All About Video

[ExtremePC]

HDMI
The advantange of HDMI is that it also carries audio when connected HDMI to HDMI end to end, but who cares because when you run it from a PC video card there is no audio being sent from the video card because its DVI to HDMI converted.

HDMI is meant for consumer products to connect togther using video resolution standards like 480p/i 576p/i 1080p/i, plus the associated ac3 audio. Personally this is rather silly because when you think about it this mean a HDMI out connector on a dvd player /STB connected to the HDMI in of a tv/plasma/projector is sending ac3 audio to the tv/plasma/projector which can't actually decode the AC3 audio because the audio should have gone to the AMP instead!

HDMI will eventually be okay when you actually patch the cables between the "source" devices and an AMP and then connect the AMP's video out to a display device, untill then its pretty much an indusrty "in joke" that has an expensive punchline!

LCD and Plasma displays
The display is broken up into tiny dots called pixels aranged in a vertical / horizontal grid. Each pixel is turned on or off and told what colour to be. The native resolution is the number of these pixels vertically x the number of pixels horizontally and is espressed as a ratio X x Y. Its this X x Y ratio that determins whether a display is 16x9 or 4x3. There is also a “trick” some manufacturers have used to make chaep 16x9 displays by having only enough pixels to make a 4x3 ratio image but making the pixels physically “long” horizontally instead of square so they look like a 16x9 screen when you have them sitting next to each other. This trick is used mainly in plasma screens.

CRT displays
Cathode Ray Tube screens are the TV we are all used to seeing, they are made up of a glass tube with three colour guns at the back of the tube that fire red, gree and blue electron beams at a perforated mask on the inside surface of the front of the screen. There is not really such a thing a resolution with these displays as there are not really any dots just lines. The best analogy to explain how they work is like this. If you took a loong piece of string and coloured it red green or blue at appropriate places then layed it on a flat surface looping it over itself vertically all those places you coloured red green or blue would form a picture. CRT tv's do this 50 times a second and lately up to 100 times a second in 100hz tv's. Each time you doubled the piece of string back you created a “line”. Typically the best TV's are capable of 550 odd lines.

550 lines x howver many colour changes you place on each line would be the best correlation to resolution.

As you can see 550 lines is well below 720 so by default even a 720p image will undergo significant compression. When a manufacturer states that their CRT tv can do 720p or 1080i what they are really saying is that they have included compression algorythms that will take a 720p or 1080i signal and compress it to fit their number of lines.

SD and HD standards
There are 2 basic SD resolutions and 2 basic HD resolutions

There are also two variants per resolution, i(interlaces) and p(progressive, really just means non interlaced). i stands for interlace and p stands for progressive.

The SD resolutions are 480 and 576.

The HD resolutions are 720 and 1080.

There is also something called refresh rate this is usually either 50 or 60 Hertz (ie 50 or 60 times a second) the refresh rate ties in with the i or p type of the image

The p at the end of the resolution means that each frame of the image is show in full either 50 or 60 times a second

The i at the end of the resolution means that each alternate frame shows only half the image (a full line of pixels then a blank line of pixels then a full line of pixels then a blank line of pixels etc) for every 50th or 60th of a second.

Basically the resolutions breask down like this:

480 = (854 pixels x 480 pixels)

576 = (1024 pixels x 576 pixels)

720 = (1280 pixels x 720 pixels)

1080 = (1920 pixels x 1080 pixels)

Native Resolutions
A display with a native resolution of 1366 X 768 or 1280 x 720 or even 854 x 480 gives a true 16x9 aspect.

A simple thing to do when choosing a display is to bring a calculator and do the maths. ie

(width of the display / 16) X 9 = height of the display.

16x9 native resolutions vs SD / HD
An SD or HD image on a native 16x9 display gets compressed or stretched vertically and horizontally by the same factor. What this mean is that if your display has a native resolution of 1366 x 768 and your viewing a 480p image the display will stretch the image by a factor of 1.6 vertically and horizontally. (854 x 1.6 = 1366, 480 x 1.6 = 768)

Non 16x9 native resolutions vs SD / HD
When you display an SD or HD image on a display with a native res that is not true 16x9 what usually happens is that the display will stretch or compress the image to fill the available pixels in one of three ways.

In the case of 720p it will stretch the image from 1280 x 720 to 1280 x 768. There are three methods that are used:
1) it streches the image horizontally making the image seem just a little "fat"
2) it stretches the image vertically and horizontally but cuts off the sides of the image that go beyond the pixel count.
3) leaves black bars at the top and bottom of the image.

Why is this important?
I don't like black bars or "fat" people on my screen when they shouldn't be there!

So why does my Display say it can do 1080i?
For displays that are true 16x9 the image gets compressed by a factor relevent to the native resolution of the display ONCE.
For displays that are not true 16x9 the image gets compressed and then stretched AGAIN by one of the three methods described above.

Exceptions
The exception is when the material is shot in even wider than 16x9 format which will give black bars at the top and bottom of even a 16 x 9 display.

Conclusion
If you are going to buy a display make sure it really is 16x9 and avoid CRT technology.

Use VGA whenever possible and have it set to the native resolution of your display.

[Mike]

Very informative Mike - thanks for that

Bonus points to you

[woohoo]

Thanks Mike - extremely informative. Made a number of things a lot clearer for me. But all that information and you know you're inviting questions...

I haven't evolved from the old CRT era yet, which is especially sad because I've got a 51cm, 60Hz Sony with only RCA jacks for input. I'll make the break one day, but in the meantime can you offer any advice on the resolution I should be using? I used to be on 600x800, but then went for 576x720, because I read somewhere that was closer to the actual PAL signal. I don't think I can see a difference anyway with the quality of the TV picture. Thanks again

[ExtremePC]

Questions are quite okay, thats whay forums are for!
Your probably running the most appropriate res now for a CRT. Standard PAL is 720x576 this should be the res a 4x3 CRT should be run no matter what the connector method is.