Digital TV DX

[gr8oldies]

Let me start by saying I know very little about digital TV, but I do know both tropo and e-skip TV DX has been reported.

I understand that once the transition to digital is complete, stations will "map" to a channel that may have been their previous analog channel or a completely different channel. So channel 41 may map to channel 7.

Would there be a way to defeat the "mapping" for DX purposes? Could I defeat my "mapped" channel 2 to get a "real" channel 2 during a DX opening?

Just curious how this is going to work...or not work.

 

[Matt]

It's my understanding that the rules for tuner manufacturers do not specify that they have to be PSIP (the scheme that allows the tuner to "map" to the old channel number) compliant. You'll likely find one of two scenarios develop...

1) The high-end tuners might allow you to turn off PSIP and tune to a "real" channel rather than a virtual one.

2) The real el-cheapo tuners might not be built with PSIP anyway.

Either way, DX-ing digital TV signals will be an entirely different animal from DX-ing their analog counterparts. Digital is not exactly an all-or-nothing arrangement, but it is pretty close to that. Either the signal is there in sufficient quantity for the detector to recognize and decode...or the detector will figure what it's seeing is just random static and ignore it.

In between those two extremes is a narrow window where you might get enough information for a frame or two, but unless your tuner has a rather complicated scheme called "forward error correction", you'll probably blink and miss that altogether.

Also, be VERY glad that most two-meter radio amateurs are careful about their second-order harmonics. One of those signals will make a digital TV receiver show things that no TV set should ever show. Ditto for police cruiser radios, but most of those either have or will soon move to the 800 MHz band, above UHF broadcasting.

Any way you look at it, TV is going to be a different animal come February of 2009.

Later....
Matt Smith
WGSR-TV....and planning for digital

 

[DeadElvis]

Would there be a way to defeat the "mapping" for DX purposes? Could I defeat my "mapped" channel 2 to get a "real" channel 2 during a DX opening?

Just curious how this is going to work...or not work.

I can defeat, to an extent. It's just a little cumbersome. On my TV, I would have to go into the "add channel" function, and manually input analog channel 2. Then, I am golden. When Nashville is in (which is not uncommon here in Memphis), I have to do that if I want to get the analog of the local channel 2, and the current channel selection does not have analog 2 (I might lose it if I run an automatic channel search, which I do quite a bit during a nice tropo opening). If a DTV signal is there, it would remap to the virtual channel.

Now, I am only passing TV DXer; when the bands open, I tend to be on the ham bands. But, that being said, I really enjoy DTV DXing, simply because it is so easy. If the station decodes, that's it; it's IDed via PSIP. None of this white-knuckle stuff waiting for an ID.

I'm in the minority, by I look forward to 2009.

DE

 

[BobOnTheJob]

I'm in the minority, by I look forward to 2009.

DE
[/quote]

I do too...mainly because the HDTV signals will be more reliable when the analog signals are not there. There are too many instances of HD and analog being a little over 110 miles apart on the same channel. The analog is full of snow & the HDTV won't lock because of analog interference. I've been watching HDTV for a year now & except for some missing signals because of co-channel interference, I like what I see.

That said, WHAS DT 55 Louisville used to come in great here at 70 miles...until WCLJ DT 54 at 12 miles went to full power. Now it's gone. I want that one back...anyone have any thoughts? The other signals from that antenna farm are perfect.

 

[DXER1]

BobOnTheJob You bring up some very good points on the digital signal being lost.
One good example is Dallas and South of Dallas
But be glad you don't live in Dallas of just south of Dallas, since WFAA's DTV is on CH9 and KCEN down in Waco managed to somehow end up on CH9 as well. I think that FCC screwed up on this one. And get this both X-mittters are at 90 miles of each other as the crow flies. SO they are a little to close for comfort.

 

[jd]

But be glad you don't live in Dallas of just south of Dallas, since WFAA's DTV is on CH9 and KCEN down in Waco managed to somehow end up on CH9 as well. I think that FCC screwed up on this one. And get this both X-mittters are at 90 miles of each other as the crow flies. SO they are a little to close for comfort.

I'm still amazed about that one, since there has to be a clash over the largest city in KCEN-DT's coverage area.
WFAA-DT coverage: www.fcc.gov/fcc-bin/FMTV-service-area?x=DT276585.html
KCEN-TV coverage: www.fcc.gov/fcc-bin/FMTV-service-area?x=DT614459.html

WFAA-DT was first to arrive, having filed their original application back in 1998. Four years after that KCEN filed for digital facilities on the same channel, so there was plenty of time to straighten it out.

 

[w9wi]

I understand that once the transition to digital is complete, stations will "map" to a channel that may have been their previous analog channel or a completely different channel. So channel 41 may map to channel 7.

