Introducing Digital Television
Starting in 2005 the U.S. mandated that every maker of large screen TV sets include compatibility for Digital Television (DTV) broadcasts and starting in 2006 or 2007 all sets of any size will have these features as DTV is due to take over full time by the year 2012.
So, what is DTV, why is it so important and how is it different from HDTV?
The first thing we need to do is refresh our memories about how TV and video works, which we covered in our first Digital Video and Television special back in 2001 (feel free to click on that link and read the technicalities).
Your current TV is technically know as analog TV and you can compare this to an audio cassette or old phonograph record, complete with hiss, pops, glitches and skips.
The TV signal is made by scanning a still picture into a series of alternating lines, a process technically known by the term interlace (represented by the letter “i”). American TV is 525i and European TV is 625i and since Europe may not be going digital for a while this will be our only reference to the vastly different systems found in Europe!
Above odd scan lines, then even scan lines. Persistence of vision blends these interlaced images into a fused, single picture.
The screen sizes are the same, but the US breaks up the image into 525 scan lines – think of a photograph if you will. Cut that photograph from left to right 525 times and then piece it back together and that is how your TV image is made, except we send that image in two groupings. One grouping is all the even lines (0 to 524) and the other grouping is all the odd numbered lines (1 to 525). Of these lines only 480 are actually visible on your TV screen, the rest are behind the mask or stretched around the edges of the picture tube.
To send this image to your home TV set they have to reduce the quality or resolution (sharpness) of the picture from what the original TV camera captured. The original signal was anywhere from 400 to 1,200 on the resolution scale (known as lines of resolution, which are not the same as those 525 scan lines). What gets broadcast is exactly 330 lines in the U.S. (Europe gets closer to 400 lines). They also cut down the color to a bare minimum and send it out behind the main signal as an afterthought, which is why you can see color shows on a black and white TV set (yes, there were once only sets that saw black and white – your mom, dad and their pet dinosaur used to watch TV this way)!
Now they put all those two groups of scan lines in a long row and send out the exact video signal over the airwaves via a radio transmitter. The problem with this idea is that your TV set must reconstruct that picture from this long amount of signal information that must be total and complete. If an airplane crosses the path of the signal and part of it gets broken up your set has a nervous breakdown, flickers, rolls, bends and you get a visual glitch. If you are too far from the station you get little signal and a lot of snow, maybe no color.
Above is a representation of a NTSC color broadcast composite signal with Luminance (Y), the big jagged wave, sync, the smaller wave under the Luma, the audio at the very bottom, the large, even carrier wave and the smaller, even sub-carrier, plus the phase-angled Cr and Cb components that pace the lower sub-carrier by only a small amount, as they are close in frequency to this wave...
That is how analog TV works and it has worked quite well since the 1950’s.
Some DTV and HDTV sets can easily display images are interlaced (i) with every other lines being scanned in different frames (60 times a second to make 30 pictures of full motion image) or do progressive (p) scanning. Some DVD players provide progressive scanning. The ABC TV network uses progressive scanning for their HDTV broadcasts.
The concept of solid scan lines was required for black and white sets from the 50’s, but with the advent of color TV in the early 1960’s solid scan lines became overkill because these new color sets uses little dots to form the image instead of a solid screen face. Color TV sets make use of the concept of both rows (scan lines) and columns to create a matrix of dots or boxes that when viewed from a few feet away form a solid image. Pictures in newspapers and magazines are made this way. Go take a look. Little dots form the image on those pictures. But, to be compatible with black and white set the entire scan line information is sent, when in reality only about 50% of that was now required for making an image under the color set dot matrix concept.
Now for digital TV (DTV) they take that same picture and compress it into a smaller size by removing the redundant information. Sky, for example. Blue sky. If half the screen is blue sky why include information for all that area? Just include a few samples of blue and some information to set the boundaries as to where one shade goes, then where another shade goes. This way you cut the amount of picture information almost in half!
DTV takes advantage of the color set dot matrix, of which a single screen area (made up of three little blobs of red, green and blue colors) known as a picture element or pixel.
