Techno-babble

Various Ramblings about HDTV

All full service analog television broadcasting in the USA is scheduled ended on June 12, 2009. Digital Television (DTV) has become the only "full service" television broadcast system used in the United States. In our market, KBTV (4), KFDM (6), KBMT (12), and KUIL (34) have turned off our analog transmitters.  We continue to broadcast our normal programs, but only on our digital channels.  If you have not yet purchased a digital TV set or a converter bo for your analog set, you will not be able to watch any of your local full service television stations (4, 6, 12, and 34) on your analog TV set.  Note that there are several low power TV stations in our market that may continue to broadcast their programs in analog.  Low power TV stations are hard to pick up if you live far from their transmitters; so many people don't even know they eist. 

The Federal Communications commission has a website which contains a lot of information about digital television (www.dtv.gov). You can see maps of the coverage area of the local TV stations, in addition to other useful information.  Another good site is www.antennaweb.org which lists every station that you can watch as well as antenna pointing information.

DTV (Digital TV) refers to how I transmit the pictures to your home and how you receive them.  DTV is the new method of sending television signals to your home (the old method was called "analog"). You must have a digital tuner in order to pick up digital signals.  This holds true regardless of where the digital signal comes from, either over-the-air ("free TV"), cable, or from satellite.  Although there are major differences between an over-the-air "digital tuner" and the "converter bo" you get with cable or satellite, they are really just different versions of the same thing: a digital tuner.

HDTV (High Definition TV) refers to picture quality, not how the picture gets to your TV set.  HDTV is supposed to mean a picture that is shaped like a movie screen (in a 169 format) with much higher resolution (clarity) than is possible with an analog picture. 

SDTV (Standard Definition TV) also refers to picture quality, and is supposed to mean a picture that has about the same clarity of your current analog TV set (43 format). SDTV can also be in the 169 format (such as "wide screen" DVD movies).

EDTV (Etended Definition TV) is a term describing a picture that uses the "progressive scan" (480p) display method (such as a DVD player). The picture from an "interlaced scan" (480i) DVD player would be called SDTV.  The difference between SDTV and EDTV is not worth the effort required to tell them apart, in my humble opinion.  An EDTV picture does look better than an SDTV picture, but it is not HDTV.

DTV Picture Formats
Just because someone is transmitting DTV does not automatically mean that the picture you see on your digital set will be High Definition TV. There are 18 different picture formats that can be transmitted, but only 6 of them are High Definition. The remaining 12 picture formats are different forms of SDTV.  Most of these SDTV formats result in a better picture than what you see on your current TV, but they are not HDTV. Some SDTV formats are for compatibility with the computer industry.

For a brief introduction to the various picture formats, visit http://www.avdeals.com/classroom/what_is_dtv.htm . For "the full story" about DTV technology, visit http://www.avdeals.com/classroom/what_is_dtv_full.htm.  Both of these sites are a little old, but most of the information presented is valid.

The signal I transmit on my digital channel is made up of ones and zeros, just like your home computer and all other "digital" equipment use.  Since DTV is digital data, I can put a lot of stuff in there.  I could broadcast up to 6 SDTV stations on my digital channel.  Or I can stuff one HDTV station and 1 or more SDTV stations onto my digital channel.

Note that I have one, and only one, digital channel (broadcast channel 21).  As stated above, I could be showing up to 6 different programs at the same time. These different programs are called "stations" to differentiate them from my digital broadcast channel.  These "stations" would be called "6.1", "6.2", "6.3".... "6.6".

KFDM has currently elected to broadcast one HDTV station, and one SDTV station. The HDTV station is the CBS network with local channel 6 programs.  In other words, the normal channel 6 programming you used to see on analog channel 6.
The SDTV station is currently the CW network. Both stations are always there. You just have to select which one you want to watch.

If you punch in 6 dot 1 (6.1) on your remote control, you will see our main DTV station. Punch in 6 dot 2 (6.2) and you will see the CW network.

Your HDTV set may use a dash (-) instead of a dot (.), but you get the idea.  If your set uses an "on screen channel guide", you can select these channels from the menu.

When we are not broadcasting true HDTV on our main DTV channel, we will be showing an analog picture that we have converted to digital. Converting our analog picture to digital does not make it HDTV. There is no snow, impulse noise, or picture degradation in a digital picture like there was when in the analog transmission system. But it is not HDTV. It is SDTV.

Currently, our only source of HDTV programming is the CBS television network.  We have no local HDTV capabilities, and I don't epect we will purchase any for a year or more.  You can go to http://www.cbs.com/info/hdtv and see the CBS primetime lineup with all the HD shows highlighted.

These are the most often used of the 18 possible digital picture formats.  1080i and 720p are High Definition picture formats, while 480i and 480p are SD formats.  The number portion of these formats is the number of active video lines used to make the picture.  The letters represent the method used to "paint" the picture on the screen.

