changing its location on the screen will be shifted in position by 1/60th of a second in every other line. This situation was ideal for CRT "picture tube" displays, mostly due to the persistence time or "latency" of screen phosphors. LCD and the like have no such situation, so it must be handled.

Great firmare will determine this condition, ( correlational edge bicubic de-interlacing for example ) and that pixel detect and fix math creates a smoother edge in clean motion blur.
Unfortunately, for us, some display do a great job at this, making a film-like look, and some are horribly digital jaggy about it. Display Processing Firmware matters.

But, it doesnt stop there. what matters perhaps more, is how the codec or compression is done, and lossy compression always has some very visible injected artifacts, blocky stuff that does more harm to the image of a frame than de-interlacing does. So...
Compression of the video frame is very important, since each pixel is not sent for every frame. Instead, each video frame image is run through som math that determines what is different in the next frame, and basically those things that are different are sent. For a News show perhaps only the head movement and eyes and mouth of the person talking are different between frames, the background is the same as the previous frame... and that info is all the next frame packet compression gets. ( this is a way over-simplification but that will do for now )
The problems get far worse when everything in the frame is different like in rapid scene cuts.

Why all this background?... you need to appreciate the huge amount of conversion that happens before your display gets its video input. By now you get that an HD video frame displayed on a flatpanel as 1920 x 1080 may well have been compressed, decompressed, chopped up into some format and reassembled a couple of times before you see it -- All this is handled for you and you have little choice in the matter, except for choice of inputs and devices with thier processing chipsets.

You can choose buying an HD display that has the right stuff in it, since that is where 1080i and 1080p signal processing matters most. HD1080i.com will explore all this in detail in early 2007, so you will know why before you buy. When it is all done properly it is truly a thing of beauty.


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6:14 AM  

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