Some Explanation of HDTV Terms

for U.S. type Television Systems

Forward

While I am a degreed Electronics Engineer, I myself have had some confusion regarding HDTV terms and some of the other video terms such as 480i, 480p, 1080i, progressive scan, and others.  Therefore after some research, I am offering at least some simple explanations of these terms.

Electronics Signal Improvement

While in College before I received my Electronics Degree, I was able to see a college radio station broadcast booth.  The picture that I saw was worth a thousand words, and the image sticks in my mind to this day.  Radio stations of course need good quality audio.  The tape running in the radio station booth was much wider and ran much faster than audio tape in my home audio tape system.  What did that mean to me?  It means that there was "more information" available on the larger size tape, and since it ran faster, that made yet even more information available.  So, the term that seemed to explain this best was: "more information per unit of time" will give a better audio signal.  Since the radio station tape was large and ran fast, it had "more information per unit of time" available on what that original audio signal was - and that was the important quality that made it the best audio signal - or at least much better than in our home systems.  Bigger was better.  The more information you have about an audio signal the higher the quality will be.

If this seems confusing, consider for a second that perhaps electronics is really not much different than us humans.  For example, if we are trying to learn what happened at an event with two witnesses, and one witness saw the event only briefly from a distance while wearing dark glasses, but the other witness saw the event clearly and much closer and for a much longer period of time, the second witness likely will describe the event much more clearly.  The second witness simply will have more information regarding the event, and thus their description may be much more complete and perhaps much more colorful.  More good information about a subject often leads to a better description of it afterwards.

To be sure we live in a complex world nowadays.  We know that there are schemes that also can improve audio signals somewhat.  Dolby systems, DBX systems, and more recently digital compression algorithms in MP3 and mini-disk recorders.  So, it can get confusing to guess which signal or system will give the best quality.  But if all else is equal, the "more information about that signal per unit of time" will always help yield best results.  So, for example, would we trust CD quality that stores 10 Megabytes of information per minute of song time to be better than the average MP3 quality level of 1 Megabyte per minute?  Yes, we would expect the CD system to sound better since it has 10 times more information available about how that song was originally played or sung.  Since MP3's use digital compression and algorithms, the CD will not be 10 times better.  On the other hand, 10 times more information held by the CD will not be equaled by the much smaller information standard MP3.  "More information per unit of time" is generally the winner.

note:  for those die-hard MP3 fans that are appalled by this, yes we agree that if one does compression only on MP3's to eliminate not-needed information, that the MP3 would in fact be smaller and would be the same quality as the CD.  However, we think that this compression is more like 3 to 1 and not 10 to 1 as most MP3's are recorded.  So the normal MP3 would have some compression of data which would not hurt the signal but also some signal quality that was thrown away to make it a smaller file.

At this time, it might be important to add that "one cannot go backwards" with signal quality.  Once the quality is lost for a signal, unless it is somehow reconstructed in a very complex manner, it is simply "lost forever".  So if we take a signal that is also poor and try to expand the information on it afterwards to 10 times as much information, there will be no change in quality. The original more complete information has already been lost.  Let us take our example of our two witnesses above.  If we take witness number one who only saw the event briefly in an unclear manner, whether that person writes a long report on the subject or a very short one it might not matter.  The more complete information was never there in the first place, so writing more and more about less and less does not work.

HDTV Video Improvement

What does this have to do with HDTV you might ask?  Well, standard Television signal, at least in the US, can also be called 480i.  The "i" in 480i stands for "interlaced".  This means that for the standard US style TV, that the TV set will have 480 lines going down the screen, and will interlace them - meaning that it will only show every other line (240 of the 480) during the first 1/60 of a second when it scans the screen; then it will "light up" the alternate line (other 240 lines) the next 1/60 of a second.  So every 1/30 of a second, one would see a full screen of 480 lines of TV picture.  Why is HDTV better?  Well HDTV can simply be called 1080i (a common HDTV version) - meaning that it will show you 1080 lines going down the TV screen instead of 480.  Again the "i" stands for "interlaced" meaning that on one scan every second line is "lit up" on the TV and on the next scan, the alternate lines between those are "lit up".  So - HDTV will be a better signal - since it gives us "more information per unit of time" about what the original video signal was.  Another way of saying this is that the signal simply allows more detail than the original TV signal - since it is simply "bigger" and carries more information.

There are also some other HDTV standards in addition to 1080i.  They are 720i and 720p.  Again, they offer more lines of resolution vertically on the TV screen than the standard 480i normal TV set picture.