     Any picture you see on a television set is made up of horizontal "lines" of video, which are painted on the screen from the top to the bottom.  In the interlaced format, the odd numbered lines (1, 3, 5, ...1079) are painted on the screen, then the even numbered lines (2, 4, 6, ...1080) are painted in between the odd numbered lines.  In the progressive format, all lines are painted in numerical order (1, 2, 3, 4, 5, ...720).  A better eplanation of how television works is available at http://electronics.howstuffworks.com/tv.htm.  An article on how HDTV works is available from the same people at http://electronics.howstuffworks.com/hdtv1.htm. Both articles are rather long, but worth the effort.

The progressive scan format usually makes a more pleasing picture when rapid movement is involved.  The problem with the progressive scan format is that it takes more "bandwidth" or a faster "data rate" than does the interlaced format for the same number of active video lines.  We broadcast in 1080i mostly because the CBS network uses the 1080i format.

You would think that 1080i would look a lot better than 720p since there are 360 more video lines in the 1080i picture.  But the advantages of the progressive scan format almost makes up for the missing video lines when there is movement in the picture.  You really have to look close to see that the1080i picture actually does look better than the 720p picture.  But it is there if you look hard enough.

All of our High Definition programs are broadcast using the 1080i format.  When we are not broadcasting High Definition, the picture you see is in the 480i format (SD).

If you have watched our digital channel you may have noticed that your digital set says that we are always broadcasting a 1080i signal, even when you can see that we are obviously broadcasting an SDTV picture.  We have set our digital encoder (the device that makes our digital picture) to "insert" our 480i picture in the middle of a 1080i signal.

We do this to keep your TV set (and my encoder) from having to switch formats every time we change from a High Definition picture to an SDTV picture, and back again to a High Definition picture (every commercial break).  It can take 2 seconds or more for your set to recognize the command and then change the display format.  It also takes a few seconds for my encoder to change the format.  This really looks bad as the picture on your set usually freezes and "piilates" during the change. Therefore, we don't change encoder formats very often.

No doubt you have noticed that our SDTV pictures always have "side bars".  Some stations stretch their 43 analog pictures to fill the 169 screen. This "stretching" of the video results in a distorted picture, and it also reduces the resolution (clarity) of the picture.  The edges of the picture are stretched more than the center to limit this distortion, but we still didn't like the resulting picture.  Also, the top and bottom of the picture usually gets "chopped off".
There is one other reason we don't try to fill up the screen when broadcasting an SDTV picture.  If those sidebars are missing, you know that the picture is true HDTV, not some SDTV picture stretched to fill the screen and pretending to be HDTV.
There is one eception to this rule.  Sometimes we have a program (usually a movie) that is only available in "letter bo" presentation.  We will stretch the picture to fill the 16:9 screen for these programs.  If we did not do this, the "letter bo" picture would show up on a 16:9 set with black bars on the top and bottom of the picture, as well as both sides.  In other words, you would get a "postage stamp" picture on your HDTV set.  This is not acceptable to most viewers, or to us.
We change the aspect ratio of our encoder to fill the HDTV screen for this reason, although it is not in HD. The letterbo mode does not distort the picture much, but it does decrease the resolution a tad.

A High Definition picture should look the same regardless of where it comes from (Over-the-air, cable, satellite, or an HD-DVD).  The only difference between over-the-air, cable, and satellite HDTV pictures would be which compression scheme is being used.  All HDTV pictures are compressed, which is a method used to reduce the amount of digital data that must be sent in order to make an acceptable picture.
Satellite and cable systems usually use a higher compression scheme than does broadcast.  Sometimes this causes a noticeable reduction in the picture quality, and sometimes it causes the picture to freeze intermittently.  Most of the time, the picture is fine.  Broadcast digital pictures can also eperience the same problems when there is a lot of fast motion in the picture, but it normally does not occur as often.

All digital sets have a "native display format" (also called "native resolution").  It is much easier (and LOTS cheaper) to change video formats in digital memory than it is to design the set to actually be capable of displaying the 18 possible video formats.

In general, the higher the number in the "native format", the better the picture will be.  1080 is currently the highest "native format" for HDTV sets.  There are now displays (TV sets) that claim to be "1080P" (1080 lines, Progressive scanning).  Although TV stations can not broadcast in 1080P, there is nothing stopping cable or satellite systems from delivering 1080P programming.  It won't hurt anything to get a set with a 1080P native format, but you won't see much improvement over a 1080i display until some 1080P programming becomes available.  This should occur before the set needs replacement (in about 10 years).

Your old analog set also has a "native display format".  Over fifty years ago, the National Television System Committee (NTSC) set the US analog broadcast video standard as 525 interlaced video lines, 30 frames per second.  All analog television sets sold in the United States were built to display pictures using this standard.  Other parts of the world use different video standards such as PAL and SECAM (among others).
All US analog television stations* as well as cable and satellite systems must send their pictures to your analog set using this NTSC "standard format".  In the case of "digital cable" and "digital satellite" the STB (Set Top Bo) changes their digital signal to an analog picture for display on your analog TV set.  A "digital converter bo" picks up our digital broadcast signal and changes it to analog so your analog set can display our programming.

* The only analog TV stations still on the air are all "low power" TV stations (LPTV), and they do not have to change to digital broadcasting like every "full service" television broadcast station.  They can if they want to, but the FCC does not require them to "go digital" at this time.  Who knows what the future will hold?

Last update: September, 2009

Part Six

See Archives