HDTV and Resolution: Full Technical Details

By: David Johnson

The quality of the picture displayed on the TV screen is primarily the result of the television's "resolution." Put simply, TV 'resolution' refers to how many horizontal lines are displayed on the TV screen. There are two methods that can be used to display the lines on the screen - either "Interlaced" or "Progressive Scan." The NTSC adopted the standard known as "interlaced" to provide a method of compression that achieves a higher resolution using less-costly circuitry.

The NTSC-analog standard is "525 lines - interlaced, at 30 fps" (frames per second). This may be written as: 525-i/30 fps; however, only 480 lines are used to make the visible image, the remaining lines contain information pertaining to picture synchronization and are not seen. For this reason, the stated 'resolution' usually refers only to the visible lines; ie... 480-i/30 fps.

In using the "Interlaced" method, the 480 lines are created in two fields (phases). The "scan-rate" for these (2) fields is 60hz (60 times per second). In phase #1, the first 1/60th of a second, 240 lines (the odd numbered lines - 1,3,5 etc) are scanned on the tube. In the second 1/60th of a second, (phase #2)- the remaining 240 (even-numbered) lines are scanned. Thus each field of 240 lines is scanned 30 times a second, and produces one complete frame (30) times per second; (1/60 second X 2 fields = 2/60 second = 1 complete frame, 30 times per second).

It's the total lines-per-image that indicates the resolution of the system, ie: (525-i or 480-i).

When resolution is considered, both the resolution of the transmitted signal and the (native) displayed resolution of a TV set are taken into account. Digital NTSC- and PAL/SECAM-like signals (480i60 and 576i50 respectively) are transmitted at a horizontal resolution of 720 or 704 "pixels". However these transmitted DTV "pixels" are not square, and have to be stretched for correct viewing. PAL TV sets with an aspect ratio of 4:3 use a fixed pixel grid of 768 × 576 or 720 × 540; with an aspect ratio of 16:9 they use 1440 x 768, 1024 × 576 or 960 × 540; NTSC ones use 640 × 480 and 852 × 480 or, seldom, 720 × 540. High Definition usually refers to one million pixels or more.

In Australia, the 576p50 format is also considered a HDTV format, as it has doubled temporal resolution though the use of progressive scanning. Thus, a number of Australian networks broadcast a 576p signal as their High-definition DVB-T signal, while others use the more conventional 720p and 1080i formats. Technically, however, the 576p format is defined as Enhanced-definition television.

Standard frame or field rates:

23.977p (allow easy conversion to NTSC)

24p (cinematic film)

25p (PAL, SECAM DTV progressive material)

30p (NTSC DTV progressive material)

50p (PAL, SECAM DTV progressive material)

60p (NTSC DTV progressive material)

50i (PAL & SECAM)

60i (NTSC, PAL-M)

HDTV also has formats that use the interlaced system; however, DTV also uses another system, called "Progressive Scan." The progressive system scans the total number of lines, 60 times a second; not half and half as in interlaced. This means you see the complete image displayed on your TV screen two-times more often. This results in smoother motion; moving images have less ‘motion artifacts’ with none of the visible "flicker." A progressive scan system with 480 lines of resolution is written, "480p." Interlace and Progressive Scan

As already stated, the ATSC has assigned (18) formats to Digital TV. HDTV is primarily concerned with just two of these formats at present: 1080-i and 720-p. (or Higher! - 1080p HDTV-Resolution is now a reality) The ATSC Standard for High Definition Television accepts either 1080 - interlaced scan lines, or 720 – progressive scan lines, or higher

Top Searches on
Technology
 • 
 • 
 • 
 • 
 • 
 • 
 • 
 • 
 • 
 • 
 • 
 • 
 • 
 • 
 • 
 • 
 • 
 • 
 • 
 • 

» More on Technology