Tuesday, 5 December 2006

Video modes

1080i, 420p, 720p -- What's this all about you say? Let’s start with the basics – Progressive vs Interlaced

Interlaced - Interlaced scanning was invented back in the 1920s as a way to broadcast TV signals within a narrow bandwidth. It can cause flicker and various kinds of distortion. Interlace is still used for most standard definition TVs, The technology available during the time it was invented didn’t allow for sending a full image over the air at the necessary rate to create a smooth sense of motion at the viewer’s end. This method consists splits each image into its odd and even horizontal lines in order send each group separately. The image will later be reconstructed at the destination by putting together the two groups at a quick enough rate to fool the eye from noticing the split. For a nice animated example in Wikipedia, click here.

Progressive - In contrast, progressive (or non-interlaced) scanning manages to display the entire image at one time.

If you are to enjoy your new next-gen console in all it’s HD glory, I suggest you get yourself a decent 720p HDTV (arguably better than a 1080i HDTV, see why here)

So what does this all mean?
During the broadcast of an image using progressive scan, it is displayed on your Television screen line by line from top to bottom. On the other hand, interlace scanning displays the same image, but only for every other line; it then fills in the missing rows.

Frame is the term used to describe the entire image when it is not split into odd and even lines. Field is the term used to describe the half-an-image groups made of odd or even lines. These two words therefore allow us to refer to the display rate as either frames per second or fields per second.

The different formats are defined upon three aspects:
  • Number of lines: 480, 720 and 1080.
  • Display Method: progressive or interlaced.
  • Display Rate: frames or fields per second.

Normal TV’s (or at least the ancient one I have) have a 480i display rate. This means it uses interlaced scanning and has a vertical frame resolution of 480 lines. It also has an aspect ratio of 4:3 as opposed 16:9 ratio seen in most HDTV’s.
The aspect ratio of an image is its displayed width divided by its height (usually expressed as "x:y"). For instance, the aspect ratio of a traditional television screen, like the one I have is 4:3.
High Definition Television sets allow broadcasting with a significantly higher resolution than traditional formats, such as the one my TV uses.

HDTV’s contain different formats for broadcasting, they can be identified by :
1 . The number of lines in the vertical display resolution.
2 . Whether progressive frames (p) or interlaced fields (i) are used.
3 . The number of frames or fields per second.
As I said earlier, Frames denote the display rate where Progressive scanning is used as it refers to the ‘entire image, when it is not split into odd and even lines’. Fields, however, denotes the half-image groups made of odd or even lines.

The frame rate in hertz can be either implied by the context or specified after the letter p (such as 1080p30, meaning 30 frames per second).
For example, the format 720p60 is 1280 × 720 pixels, progressive encoding with 60 frames per second (60 Hz). 720p assumes a widescreen aspect ratio of 16:9, and a horizontal resolution of 1280 pixels for a total of about 0.92 million pixels. The frame rate (in this case equal to the field rate) can be either implied by the context or specified in hertz after the letter p.

The format 1080i50 is 1920 × 1080 pixels, interlaced encoding with 50 fields (25 frames) per second. Usually assumes a widescreen aspect ratio of 16:9, implying a horizontal (display) resolution of 1920 dots across and a frame resolution of 1920 × 1080 or about 2.07 million pixels.

Information for this post was gathered from Wikipedia and here.