TV is an Optical Illusion

If all of this scanning lines down a television set is disturbing to you, perhaps it is time to explain how TV (television) works at all.  Yes, the TV screen is scanned a line at a time top to bottom.  Worse yet, for a normal "interlaced" screen (the normal TV set) the TV set scans only every second line each time down the screen - and does the in-between lines the next pass.  So, how does all of this look like a stable moving picture?  The answer is that it does not.  TV itself is an optical illusion.  While there is much discussion on the capability of the human eye, somewhere around 30 frames a second - if there is "image blur" from a TV screen as TV's will do - is what the eye may view as fluid motion.  So the human eye is mostly concentrating on each "finished screen" of the TV scan rate and really not watching the individual scan going down the screen.  Because of this, television may be considered an optical illusion - one that uses the slowness and the manner in general of how the human eye works to make a complex very fast TV scan rate look like a normal moving picture.  One has to credit the inventors of the television, that they were able to capitalize on the way the human body - specifically how the eye works - to give us what has become a great media tool - television.  Of course at times we have all seen where that illusion breaks down - such as watching a wagon wheel in a "Western Cowboy" type movie that seems to be turning backwards.  That is because again, TV and movies are optical illusions and the TV or Movie Camera can both act as a "stroboscope" and catch images only at certain points.  That can give the illusion of going backwards.  The optical illusion breaks down at that point.

Progressive Scan

Those of us who have purchased DVD's of late have seen a feature called progressive scan.  Is that good we might ask?  You bet!  Normal TV signal progressive scan might be called 480p - where the "p" stands for progressive scan.  How does this work?  Well, at one time, most people thought the human eye could only detect 24 or 30 frames a second, and hence that is the speed for some motion pictures or television.  However, that is the speed at which we tend to see the illusion of "fluid motion".  A second question might be - would a better picture seem "clearer" or "more detailed" to us?  Yes!  Our eyes are in fact good enough to see more detail than 30 frames a second.  Recall that the normal TV picture is interlaced (the "i" in 480i) and that each other line is lit up going down the screen - and the alternate lines are lit up on the next pass.  With progressive scan (also called 480p at times - the "p" meaning "progressive scan") all of the lines of the TV screen are lit up each time the TV image is sent to the screen for display.  Our eyes detect that as a much more stable and detailed picture.

There are some dealers who sell progressive scan DVDs calling them simulated HDTV.  Can you see why?  Of course they operate differently.  But if a 480p progressive scan picture lights up twice as many lines as 1 normal TV picture of 480i, that 2 to 1 improvement may look similar to the eye as a 1080i signal that again has slightly more than twice the lines of a 480i line signal.  While 480p progressive scan is definitely not the same as a 1080i HDTV signal, it may look like almost similar quality to a person.

Which is Better or Best of the Video Signals?

One more time, we need to go back to what we said earlier.  The more information we have about an item - be it audio or video - the better quality it will be.  So, a 480i (interlaced) TV signal is the starting point and lowest quality.  480p (progressive scan) may look twice as good or twice as stable to some of us and will be a better looking signal since it "lights up" twice as many lines going down the TV screen.  720i will look better than 480i but perhaps not as detailed as 1080i.  1080i (HDTV interlaced) will have even more information and detail and will look like a still better picture.  As to where 720p would fit into this for quality, we will leave that one up to individual preference.

I am certain that I took some liberties along the way to try and simplify some of this.  But I do hope that the explanations here may benefit a few people like me that might have struggled trying to understand why one TV video signal might be better than another.

How to Tell What Signal Your TV is Getting?

This is one seems harder to tell than it should be.  I find that many manuals that come with HDTV products or progressive scan products are poorly written.  Even worse, the product itself may have its main feature HDTV or progressive scan turned off - in fear that it might be incompatible with your TV set.  Even worse still, is that I find that at least with some manufacturers, that if you email them you can still get bad or confusing or incomplete information.  What to do?  Some TV sets - such as mine - have an "info" button.  That "info" button when pressed tells you what signal the TV set is currently receiving - 480i, 480p or 1080i.  So, even if the manuals make no sense, one can play with product menus on receivers or DVD players until the TV says the signal is now what you want.  My TV set also indicates what type of signal you are getting when you change channels or some other controls.  Watch for the information and look for a button similar to an "info" button on your TV remote.  It might be quite helpful to you.

Another way to tell an HDTV signal:  At least at the time of this writing, both the HDTV satellite service I had plus the cable service were unable to put both the HDTV and the normal TV signal on the same channel number.  So, if you have both an HDTV receiver and a normal (non-HDTV) receiver - look for channels with the normal non-HDTV one.  HDTV channels may be simply a blank screen, but the audio will be there.  That may be because the signals are HDTV and a non-HDTV receiver cannot decode it.  Of course, if it is an HDTV channel, the HDTV receiver should pick up that channel.  But at least right now, I am seeing cable and satellite services that have channel "pairs" - where two channels will have the exact same program channels - but one channel number will have HDTV and the other normal non-HDTV.  For example, normal network CBS could be channel 2 - with no HDTV, and its HDTV pair might be at channel number 200.  Yet both would be running the same exact programs.  In this case, the HDTV receiver would decode channel 200 but the normal non-HDTV receiver might just show a blank screen with audio on channel 200, but work ok (no HDTV) on channel 2.

You "might" also be able on a large screen set to tell by just looking at the picture, but this is hard and may be deceiving.  On a large screen set, 480i (normal tv) may look "ok" but grainy.  Progressive scan (480p) will look much better than 480i, more stable of a picture, and perhaps also a glossy nice looking screen and seemingly better color.  HDTV (1080i, or 720p or 720i) will look good, as progressive scan does, but if you look closely, you will see more detail.  At least this is how the three formats look to me.  I of course prefer  HDTV 1080i.  But if one cannot have that, progressive scan is second best.

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