The visible picture area of a home TV set is 640 by 400 pixels (but they actually use 720 by 480 image which is cropped down by your TV set to 640 x 400) which is known in computer circles as S-VGA and was used by the IBM AT machines and the original Windows operating system. We now, of course, use a much bigger matrix for computers 800 x 600 or larger.
Now if you own a digital camera for still pictures you probably are aware of the term mega pixel. Today the average consumer knows enough to get a 3 or 4 mega pixel camera for still work.
Analog and DTV is less than a mega pixel! To be exact 1/3 of a mega pixel or 0.34 MP! It takes only 340,000 pixels to generate a TV image. On the standard home TV there are about 1 million color dots or squares made in triplets of red, green and blue (RGB). Each triplet constitutes a pixel. The intensity of the RGB colors creates close to 17 million different hues when viewed at a distance. This is full color. It is how your computer has been working since the 1990s. This is known, technically, as 16 bit color.
So they take samples of color, samples of brightness, remove excessive pixels that repeat and give information on where all these colors go to make a picture. That is the compression process and it is the same process used by your digital still camera or computer in the JPEG picture image.
Images can be stored in a variety of formats, including “raw” of which Targa and BMP are examples. A picture taken with a 2 mega pixel camera and stored in a raw format can have a size of more than 10 megabytes (10,000,000 bytes of information). This takes up a lot of space so they use a compression format like JPEG (jpg) or GIF to cut down the information and size of the file. So this same image takes up only 0.75 mega bytes (750,000 bytes of information) in a JPEG format and even less in a GIF format. These are your internet picture formats. What you lose is image quality. The resolution drops and you pick up which is called “artifacts” which is “junk” in solid colors (dark spots) and tinge of colors or smearing between color regions (a little pink from the faces of people bleeding into the blue sky or a little blue from the sky bleeding into the hair and edge of the faces).
When viewed at a distance your eye can’t see these blemishes.
So, with a digital image we now have a much smaller, more compact set of information that doesn’t have to be totally complete to form a picture on the receiving end. We turn all this information into a stream of numbers and then broadcast that stream as a series of dots and dashes over radio waves – like Morse code of the 1800’s!
The radio waves are set to a different frequency than those of most HDTV or analog TV sets, so you need a special tuner to receive the DTV signal. It is this device that is now being included in all sets over 30” and by the end of 2006 all sets of any size will have a DTV tuner. You might be able to buy a set top tuner that can be connected to your cable or antenna inputs so those without DTV tuners might be able to get DTV signals...
On your receiving end the stream of digital data about the picture is processed by a little computer that can compensate for missing elements caused by that airplane. What you might see is a picture that breaks up into little boxes for a split second, but no flicker, roll or bending of the set with that horrible “buzz” sound like you get with analog TV.
Also, with DTV you either get the signal loud, bright, sharp and clear or you don’t get it at all. There is no static ghost images with no color like you get in fringe areas.
For all intents and purposes DTV is basically a broadcast version of Direct TV, which is also a DTV signal, but sent on private frequencies and scrambled, which requires a box to unscramble. We can compare DTV to an audio CD or DVD disk, which is loud, clear, brilliant and sharp with no hiss.
Broadcast DTV will not be scrambled and anyone with a simple DTV tuner can get the signals.
The signals take up less space than analog TV signals (which are going to be re-sold for other uses such as for police, fire and maybe even telephone services after the year 2015). Some TV stations are cutting their HDTV allocations into smaller segments to broadcast multiple DTV signals.
That’s the difference between HDTV and DTV. DTV is based on the same analog picture content of 340,000 pixels with a viewing format of 640 by 400, while HDTV is based on a matrix of 2 million pixels in a wide screen format having a picture size of 1920 by 1080 using 6 million RGB dots to form the 2 million pixels.
So, once again DTV and analog TV are both 1/3 of a mega pixel (0.34 MP) while HDTV is 2 mega pixels.
