I was fortunate enough to take stock of a used Mitsubishi 1080i widescreen 46″ TV, from a friend of mine leaving town this week – it looks great sitting in my front room. I’m somewhat of a novice when it comes to high end TV, and acronyms like HDTV, DVI, HDCP mean very little to me, so to get the best out of my new purchase I had a poke around the Internet to find out how widescreen works. Here’s what I found out…
Conventional TVs display a picture with aspect ratio 4:3, which simply means that the picture height is 3/4 of that of the width. Widescreen TVs typically display an aspect ration of 16:9, where the width is much wider than that of the conventional picture. Widescreen picture height is 9/16 of the width.
Conventional 4:3 pictures can be displayed on 16:9 TV by stretching the horizontal disproportionately or proportionately with cropping of top and bottom. 16:9 pictures can be displayed on 4:3 displays using letterboxing – reducing the width proportionately and adding black horizontal lines to the top and bottom of the picture.
The resolution of the picture displayed on a TV depicts the quality seen by the viewer, and is usually quoted in pixels. Conventional TVs typically display a 640×480 image (4:3 aspect ratio). Widescreen TVs can display up to 1920×1080 pixels. Often, TV manufactures quote the resolution as 720p, 1080i, or 1080p – this is the number of vertical lines (scan lines) that can be displayed on the screen at one time. The ‘i’ stands for ‘interlaced’ and ‘p’ for ‘progressive’, both explained below.
Interlaced vs. Progressive:
If you imagine the number of pixels that go into a single frame of motion video, and that each pixel can have a vary number of colors, you will appreciate that even a low resolution picture (640×480) can contain a lot of data – about 307200 pixels per frame. When broadcasting TV over the air or cable, the number of frames per second and the picture resolution will directly affect the amount of bandwidth required to air the broadcast.
Interlacing was introduced (many years ago) to cut the bandwidth required to broadcast a TV show in half. Interlacing works by alternating the display of odd horizontal scan lines and even horizontal scan lines for a given frame. Since the broadcast contains half of the picture per frame, the amount of bandwidth is reduced by half. To give the affect of full picture motion without flicker, the TV receiver displays picture frames at 50 or 60 times a second (PAL – 50Hz, NTSC – 60Hz) – faster than the human eye.
Interlacing enabled broadcasters to push TV pictures over the air and down copper cables using less bandwidth, but at a lower quality. With the introduction of DVD players and video sources that were not constrained by bandwidth, progressive scan (or non-interlaced) was introduced. Unlike interlaced, progressive TVs display each scan line in sequence order, so the viewer sees a clearer crisper picture. High end plasma, LCD, and some rear projection TVs support progressive scan, all others expect an interlaced signal.
High definition TV – TV broadcasters are now pushing out broadcast content in higher resolutions (and 16:9 aspect ratios) so viewers can enjoy higher quality viewing on large screen size TVs. I was interested to find out that not all the broadcast networks push out the same HD format – typically broadcast HDTV is 720p, but some networks broadcast 1080i. Broadcast 1080p is almost unheard of because of the insane amount of bandwidth required.
DVD and Anamorphic Widescreen:
From what I can gather, standard DVD is 720×480 (NTSC) or 720×576 (PAL) at 4:3 aspect ratio or 16:9 aspect ratio using non square pixels. Typically 16:9 widescreen is achieved on DVD using anamorphic filming – storing a vertically stretched version to make optimal use of the vertical resolution (see here for more details on anamorphic filming). DVD players that support 16:9 can automatically adjust the width of anamorphic widescreen movies to take advantage of a 16:9 display, whereas 4:3 players will compress the vertical and add black horizontal bars. Anamorphic typically uses a 2:35:1 aspect ratio, so even 16:9 displays will see partial black horizontal lines above and below the picture.
So, in conclusion… my widescreen DVDs look great on the new TV, 4:3 DVDs look pretty good, and conventional TV is watch able in 4:3 narrow but I could really use an HDTV signal. Unfortunately my Media Center is using a standard analogue coaxial tuner to record TV, and Comcast will only provide HDTV via a set-top box. Looks like I have to wait until cable cards become main stream.