Friday, November 30, 2007

HD/SD what's it all about

Been talking about new tellys what with the HD/ digital buzz and Xmas, trouble is some people still don't understand a) what HD is and b) what size the screen is. I'll start with the second problem first.

Televisions are sold by size, that size being their diagonal length. This is great when comparing two otherwise identical televisions, not so great when comparing standard definition (SD) and high definition (HD) because they use different screen ratios.

For an SD set the ratio is 4:3 that means for every 4 inches (or centimetres, or feet, or microns, or...) of horizontal length you get 3 inches (or...) vertically. For HD the ratio is normally 16:9 and no that's not the same as 4:3 remember you need to divide each side by the same number.

So let's take a 32" television and determine the screen height and width for both SD and HD. Using good old Pythagoras a2+b2=c2 this is easy (for a given value of easy).

For an SD set it turns out to be the square root of the diagonal squared divided by 25, that is then multiplied by either 4 or 3 depending on whether you want horizontal or vertical length. For HD it's the square root of the diagonal squared divided by 337, that is then multiplied by either 16 or 9

For Excel users that's =SQRT(POWER(diagonal,2)/number))*height or width. So to find the width of our 32" SD telly =SQRT(POWER(32,2)/25))*4 for HD =SQRT(POWER(32,2)/337))*16 for heights substitute 3 and 9 for 4 and 16.

So a 32" television screen will be 25.6" wide by 19.2" high or 27.9" wide by 15.7" high for SD and HD respectively. If you wanted an HD screen with the same height as your old 32" SD you'd need to buy a 40".

It's important to note that's the raw screen size it doesn't include the bevel around it.

Next problem - what is this HD anyway? High definition by itself refers to the number of display lines on the television, just to confuse matters each 'size' also comes in one of two different types.

Like the diagonal size of the television the display lines is sold using one value, that being the number of horizontal lines the screen can display. The minimum allowed to be called HD is 720, the maximum currently broadcast is 1080*. Again, like the diagonal, knowing this number allows to calculate how many vertical lines we should have. Using our 16:9 ratio we divide the number by 9 and multiply by 16 (Excel =(lines/9)*16).

So for a 720 HD set we should have 1280 vertical lines, in the technical bumpf this will show as 1280x720, for a 1080 HD set it's 1920. If the number is less then this then you won't get a pixel-to-pixel match and the television will essentially have to guess which pixel to show - this is normally bad. If you have more then you may reap the benefit of something called over-scanning - this is normally good.

After the number you'll see a letter either an "i" or a "p" they stand for interlaced and progressive. Interlaced means the television draws every other line in one 'beat' then goes back and fills in the others in another 'beat'. For progressive all lines are filled in at once and normally results in a smoother moving picture. In other words "p" is better then "i".

Anything you display will be scaled up or down according to the televisions abilities, a 1080p broadcast will display on a 720i television although obviously not as well as on a 1080p television. This is just like the current widescreen (16:9) broadcasts on an SD (4:3) television but without the black borders

[Update - It's been pointed out to me that this isn't necessarily true a pure 1080i/p signal may not be downscaled to a 720i/p screen it's up to whatever's broadcasting the signal to downscale it]

Finally we get to the connector. Due to various copyright and blah you'll only get HD signals through an HDMI connector. If you've got an HD broadcast unit (Blu-ray player HD-DVD player etc.) and you're hooked up to an HD television with a SCART lead you are not getting an HD picture and you should go and buy a cable immediately.

An HD-Ready television has to have a minimum of 720i (but watch those vertical lines) and an HDMI connector, there also appears to be a spec around called Full-HD which is the same but with a minimum of 1080i (again watch those verticals).

In next week's issue LCDs vs Plasmas (vs CRTs) :-P

*in techie-talk seven-twenty and ten-eighty

6 comments:

Don B said...

WOW! My head hurts, but as my 1990s tele is on the blink I'd better pay attention if I want to understand my next quantum leap into technology (at present I can't even get a picture from my DVD on my TV - suspect I haven't got a cable connected properly).

I need to be able to know either that the spiel by the Curry's salesperson is rubbish or that the assistant at John Lewis is very knowledgable and I can probably trust him/her (odd that at JLP nearly always a youngish male in his late 20s and at Curry's nearly always a 16 year old girl).

I have printed off your TV primer for dotty academics like me.

FlipC said...

At least for a headache you can always take some aspirin, to determine whether the salespeople are telling you the truth produces a headache only curable by learning about the subject yourself (which you shouldn't have to)

I hope this bit helps, I suppose I'd better write up the LCD vs. Plasma entry next for you :-) Though my brain is a little fuzzy at the moment for reasons you'll discover.

Anonymous said...

Iv been looking around the internet for some good quality scart cables so far they have either been really cheap and rubbish or expensive and good but I don’t want to be forking out loads of money every time I need a scart cable. I have ordered a few 1 metre scart cable for my TV and consoles.

FlipC said...

Hey clarksburg thanks for reading. just some thoughts -

First off make sure 1m is long enough for what you want; sounds daft but if you've a few things on a stand and are looping out and back and around it's surprising how much you actually need.

Secondly gold plating - unless your previous cables suffered from tarnish or you've a totally high end system with gold sockets then gold plating does squat either way in terms of signal.

What is more important is a fully shielded/screened cable. This will stop interference between the wires and between the entire cable and other wires or outside influence.

Thirdly I prefer the straight connectors, somehow I always end up with one item on the 'wrong' side which would mean twisting the connector around.

Hope that was of interest

Anonymous said...

Iv been looking around the internet for some good quality scart cables so far they have either been really cheap and rubbish or expensive and good but I don’t want to be forking out loads of money every time I need a scart cable. I have ordered a few 2 metre scart cable for my projector to my xbox, its amazing.

FlipC said...

Ookay from the formulaic repeat and the fact that Google has you pasting the same text into other areas I'm guessing you're a bot.

Result - one filled in contact form sent to Lindy.