Basic info on interlaced/progressive and refresh rates needed

[djorkboy]

Ok, I'm well prepared to feel like an idiot for asking what might be such a basic question (well, set of questions) but I want to get all this clear in my head once and for all, so here goes.

Can anyone tell me:

1. What does Interlaced and Progressive mean really? Is it just that progressive scans one line after another all in one pass where interlaced scans all the odd numbered lines in one pass then all the even numbered lines in the next pass, then back to the odd numbered ones etc?

2. How does interlaced video relate to refresh rates? why does 1080i have a refresh rate of 25 or 30 when 1080p has a refresh rate of 50.

3. I have a X1650Pro card with 1xDVI 1xVGA and 1xS-video port. I use VGA to connect to my LG Rear Projection HDTV, can I set my card to output 720P at 50hz through this VGA or do I need to use some sort of Component cable to use the HDTV functions of the card? or should I be using the DVI output with the VGA adaptor?

I tried to find a "basic guide to HDTV standards" somewhere but all I could find were pages that referred to HDTV only, not in comparison to computer outputs.

[giantal]

1. What does Interlaced and Progressive mean really? Is it just that progressive scans one line after another all in one pass where interlaced scans all the odd numbered lines in one pass then all the even numbered lines in the next pass, then back to the odd numbered ones etc?

Yes you got in one.

2. How does interlaced video relate to refresh rates? why does 1080i have a refresh rate of 25 or 30 when 1080p has a refresh rate of 50.

25/30 are really the reported effective refresh rates. They are really 50/60 produced as per your interlaced description above.

3. I have a X1650Pro card with 1xDVI 1xVGA and 1xS-video port. I use VGA to connect to my LG Rear Projection HDTV, can I set my card to output 720P at 50hz through this VGA or do I need to use some sort of Component cable to use the HDTV functions of the card? or should I be using the DVI output with the VGA adaptor?

I have a X1300 which had some trouble syncing to my Panasonic 32" LCD at 50hz. I believe this was a TV limitation through the vga connection, however DVI using 720p@50 (which is should show as an option in the latest catalyst drivers) worked wonders. Just be mindful that it may be subject to overscan issues, IMO no problem for TV viewing.

A side note, if catalyst is not allowing 50hz from the control center I have found that the TV setup in MCE will allow you to select 50hz regardless if you choose ALL Modes. This has the added advantage of having a different resolution for desktop and MCE viewing.

[ExtremePC]

Its slightly more complicated than that and to explain it further you have to treat analog and digital signals differently (not the TV trasmission type but analog and digital as in CRT displays and LCD/Plasma displays)


Analog video does not have a resolution per se but rather have a vertical line count and this is actually where we get the standards for 576, 720, 1080. All those numbers represent the vertical number of lines. The refresh rate is the "speed" the vertical lines take to fill a screen top to bottom. All DVD's and most TV both digital and analog signals are "encoded" to begin with as an analog component standard.

There is another refresh rate that has to do with the horizontal "speed" for each line that is a measure of how fast the signal changes to give you different colours and intensities across a single line.

Digital displays use a dot count where each pixel has a data value for colour and intensity and is specific for the given piece of video being displayed at a particular native resolution and refresh rate that is relative to displaying an entire field (I'll elaborate later about fields) onscreen at a time.

So far I've talked about whats in the actual signal difference between analog (component) and digital (VGA, DVI, HDMI), next is how they relate to the actual display.

The reason I am mentioning the background above is becuase the refresh rate is handled differently depending on the display and input type and the major differences are in the handling of interlaced signals more than progressive.

I have to mention what a field is so the rest of this makes sense. In Interlaced video a frame is made up of alternating lines of video kind of like a venetian blind where you have a single line of video then a blank line then another single line of video then a blank line, there are as many of these lines in the video relative to the line count or 576 lines, 720 lines, or 1080 lines. ie. for a 720i signal there are 360 lines with the video data displayed with 360 blank lines in between. a whole screen full of these lines is called a field. In interlaced video two of these fields shown really fast after each other make up a single frame of video. So if the signal is being "sent" at 50Hz you only get a whole frame every 2/50ths of a second or really and effective refresh rate of exactly half the refresh rate of the transmission ie. 25Hz

There are three main methods that all displays use to handle interlaced signals, in the examples below I'm talking about a 50Hz signal:

1. Each field is "painted" down the screen on alternate lines, line by line and the entire field (or half the picture) takes 1/50th of a second to finish, then the second field is painted line by line on the other set of alternate lines down the screen in 1/50th of a second. Because this is happening so fast the eye can't tell that its only being shown half the picture at a time but the time it takes for an entire picture to be shown is 1/50th + 1/50th or 2/50th of a second. this works out to be 1/25th of a second for a 50Hz signal or and effective refresh rate of 25hz.

2. Similar to no. 1 but all the pixels for a field are tuned on for the alternate lines at the same time instead of being "painted" line by line, they are still being shown in 1/50th of a second for each field so a complete picture is only displayed for a 50Hz signal in 2/50ths of a second or again an effective display refresh rate of 25Hz.

3. The data from the first field is stored in the 1/50th of a second and nothing is displayed, the data from the second field is added to the first in the next 1/50th of a second and the a whole picture is finally displayed but only after 2/50th of a second or at 25Hz.

4. There is another "con job" method that is used in either cheap or expensive displays and that is the data from the first field in the first 1/50th of a second is also displayed on the 2nd 1/50th of a second and the second field of data is ignored completely. this can be done either like method 1,2 or 3. When a display does this you can tell by really bad jerky movement on the scrolling info bars you see on the top or bottm on some video or TV transmissions and is refered to as the "ticker tape effect".

Progressive on the other hand is much much simpler but much much harder becuase there is exatly double of the amount of data that needs to be sent and processed every 1/50th of a second for 50Hz or 1/60th of a second for 60Hz signals. There are no fields in progressive video only frames as the entire picture is displayed at the same time every 1/50th or 1/60th of a second.

I can now elaborate why a 576p or 720p signal can look much better than a 1080i signal. When you compare a 576p signal with a 1080i signal with both at 50Hz and looking at the image being displayed in a single 1/50th of a second the 576p signal will be showing a complete 576 lines all joined up together with no gaps but a 1080i signal will have only 540 lines of picture data displayed on top of that the picture data will be only half the picture and it will have a gap in the picture every second line.

Coversely it takes 1080i 2/50ths of a second to display an entire picture (and thats only after tricking your eyes into seeing two pictures as one) where a 576p signal will show you a complete pictue in 1/50th of a second. With a 720p signal the difference is even larger because it shows you 720 lines with no gaps at the same time or the entire picture in half the time.

Fast moving objects move more smoothly in progressive video signals because they apear less spaced out along the path of movement onscreen per complete frame because progressive video is able to capture exaclty double the amount of "sample" points as the object or objects move. Its also why the "ticker tape effect" is much less prevalent in progressive video.

[djorkboy]

Wow, thanks guys, that is exactly the sort of info I am after.

Someone should make a sticky out of ExtremePCs post as a definitive introduction to how DTV and HDTV works.

[ExtremePC]

Originally Posted by djorkboy

Wow, thanks guys, that is exactly the sort of info I am after.

Someone should make a sticky out of ExtremePCs post as a definitive introduction to how DTV and HDTV works.

Maybe, but I'd have to clean it up a bit first and elaborate on some stuff I oversimplified.