Would there be a way to defeat the "mapping" for DX purposes? Could I defeat my "mapped" channel 2 to get a "real" channel 2 during a DX opening?

It's up to the receiver design.

At the minimum, you could go into the TV's setup menu and "rescan" the channels. Unfortunately if it has to rescan all 68 channels it's going to take awhile! (even after transition it's still going to scan channels 52-69 even though there won't be any stations there...)

Some receivers will allow you to scan a specific RF channel. You could tell it to rescan RF channel 2, which should only take a few seconds.

You could theoretically build a receiver that could be programmed to ignore the remapping data altogether. The Hauppauge WinTV-D card does; if you want it to remap you have to enter the remapping data yourself, manually. But I doubt there will be many sets sold with that ability.
_________________________________________________
(quoting Matt

In between those two extremes is a narrow window where you might get enough information for a frame or two, but unless your tuner has a rather complicated scheme called "forward error correction", you'll probably blink and miss that altogether.

Actually, all DTV tuners will have forward error correction. I think most viewers would find DTV reception uselessly unreliable without it.

I expect almost all DTV receivers to use a small number of mass-production chipsets. The difference between a cheap set and an expensive one will be mostly the quailty (and size!) of the display.

Also, be VERY glad that most two-meter radio amateurs are careful about their second-order harmonics. One of those signals will make a digital TV receiver show things that no TV set should ever show. Ditto for police cruiser radios, but most of those either have or will soon move to the 800 MHz band, above UHF broadcasting.

Actually the 2nd harmonic of the 2m ham radio band falls in the 216-400MHz military communications band. The 4th harmonic falls in TV channel 31. The 5th harmonic would fall in a channel above 51 that will be unoccupied (by TV) after transition. Ham interference to DTV would have the same effect as any other interference: if minor it would cause reception to break into blocks; if more serious it would stop reception altogether - you'd get a black screen.

Police harmonics from the "VHF-High" band could affect two UHF channels in a similar method to ham harmonics. Harmonics of UHF police radio (and as you say, 800MHz) would fall outside TV bands. VHF-Low would be the most troublesome this way, but there aren't many police agencies left down there.
_________________________________________________
(quoting jd)

WFAA-DT was first to arrive, having filed their original application back in 1998. Four years after that KCEN filed for digital facilities on the same channel, so there was plenty of time to straighten it out.

I think the theory is that because both stations are using less than the 160kw maximum for high-band VHF DTVs, their coverage areas don't overlap. I realize theory may not coincide with practice!

The problem goes away on 2/17/2009, when WFAA-DT returns to channel 8. (KCEN-DT stays put on channel 9)

 

[BobOnTheJob]

But be glad you don't live in Dallas of just south of Dallas, since WFAA's DTV is on CH9 and KCEN down in Waco managed to somehow end up on CH9 as well. I think that FCC screwed up on this one. And get this both X-mittters are at 90 miles of each other as the crow flies. SO they are a little to close for comfort.

I'm still amazed about that one, since there has to be a clash over the largest city in KCEN-DT's coverage area.
WFAA-DT coverage: www.fcc.gov/fcc-bin/FMTV-service-area?x=DT276585.html
KCEN-TV coverage: www.fcc.gov/fcc-bin/FMTV-service-area?x=DT614459.html

WFAA-DT was first to arrive, having filed their original application back in 1998. Four years after that KCEN filed for digital facilities on the same channel, so there was plenty of time to straighten it out.

I think I can top the WFAA/KCEN coverage overlap. Look at WKPC DT 17
http://www.fcc.gov/fcc-bin/FMTV-service-area?x=TV276655.html
Now look at WHAN LP 17
http://www.fcc.gov/fcc-bin/FMTV-service-area?x=CA203199.html
The ENTIRE WHAN LP service area is within the WKPC DT service area. And yes, they are both on the air & have been for some time. They say with a good antenna, you can see the LP 2-3 miles out. I used to see it at 57 miles.
The two sites are about 22 miles apart.

The FCC sometimes doesn't have the sense God gave a cement goose.

 

[DXER1]

But be glad you don't live in Dallas of just south of Dallas, since WFAA's DTV is on CH9 and KCEN down in Waco managed to somehow end up on CH9 as well. I think that FCC screwed up on this one. And get this both X-mittters are at 90 miles of each other as the crow flies. SO they are a little to close for comfort.

