Refresh Rates and Progressive / Interlaced Video - A Not Quite Short Explanation

All video signal come in one of two forms, Interlaced or Progressive and which is being used for a given piece of video is signified by placing either an i for Interlaced video or a p for progressive video after the number that signifies the vertical line count for a given format.

Analogue Video does not have a resolution per se but rather it has a vertical line count. This is actually where we get the video standards for 576, 720, 1080. All those numbers represent the vertical number of lines for the video formats that bear the same name. It must also be noted that the line count while compatible with the vertical dot count in digital displays is not the same thing at all and the former must be converted to the later before being displayed on a digital display like an LCD or Plasma screen.

Digital Video on the other hand does have a resolution made up of the number of dots that make up an image vertically and horizontally, i.e. 1280 x 720. Digital Video also has the same Progressive / Interlaced and Refresh rate properties as Analogue Video depending on how it was either encoded in the first place or how it is processed for transmission or display.

The term Refresh Rate is the "speed" the vertical lines or lines of dots take to fill a screen top to bottom. All DVD's and most TV programs, both digital and analogue are "encoded" to begin with, in an analogue component video standard. Again it must also be noted that the analogue horizontal line data while compatible with the horizontal dot count in digital displays is not the same thing at all and the former must be converted to the later before being displayed on a digital display like an LCD or Plasma screen.

There is another refresh rate that has to do with the horizontal "speed" for each line in a frame or field that is a measure of how fast the signal changes to give you different colours and intensities across a single line or line of dots. Digital displays use a dot count where each pixel has a data value for position, 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 or frame on screen. (I'll elaborate later about fields).

So far I've talked about what’s in the actual signal difference between Analogue (i.e. component) and Digital (i.e. VGA, DVI, HDMI), next is how they relate to the actual display. The points above are a quick background because Refresh Rate is handled differently depending on the display type in conjunction with the different signal types being sent to it. There is greater difference in the handling of interlaced video over progressive video.

With 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 and there are as many of these lines in the video relative to the line count of 576, 720, or 1080 lines. I.e. 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. 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 original signal. I.e. 25Hz

There are three main methods and one “con job” that might be used in a display to handle interlaced video signals, the examples below relate specifically to a 50Hz video signal for simplicity.

Method 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, the screen is them wiped clean and 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. The time it takes for an entire picture to be shown is 1/50th + 1/50th or 2/50ths of a second, this works out to be 1/25th of a second and has an effective refresh rate of 25hz.

Method 2. Similar to Method 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 1/50ths of a second for each field so a complete picture is only displayed in 2/50ths of a second or again an effective video refresh rate of 25Hz.

Method 3. The data from the first field is stored in the first 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 have elapsed, yielding an effective video refresh rate of 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 1/50ths of a second is also displayed on the 2nd 1/50ths of a second, the second field of data is discarded or ignored completely. This can be used in either method 1,2 or 3. You can tell when a display does this by the strobing movement on the scrolling info bars you see on the top or bottom on some video or TV transmissions and is referred to as the "ticker tape effect".

Progressive video on the other hand is much simpler but much harder because there is exactly double of the amount of data that needs to be sent and processed every 1/50ths of a second. There are no fields in progressive video only frames as the entire picture is displayed at the same time every 1/50th 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, 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 and on top of that the picture data will be only half the picture and will have a gaps in the picture every second line.

Conversely it takes 1080i 2/50ths of a second to display an entire picture (and that’s only after tricking your eyes into seeing two pictures as one) where a 576p signal will show you a complete picture 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 appear less spaced out along the path of movement onscreen per complete frame because progressive video is able to capture exactly 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.