In the case of both DTV and HDTV the receiving process is much the same, except the channels or frequencies used for each channel are different. As mentioned above some stations are cutting their HDTV allocation into multiple DTV broadcasts, so obviously you can’t see those multiple broadcasts using an HDTV tuner. You have to have a DTV tuner to find the exact area where the signals are located!
Once your tuner finds either a DTV or HDTV image it is processed by a little computer that reads the data stream and reconstructs the image based on the compressed bit map information putting colors into the right area until the full color image is “rendered” in memory. At that point the TV set then does the work of any TV set! It turns this image into a series of scanned lines and paints the picture across the face of the TV set just like an analog TV set!
Some sets may not do this, so an ABC TV HDTV broadcast will get received as progressive by the computer inside the HDTV set makes the image by converting it to interlaced, which probably doesn’t mean a bag of beans.
So, analog TV is a 640 x 400 image made up of 525 interlaced scan lines with a sharpness rating of 330.
Analog VHS video (traditional home consumer format) is 640 x 400 image made up of 525 interlaced scan lines with a sharpness rating of 240.
Analog Hi-8 video is a 640 x 400 image made up of 525 interlaced scan lines with a sharpness rating of 400.
DVD is a 640 x 400 image made up of JPEG/MPEG file compressions that is converted to 525 scan lines that may be interlaced or can also be sent out as progressive, with a sharpness rating of 480.
DV (digital home video) is a 640 x 400 image made up of AVI file compressions that is converted to 525 interlaced scan lines with a sharpness rating of 530.
A modern TV set is often digital, may have both a digital and analog tuner, may accept both progressive and interlaced scans and displays a picture that is 640 x 400 with a sharpness rating of 500+.
A DTV image is 640 x 480 made up of JPEG/MPEG file compressions broadcast over the air by radio waves and having a sharpness rating of 500 or better. It is converted by a TV set and turned into 525 scan lines that can be either interlaced or progressive with a sharpness rating of 500 or better.
An HDTV set may or may not receive DTV signals, but generally will receive both HDTV and analog TV signals. A true HDTV set has a matrix of 1920 x 1080 with 2 million picture elements (there are some low cost HDTV sets with a 1280 x 1080 matrix which far less pixels) and has a sharpness rating of 700 or more. Most can receive both 1080i and 720p broadcasts of HDTV shows, plus regular analog TV. Some also get DTV signals. Starting in 2006 all TV set will have both DTV and analog TV tuners.
Some DTV sets may also have tuners that allow you to receive HDTV broadcast, but you will not get the full screen size nor sharpness of a true HDTV set.
DTV is basically twice as sharp as analog tv and about half as sharp as HDTV. It has the clarity, color and stability of HDTV, but without the extra sharpness nor the wide screen and 5 track surround sound.
Our 2005 Video Special continues with these offerings::
Video Special 2005 |
Intro To Digital TV |
DVD or DV Camcorders?
High Definition DVDs |
Future DV Mediums |
Aiptek 5900 MP4 Camera
New HDV Technology from CineForm and RedRock Micro
Three Low Cost DVD Camcorders
64 Bit Computer Technology |
New Fall 2005 US Prime Time TV Shows
Our video special continues with these offerings from 2004:
Camcorders 2004 |
Capture Cards 2004 |
Digital Theatrical Movies |
JVC HD Camcorder |
Sigma SD9 and Foveon Technology
Color Imaging Technology |
2004 Network Prime Time TV Shows
From our archives with these offerings from 2003:
Hi-Def TV |
Promote Ur Videos |
Future of DVD |
Computer DVD Burners
New US TV Shows for 2003-2004 Prime Time
From our archives we have these articles from the 2002 Issues:
Doing Video On Your Computer |
The Pinnacle Capture Card |
ATA Hard Drives
Hard Drive Terms |
Western Digital Drives |
Producing A Scripted iMac Video
Audio For Video |
Lighting For Video |
Digital VHS |
Removable Hard Drives
From September 2001 Issues:
Buying a Camcorder | Producing A Cable Access Show |
Producing Broadcast TV
A Technical Look and History of Film, Video and TV |
Stream Video and Webcasting