Now that we've covered the basics of what standard definition television — or
NTSC video — is, let's move on and define High Definition Television (HDTV).
"High definition is a 16:9 aspect ratio digital signal with various lines of resolution,
all greater than 480."
This definition is broader than standard definition television because HDTV encompasses more
than one type of signal. Since most television stations have narrowed the choices down to two
different signals, we'll cover them exclusively.
- 720p - A 16:9 aspect ratio progressive scan digital signal with 720 lines of resolution,
shown at 60 frames per second.
- 1080i - A 16:9 aspect ratio interlaced digital signal with 1080 lines of resolution,
shown at 60 fields per second.
Obviously your old television isn't equipped to read or display an HDTV image for numerous
reasons:
- Older televisions can't input a digital signal;
- Even if you convert the signal to an analog signal first, the older television isn't equipped to convert the widescreen 16:9 and map it into its 4:3 aspect ratio,
- Even if you have an older widescreen rear projection television, the television isn't equipped to change the higher resolution image to 480 lines of resolution.
Fortunately, newer digital televisions, whether they are true HDTV sets or otherwise, not only handle an HDTV signal but are backwards compatible. We will cover how different display devices show HDTV and delve deeper into this subject in our advanced level section. For now let's go over the basics of the two signals.
720p
A 720p High Definition signal is displayed in progressive scan, meaning that the entire
image is drawn at once. Because HDTV is a digital signal, it will not only have vertical
resolution; it will have horizontal resolution as well. To ensure that your television or
monitor can fully resolve a 720p signal, you should make sure that the display device
has a resolution of at least 1280 x 720 (see Figure 1).
Figure 1 - 720p
1080i
A 1080i High Definition signal is displayed as an interlaced signal, just like
standard definition television. This means that only half the image (540 lines) is
displayed at any one time. Just like with 480i (NTSC video), the odd lines are displayed
first, followed by the even lines. And, like 720p, 1080i is a digital signal, so it
has a horizontal as well as vertical resolution (see Figure 2).
Figure 2 - 1080i
So which format of HDTV looks better? Consider this: The human eye is more sensitive to vertical resolution than it is to horizontal resolution. Because 1080i is shown as an interlaced signal, only 540 lines of vertical resolution are seen at any one time. The other 540 lines of resolution are shown 1/60th of a second later, and your eye combines the two to create 1080 lines in your mind. 720p shows all 720 lines at once, every 1/60th of a second, to create a flicker-free, super high resolution image.
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