View Full Version : How do you DX digital TV ?

John Wilke K9RZZ
03-02-2007, 01:54 AM
Seems like a dumb question, but the only experience I have with digital TV is off the Free To Air satellites and to find new stations there, you have to set up the receiver to scan, then come back later to see what it found. The receiver has a built in signal strength meter that is used to peak on the satellite, but you can't tell what's actually there until the scan is done.

How does it work over the airwaves?? :confused:

John K9RZZ

03-04-2007, 11:34 AM
I watch the tropo maps and check out the analogs on TV (or I check FM for signs of tropo or enhancement). Then I tune my DXable channels on the STB looking for something.

But after analog shutoff I won't have the analoges to use for references, so the task gets harder. I'll have to rely on the few open FM channels I have left for signs of tropo.

Robert Grant
03-04-2007, 05:32 PM
For tropo, it is similar to how you would DX analog tv.

You will need a TV set with a digital tuner (almost any set currently for sale in the USA, 27" or larger, has one, look for the letters "SDTV" or HDTV tuner built in" - NOT "HDTV ready" - on the box or the set itself), a set top box, of a digital tuner device for your set.
Plug the antenna of choice into the antenna jack (any antenna that worked well for analog TV will work fine for digital TV), and check for DX.
Find which "actual channels" your local DTV stations transmit on (go to fcc.gov, click on search near the top of the site, click on "TV station query" in the bar at the left, type your coordinates and the location boxes and "100" in the "find in km radius" box).
Note that the snow looks different - more intense, larger "flurries" than on a truly vacant channel - this is hypersnow, the signature of a DTV signal as seen on an analog TV.
In a tropo opening, look for channels that have hypersnow when tuning in analog. Switch to digital, and, if you have direct actual channel entry, tune that channel in digital mode.
Seeing hypersnow does not guarantee you will be able to receive the digital transmission, it has to be much stronger to receive in digital than it has to be for hypersnow to be seen in analog, it must be a little stronger than all other signals present on the channel, and it has to be relatively free from multipath.
Hence, what would qualify as "DX" in digtal TV may not be as unusual in analog. It is more of a challenge.

Good Luck, N8NU.

Robert Grant
03-04-2007, 05:34 PM
Note than when (and if) the analog transmission cease, that an analog tuner will still be very useful for revealing the presence of tropo, the presence of hypersnow on many channels will be obvious, even from stations you won't be able to identify.
Don't throw out your analog set!!


John Wilke K9RZZ
03-05-2007, 10:28 AM
So, you need an analog set to detect stations? It can't be done with the digital set?

Will the digital TV set 'decode' the distant station, or do you need to take a guess at who it is and set up the decode?

For example, on the satellites, even if I'm aimed right with the antenna, and on the right frequency, with the right polarization, I will see nothing without having the receiver figure out the bit rate, video number, and audio number of the signal (or know it ahead of time and enter it). For any given frequency and polarization off the same transponder, there might be a dozen different TV stations to be seen, so you've got to have the numbers right!

So, with over the air digital TV, I'm still not clear. Does the TV do the searching, or are you guessing who it is and punching in the decode numbers??


John K9RZZ

Robert Grant
03-05-2007, 09:03 PM
The answer may cause you MORE confusion.

It differs from one DTV reception device to another.

You do not necessarily need an analog set to detect the presence of a digital TV transmission. The analog set is merely a convenient way to do this, as (on many sets) you will notice the presence of a digital signal at a much lower signal level than with only a digital tuner, and an analog set can tune through the channels as fast as you can turn the channel knob (a digital tuner has to lock on to a DTV signal, which takes seconds, making even non-DX channel surfing more slow than it was in analog).

DTV channel mapping can be automatic, but is not quite as "automatic" as, say, a direcTV box.

My own experience with four Terrestrial ATSC receivers:

1) My own Sylvania 6427GFF: One can punch in any channel on which one suspects there is an ATSC signal. If there is no viable signal, the set gives up after several seconds and displays "NO SIGNAL", if there is a signal, this will produce the picture and sound of that station's main (.1) ATSC program, and will display the virtual channel of that station (e.g., you have this set straight out of the box, hook up antenna, and aim for the WDJT tower. You are in DTV mode, and press "4-6" on the remote, the set picks up WDJT-DT on that actual channel, David Letterman pops up on the screen, but your channel display suddenly reads "D58"). If you want to keep this channel in memory, you add it, and, from now on, you can get to them by entering either "46" or 58".
This set also has an automatic DTV search, and the option of deleting any channel from the memory. This is good, since without being able to add and delete from the memory, it would be hard to have two stations from different directions in the menu (e.g., in my case, WBGU-DT, actual channel 56, and WTVS-DT, virtual channel 56)

2) My RCA ATSC-11 set-top-box: One can punch in the actual channel of the ATSC station, but, this AUTOMATICALLY inserts that channel into memory, and it can only be removed by starting a new automatic channel search (during which it dumps ALL of the channels from memory, and takes more than a minute to scan).
One quirk about this one it that it will allow the memorization of more than one station with the same virtual channel. During one trop opening, it decoded both WDIV-DT (actual 45, virtual 4) and WCMH-DT (actual 14, virtual 4). Choosing channel four would give you WCMH 4.1, then pressing the up-channel button would give, in order, WCMH-WX 4.2, WDIV-HD 4.1, and WDIV-WX 4.2 (!).