I'm still amazed about that one, since there has to be a clash over the largest city in KCEN-DT's coverage area.
WFAA-DT coverage: www.fcc.gov/fcc-bin/FMTV-service-area?x=DT276585.html
KCEN-TV coverage: www.fcc.gov/fcc-bin/FMTV-service-area?x=DT614459.html

WFAA-DT was first to arrive, having filed their original application back in 1998. Four years after that KCEN filed for digital facilities on the same channel, so there was plenty of time to straighten it out.

I think I can top the WFAA/KCEN coverage overlap. Look at WKPC DT 17
http://www.fcc.gov/fcc-bin/FMTV-service-area?x=TV276655.html
Now look at WHAN LP 17
http://www.fcc.gov/fcc-bin/FMTV-service-area?x=CA203199.html
The ENTIRE WHAN LP service area is within the WKPC DT service area. And yes, they are both on the air & have been for some time. They say with a good antenna, you can see the LP 2-3 miles out. I used to see it at 57 miles.
The two sites are about 22 miles apart.

The FCC sometimes doesn't have the sense God gave a cement goose.

I understand what you are trying to say, But
these stations are both Digital, not one analog and one digital as you are describing.
These signals give people in Hillsboro fits since they may want to watch WFAA out of Dallas but have to fight KCEN out of Waco's signal and vise versa. Hillsboro is the halfway point between WFAA and KCEN's tower sites. If you look at WFAA's signal contour, the edge of the signal touches the north side of Waco, which is about 20 or so miles from KCEN's transmitter.

 

[tomservo]

Let me start by saying I know very little about digital TV, but I do know both tropo and e-skip TV DX has been reported.

I understand that once the transition to digital is complete, stations will "map" to a channel that may have been their previous analog channel or a completely different channel. So channel 41 may map to channel 7.

Would there be a way to defeat the "mapping" for DX purposes? Could I defeat my "mapped" channel 2 to get a "real" channel 2 during a DX opening?

Just curious how this is going to work...or not work.

Here's something you can try that may work. Find out what the actual physical channel number is of a digital station you already have programmed. Enter that channel number, but add 2 to the sub-channel and see if the TV picks up the signal there too.

I've got a Sony projection set in the living room that works that way - discovered it completely by accident. Our local PBS channel is analog 18, digital 36. Of course the digital is mapped as 18.1 and 18.2... However, I can also tune the channel by inputting 36.3 or 36.4 (I dunno why it is shifted .2 up...)

Using that method I've gotten several Memphis stations from 90 miles out with just an indoor antenna, just by plugging through the numbers 15.3, 16.3, 17.3 etc...

Checking analog is easy since it's just "Channel Number" [enter], and checking for a digital signal is "Channel Number" [dot] "3" [enter].


Might be worth a shot? ???

 

[gr8oldies]

Thanks very much for the info. I was very curious about that. I take it all of the Americas aren't dropping analog on the same day as the US. What about Canada and Mexico?

 

[PhilJSmith67]

However, I can also tune the channel by inputting 36.3 or 36.4 (I dunno why it is shifted .2 up...)

Every stream within the multiplexed ATSC DTV signal has a Program ID (PID) associated with it. Hexadecimal (Base-16) is used for PID numbering, and each "program" is typically separated by blocks of 16 (conveniently 0x10 in hex). The A/53 spec says that the first DTV subchannel should start at 0x30, instead of 0x10. In the case of your local station on 36...

0x30 is the base PID which provides PID mapping for subchannel #1
0x31 is most likely the PID for the MPEG2 video
0x34 is most likely the PID for the primary AC3 audio channels (2.0 or 5.1 English)
0x35 is (optionally) the PID for the secondary AC3 audio channels (possibly 2.0 Spanish)

0x40 is the base PID which provides PID mapping for subchannel #2
0x41 subchannel #2 video
0x44 subchannel #2 primary audio
0x45 subchannal #2 secondary audio

Now you can start to see where 36.3 and 36.4 come from. What you're doing is telling the receiver to go to "RF channel 36" (the real 36) and tune to PID 3X or PID 4X. If 18 had another subchannel, it would most likely be on PIDs 0x50, 0x51, and 0x54.

There are a whole mess of other PIDs that can exist in the DTV data. One of those streams, which I believe is 0x1FFB, carries the PSIP "virtual channel" data. This PID gives your tuner the remapping info.

36.3 => 18.1
36.4 => 18.2

It's a little more complicated than that, but the basics are that when you do a "scan" on your TV, as it lands on 36 it memorizes the PSIP mapping so the next time you punch 18.1, it knows to internally tune to 36.3. Some sets don't accept the physical channel number once the PSIP one has been memorized, which is quite a P.I.T.A. for DXing, since a typical scan can take so long as mentioned in a prior post.

There are even "null" PIDs that do nothing but fill in unused bandwidth in the fixed-bitrate ATSC DTV stream.