Mobile DTV on the cheap

[wb4jfi]

Sorry, long and boring follows.  Read only if insomnia has attacked, or 
you are a glutton for DTV information.

The DTV change had a few phases.  First, (step 1) before their DTV 
channel, WBAL was running analog TV on channel 11, a high-band VHF 
channel, probably 316KW ERP.  That gave them a certain coverage area, 
which was calculated by a computer program using the "Longely-Rice" 
(spelling?) model.  Very similar situation to W*USA(9) and WJLA(7) in 
DC.  All was good.

Step 2:
Then, the DTV gods assigned them a UHF channel for DTV transition 
operations.  The gods further called upon the great calculator 
(Longely-Rice) to come up with a ERP power level that would "replicate" 
the analog VHF channel coverage area on that new UHF transition 
channel.  Potential interference to other TV broadcasters was than 
calculated, and various power levels "tweaked" by the gods, to create as 
close to "replication" as possible.  This required substantial more ERP, 
since UHF arrows do not travel as far as high-band VHF arrows.  (I think 
W*USA was around 816kW ERP) Then, the gods rested.

Step 3:
WBAL, like many other TV stations, said, hmmmm, is there a higher ERP 
power level that we could transmit at on our new UHF DTV channel that 
would not cause more interference to others, but allow us to push our 
arrows out even further?  After consulting the great calculator, many 
stations found that they could transmit at higher levels, up to one 
MegaWatt ERP, without significant interference (or on occasion providing 
"compensation" to a potential interference receiving station).  This was 
thought good, and was called "maximization".  Many stations "maximized" 
their new UHF DTV assignments, to cover yet more peons.

Step 4.
Woe, there became a time when the gods wanted to kill off the old, 
analog systems, and recover valuable spectrum.  February 2009 was 
declared the time when all full-power analog TV transmitters were to 
stop.  Stations were given the choice of: 1) staying on their new DTV 
channel; 2) returning to their original analog channel (flash-cut); or 
3) humbly suggesting to their gods that another channel was better for 
all (supported by engineering evidence and run through the great 
calculator).  (I did all three)  The gods then calculated, fussed, and 
calculated some more, and lo, there was issued a decree that each 
station will operate upon the final assigned channel within that decree.

Step 5.
The gods' 535 bosses (in the building on the mount) decided that the 
peons had been too slow in obtaining adequate hardware, and directed 
that the gods "adjust" the day-of-analog-death until June 2009.  Many 
stations spent many more gold pieces to prepare for that death-day, 
especially if they had been granted their selection of option 2 or 3 
above.  WBAL made the decision to return to channel 11, and that was 
granted.  Yeah!, WBAL exclaimed, no longer will we need to feed the 
raveneous(sp) mouths of the two or three IOT transmit tubes, each of 
which puts out 30-50kW TPO of RF (at 30% efficiency?).  No more water 
cooling and glycol!  No more IOT tube suicide!  Instead, we can go back 
to the sanctified operation of solid-state devices on VHF, at only a few 
kW TPO!  With fewer peons now eating at our troughs, we need to save 
every gold coin that we can.  Short-term satisfaction prevailed upon the 
DTV land.

Step 6.
Oops, the new and godlike "8VSB ATSC DTV" scheme is not the same as 
analog, woe be us.  After the death-day of June 2009, some VHF stations 
find their arrows are NOT slinging as far as the old analog arrows did 
(or even their "temporary" UHF arrows), even on the same VHF frequency.  
Is it peak-to-average power requirements?  Is it the modulation scheme?  
Is it an RF "tax" on the digital signal?  Lo, maybe we can overcome this 
with more power!  And, the gods were asked to allow testing to find out 
the true nature of this miserable state of affairs, and if more power 
will overcome this new tax.  The potential of co-channel interference 
raises its ugly head once again, so careful testing is required.  Many 
said " More transmit power is ALWAYS better!!"  Alas, 'tis not always so.

There can be many contributing factors why a DTV signal is not decoded 
where analog would work fine.  Ghosting/multipath is still a potential 
problem, although the newer tuners/demods are MUCH better than earlier 
ones.  Sometimes more transmit power actually causes MORE problems in a 
multipath situation, causing stronger reflections that were not a 
problem before.

I think the bait-and-switch comment comes simply from WBAL having a 
"temporary" UHF DTV channel during the transition, which required 
dramatically more ERP to achieve analog signal "replication" (up to a 
MegaWatt).  At the end of the transition they selected, and were 
granted, a return to channel 11, which theoretically requires MUCH less 
transmit power to "replicate" their original analog coverage.  (btw, 
anyone who believes their 316kW ERP analog signal can be replicated with 
only a few tens of kW of DTV power on the same frequency seems a little 
naive).