3) My sister's Hitachi integrated tuner 51" CRT rear projection HDTV set:
Has automatic search on command, but also allows direct entry of station by actual channel as well as virtual channel. Like the RCA STB, selecting a channel automatically assigns it to memory, but, like the Sylvania, it allows deletion of individual channels from memory (if my memory serves me correctly).
Unlike either the Sylvania of the RCA STB, it does search for ATSC transmissions ON ITS OWN, and adds them to memory automatically.
One evening I notice trop was in, and was able to watch WJW-DT with their 720p newcast at about 115 miles (with an indoor 2-bay antenna). But the automatic memory revealed that the set had also managed to DX two other Cleveland ATSC stations, which were no longer detectable, by itself.
One wonders if this could be annoying to non-DXers, their set filling itself with channels that have no signal 90+% of the time.

4) Forget the brand of the product, I device about the size of a Fig Newton that plugs into the USB port on his Mac:
Depends on automatic channel scan, AFAIK, and has a menu to enter channels by frequency (this is in MHz, not by channel, 470.310, for example!), that did not seem to work too well.

As many of the newer sets are using automatic channel mapping, without any provision for manual entry, DXers with soldering irons may have to come to the forefront with a converter to convert any channel to a single RF channel to be fed to the DTV set, possibly a surplus TV tuner board, followed with an inverting upconverter to convert the surplus tuner IF frequency to a channel in the DTV receiver's range (3, for example). The DTV set's memory would only find a single ATSC signal on 3, and would then be used to decode whatever signal you have converted to 3.

good luck, N8NU

03-05-2007, 11:54 PM
So, you need an analog set to detect stations? It can't be done with the digital set?

Yes it can, but it's a PITA.

Will the digital TV set 'decode' the distant station, or do you need to take a guess at who it is and set up the decode?

The TV will decode the station, one way or another. You might like "how" it does the decoding or you might not. Depends on the box, and depends on what you prefer. You might have to buy different boxes before you find the one you like.

So, with over the air digital TV, I'm still not clear. Does the TV do the searching, or are you guessing who it is and punching in the decode numbers??

As Bob said, most don't but some do (the searching)..

With analog off the air and nothing left but DTV you'll have to rely on something to let you know when trop or Es is up. If you are waiting for Es on ch2, for example, you'll have to keep your DTV box set to ch2 (or 3,4,5,6) and stare at the TV until something decodes, or partially decodes, if it ever does decode. Exciting, isn't it?

In my opinion, a DTV or an STB needs to act like a police scanner. You punch in the channels you want, ignore your local channels and set the dang thing to auto-scan like a scanner. Then, when the box sees or finds something, it'll stop and try to decode. Then, when you're ready, you hit scan and the thing resumes scanning. How else can you DX DTV and keep your sanity??? Are you going to do a manual channel scan every five minutes? What happens if a station pops up between scans? You'll miss it.

Jeff Kadet does (or did) exceptionally well with DTV Dxing. The biggest reason is that Jeff used the Win TV-D card, which shows the DTV channels on their real channel, not remaped. If you don't program in your local channels and strong semis, you can shoot up and down the band (both U and V) looking at your good channels very fast. There's no auto scan, but you can come closer than with anything else.

How enjoyable is DTV DXing? I'll bet that most will tell you "not very". You might hear "I really like it" once in a while but the majority will do it and hold their noses. Some of the hard-core analog TV Dxers may just walk away. Like I said, it a PITA. Like flying blind. Black screen. Remapping. Maybe a few minutes (or seconds) of video out of a multi-hour Es opening. Totally different.

John Wilke K9RZZ
03-06-2007, 01:42 AM
Thanks for the informative replies.

I guess, now's the time to enjoy analog TV DXing while it's still here, because the world is going to change, and it will be gone forever! :(

John K9RZZ

Robert Grant
03-11-2007, 05:03 PM
Analog TV DX will not end in the USA on 2/27/09.

How will you get any analog stations when every analog transmitter in the USA has been shut off?

Easy. You will get analog DX from stations OUTSIDE the USA!

Canada and Mexico have no intention of analog shutdown anytime in the forseeable future (in fact, Canada might not be installing ATSC transmitters outside the largest markets).

Some other NTSC countries, due to political concerns (e.g., Cuba) might NEVER deploy an ATSC transmitter.

It will still be wise to check the lowband channels on an analog TV during the Es season.

(I have been planning to write an article about post-2009 TV DX for the VUD).

Robert Grant.