The 1080p HDTVs
I am going to try to shed some light on the 1080p HDTVs that are taking the HD world by storm. Many describe the 1080p TVs as the "Holy Grail" of high definition. But there is some important information you should know before you start shopping for one of these babies.
Display vs. Transmission
This is probably the area where most people get confused when considering a HDTV. The manner which a HDTV displays the picture and the transmission of the signal are two different things. Now that used to be not the case when all TVs were CRT based which required an interlaced scaned display to reduce flicker, but now days with the advent of fixed pixel displays such as LCD, plasma, DLP, Lcos, among others, the need to interlace the display is no longer required. Fixed pixel displays are built with a pixel matrix that allows the control of light for each pixel rather than the need of the screen to be scanned with an electron beam like a CRT. The entire bit array can be, and usually is, updated in a progressive manner. This has several advantages over the CRT, but the biggest benefit is the elimination of flicker totally.
Now the HD signal that is received by the HDTV is either 1080i (interlaced) or 720p (progressive) depending on the format chosen by the broadcasting station. Just in review a 1080i interlaced signal will divide the total frame of 1920×1080 pixels into two fields of 1920×540 pixels where the first field will contain the even lines and the second field will contain the odd lines. If this interlaced signal was to be displayed on a CRT, it could be scanned directly on the CRT, but if it is to be displayed on a fixed pixel display, it needs to be reconstructed into the fixed pixel array. This process is called deinterlacing.
The important thing to remember is there is a difference between the signal received by the TV and the method the TV updates its display. 1080p and 720p TVs are capable of receiving an interlaced signal or in some cases a progressive signal.
Now here is where it gets sticky. While we are going to discuss the 1080p displays receiving a 1080i signal, 720p displays will operate in a similar manner. There are two methods used to convert an interlaced input signal to the fixed pixel display array. One is to weave the two fields together (film mode) to construct a 1920×1080 progressive frame and the other is to scale each 540 line field (video mode) up to 1080 lines for display. Weaving will allow a frame to be reconstructed as it was originally scanned when the source is film, like a movie. Scaling each frame will keep the temporal resolution when the source is interlaced video, like a live sports broadcast.
The issue here that is important is some 1080p TVs will use both methods depending on the type interlaced signal that is received by the video processor actually detecting film material or interlaced video and switching between the two modes automatically. This will allow full 1920×1080 pixel resolution when a movie is being watched and the full temporal resolution of 60 frames per second at reduced resolution when interlaced video is received. It should be noted here that when in the film mode the input frame rate is 30 per second and when in the video mode it is 60 per second. Reducing the temporal resolution of the display when watching a movie does not matter because the movie was shot at 24 frames per second and the increased 60 frames per second is unnecessary. By using the video mode when watching a live sports broadcast will result in smoother motion. In case you were wondering, while there are video processors that can smooth out video frames woven and even construct the extra frames required for 60 frames per second, they are not within the price range of consumer electronics.
It is important to know how the 1080p HDTV will deinterlace an interlaced signal. Some sets do not weave the two fields ever and will only scale the two 540 line fields received to 1080 lines for display thus reducing the resolution to half what the original film frame contained. Also it is difficult to know which sets use this cheapo method of deinterlacing because this is not advertised by the manufacturers.
Some 1080p sets will accept either a 1080i or 1080p as well as 720p HD input signal and some sets will accept only a 1080i as well as a 720p HD input signal. This is where many people get confused and will refer the latter sets as 1080i HDTVs. Here again getting confused with the type TV it is with the type signal it will receive.
Having a 1080p input is not necessary for TV viewing, but would be useful for connecting the HDTV to a computer or game box. If you are going to use your 1080p HDTV with a PC or game box, then by all means get a 1080p HDTV that will accept a 1080p input, but if you are going to just watch TV and high definition DVDs, then the 1080p input is not necessary and will not provide any better picture. What’s that you say? How about the Blu-ray DVD players that offer 1080p output? Well, ok, if you have a HDTV that will accept a 1080p input at 24 frames per second and will refresh the display at a multiple of 24 Hz, then some improvement could be had, but if your HDTV’s display refresh is at 60 Hz (most are) then there will be no improvement. Well wait, if your HDTV uses only the video mode for deinterlacing and the HDTV will accept a 1080p input then assuming the DVD player outputs complete reconstructed frames, then there would be an improvement as well.
As you can see it is important to find out as much as is possible about any potential 1080p or 720p HDTV before you purchase. But how does one find out? I have made several posts on this subject that refer to tests conducted by Gary Merson and that will be a great place to start. Just follow these links:
Are You Getting All Your Resolution?
Are You Getting All the HDTV Resolution You Expected?
Are You Getting All of the HDTV Resolution You Expected? Round 2
Are You Getting All of the HDTV Resolution You Expected? Round 3