The problem is that the 8VSB DTV signal does not always want to play 
nice and properly replicate with the calculated, supposedly equivalent, 
ERP.  Those stations (like WRC) which chose option 1 above (stay on the 
new DTV channel) for the most part knew what their coverage would be, as 
all they did on June 2009 was turn OFF their analog transmitter, and 
continue to use their new DTV facility just like during the rest of the 
transition period.  Stations like WBAL, W*USA, WJLA, had to rely on the 
great calculator to come up with a new - unproven - DTV ERP level that 
"should" replicate the previous analog coverage.  The great calculator 
seems to be a little off for many of them.

I'm not sure that there was much talk about "digital gain" as far as a 
transmit plant is concerned.  I don't remember very much discussion of 
"digital gain", but I retired before the end of the transition.  
"Digital" signals like 8VSB (really digital data encoded onto an ANALOG 
- RF - signal) do require a certain S/N level before they can be 
decoded.  But, they also require other factors as well, such as being 
able to find the pilot, and a hopefully flat, unvarying spectrum.  You 
can have a VERY STRONG DTV signal as far as pure RF level, and still not 
decode a single frame of video (or audio).  At my old home in Falls 
Church, I could literally see the tops of the towers and antennas in NW 
DC, but had a lot of problems decoding DTV.  Plenty of RF, as verified 
by the spectrum analyzer.  It was those darned suck-outs in the spectrum 
caused by multipath that killed decoding.  I could move the antenna one 
way, and get WRC, another way and get WETA and W*USA, yet another way 
and get WJLA.  I got to where I could predict successful decoding based 
on seeing the spectrum bumps/dips come and go.  Interestingly, W*USA and 
WJLA were NOT received at the same antenna pointing at my house, even 
though they were combined onto the exact same (wideband panel) transmit 
line and antenna, ran the exact same RF power, and were only a few 
channels apart (34 vs 39 if I remember).  I had the same problem when 
Gannett was in Rosslyn.  We could easily see all the transmit towers in 
NW.  But, decoding the DTV signals required careful antenna alignment.  
When we moved to Tysons, things were better.  Rosslyn had nearby 
buildings which caused bad reflections, while Tysons had fewer 
buildings, and was a straight shot and partly down the Potomac.

I've been out of it for four-1/2 years now, so some stuff has changed.  
But, the difference between VHF and UHF transmit powers required to 
"replicate" a station's coverage is basic, and still accurate.  VHF = 
lower ERP, UHF = higher ERP, for the same coverage area.

Chip, your comment about no station having higher power during the 
transition than after, unless VHF to UHF, appears backwards to me.  
WBAL, W*USA, WJLA all had higher power during the transition (on their 
UHF DTV channels), than post-transiton when they moved back to their VHF 
channels.  As did many other original analog VHF stations around the 
country.  If you can, borrow a spectrum analyzer and look at the RF to 
see if it is a level problem, or distortions in the overall signal 
spectrum.  If you could see a decent analog signal from W*USA and WJLA 
on channels 9 & 7, you should be able to decode their DTV signals as 
well, as far as signal level goes.  Or, have you done that already?  
It's a pain that a multi-thousand dollar piece of test equipment is 
required to troubleshoot basic television reception.  Ah, but digital is 
always better!

As to why the Best Buy gurus sold UHF-only DTV antennas, who trusts 
them?  I could easily stump almost all of them, even on my worst day.  
Most did not even understand the basic question: "What is the native 
resolution of this TV?"  I kept on asking anyone who was looking at OTA 
antennas: "What about the end of the transition when 7 & 9 go back to 7 
& 9?" Huh????  Here in Charleston, things are different.  I don't think 
there is a single full-power VHF DTV transmitter.  The ones that I am 
interest in are all UHF, and unfortunately far away from me.

Bill (and others), feel free to chime in and correct me where I erred..
Terry


On 1/19/2011 8:48 PM, Chip Fetrow wrote:
> No station had higher digital power during the transition than they 
> did in the end, unless they were on VHF and moved to UHF.
>
> I think the confusion comes from the old analog power, and the much 
> lower digital power.  "Digital gain" was supposed to allow for much 
> lower power, but now look at what is gong on.  Many, if not most, 
> stations have either applied for or already received power increases.  
> Still, they don't have the coverage they once did.  If they did, I 
> would have OTA TV at my house.
>
> --chip
>
> On Jan 17, 2011, at 9:26 AM, tacos-request at amrad.org wrote:
>
>> Message: 2
>> Date: Sun, 16 Jan 2011 14:06:17 -0500
>> From: Bob Bruhns <bbruhns at erols.com>
>> Subject: Re: Mobile DTV on the cheap
>>
>> I am still playing with a small VHF-UHF rotatable on an old lamp stand
>> in a back room in Herndon.  Before the switch I could get WBAL, but
>> since the switch, no.  I read that stations were running huge power on
>> digital before the switch, and they dropped way back afterward - like
>> 300 KW to 7 KW???   If so, that was bait and switch.
>> [...]
>>   Bob, WA3WDR
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