DTV on Channels 2-6 Won't Work - Why?

[landtuna]

As an old 1960's-era navy radioman I thought I had a pretty good understanding of radio frequency attributes but recently I've read in several threads about the problems associated with digital TV in RF 2-6. Digital is new to me but I can't think of a reason why this would be so.

Can anyone explain this?

 

[fybush]

Three quick reasons why it's problematic:

1. Low-band V is extremely susceptible to electrical noise (he says, as he watches the NAB nightlight video from 60 miles away on WGRZ-TV 2 through a haze of electrical noise), and that sort of noise wreaks havoc on DTV decoding.

2. Low-band V is extremely susceptible to E-skip (he says, as he watches WPBT-2 from Miami, 1100 miles away, breaking through WGRZ), which of course rips up reception of local stations and wreaks havoc on DTV decoding.

3. Channel 2 = 54-60 MHz = wavelength of 5.5 meters, give or take = big, big antenna needed for clean reception, something increasingly unwelcome in today's homes. (He says, gazing outside at the enormous VHF antenna on his roof that's bringing in channel 2s from Buffalo and Miami...)

 

[KR4BD]

Here are some reasons...

1. The FCC has drastically reduced the amount of power DTV stations can run. For VHF stations, this reduction is very obvious.

2. Most of the DTV stations operate on UHF, requiring smaller antennas for good reception. Most indoor "DTV antennas" being pushed on the public right now are tuned for UHF ONLY. Stations using VHF channels are all finding even their local viewers can't see them because they are using UHF (and not VHF) antennas to receive their signals. If you are VERY CLOSE to a VHF station, any antenna will work, however.

3. Many of the converter boxes needed to be retuned after the June 12 switch because many channels switched from temporary channel assignments to their permanent ones. For some, rescanning the box is an easy fix, but for others, they must be manually retuned which may require someone with a little more than casual knowledge to do.

4. I am convinced that the FCC made a huge mistake when they used a dual channel system during the transistion. Many VHF stations used UHF channels (and vica-versa) for their DTV signals until they were allowed to return to their original channel with much less power than they had before.. Some folks just don't understand that their local channel 27, which has used that channel for 50 years, is now on Channel 13 with a DTV signal but still calls itself channel 27! Very confusing for many!

 

[oldiesfan6479]

(He says, gazing outside at the enormous VHF antenna on his roof that's bringing in channel 2s from Buffalo and Miami...)

Scott--

If you didn't have an enormous rooftop antenna, we'd throw you out of the
Radio/TV Geek Club.

 

[fybush]

4. I am convinced that the FCC made a huge mistake when they used a dual channel system during the transistion. Many VHF stations used UHF channels (and vica-versa) for their DTV signals until they were allowed to return to their original channel with much less power than they had before.. Some folks just don't understand that their local channel 27, which has used that channel for 50 years, is now on Channel 13 with a DTV signal but still calls itself channel 27! Very confusing for many!

How would you have done it better, keeping in mind that most of the very old, established stations on 2-6 (and many on 7-13) had to move up to UHF for technical reasons, and that UHF stations on 52-69 had to be displaced?

 

[KeithE4]

As an old 1960's-era navy radioman I thought I had a pretty good understanding of radio frequency attributes but recently I've read in several threads about the problems associated with digital TV in RF 2-6. Digital is new to me but I can't think of a reason why this would be so.

Can anyone explain this?

My guess is bit-error-rate (BER) issues due to the increased noise, both atmospheric and man-made, on the lower frequencies. If there are too many data bits that cannot be decoded by the tuner/converter box, then the picture will start pixellating. The coding scheme (Viterbi?) used by the digital TV system can correct for a lot, but it's not perfect.

But if 100 kW ERP transmitters were allowed on VHF 2-6, I'd guess that we'd have much less of a problem. Digital is still better down in the noise than analog with the same power level. But we don't have the same power level anymore (20 kW now?).

 

[landtuna]

Three quick reasons why it's problematic:

1. Low-band V is extremely susceptible to electrical noise (he says, as he watches the NAB nightlight video from 60 miles away on WGRZ-TV 2 through a haze of electrical noise), and that sort of noise wreaks havoc on DTV decoding.

I knew about the lo-band noise but didn't realize it impacted decoding that much. DTV reception still seems to be somewhat magical and needs a sorcerer more than Joe the TV tech.

2. Low-band V is extremely susceptible to E-skip (he says, as he watches WPBT-2 from Miami, 1100 miles away, breaking through WGRZ), which of course rips up reception of local stations and wreaks havoc on DTV decoding.

I've never practiced DXing TV so didn't realize the skip was all that severe. I've never been able to receive analog stations 120 miles south (with transmitters at 9000 ft ASL) let alone 1,100 miles away.

3. Channel 2 = 54-60 MHz = wavelength of 5.5 meters, give or take = big, big antenna needed for clean reception, something increasingly unwelcome in today's homes. (He says, gazing outside at the enormous VHF antenna on his roof that's bringing in channel 2s from Buffalo and Miami...)

AHA! When DTV came to pass I took that huge VHF/UHF outdoor antenna I had stored away years ago and plopped it on top of my back porch. It feeds five TV's through an antiquated coax network in "this old house" pretty well. Still can't get Miami though.....

Sorry I missed your pass through Tempe at our famous hot dog place. Could have been a great training session. ;D Maybe one day I'll pass through my old digs near Long Pond.

 

[landtuna]

Some folks just don't understand that their local channel 27, which has used that channel for 50 years, is now on Channel 13 with a DTV signal but still calls itself channel 27! Very confusing for many!

We had a very spirited discussion on this topic some months ago. The tech side lost out to the branding side. I see their point but continue to believe the actual RF "channel" designation should be used. Although only one of my four converter box brands display the RF channel while scanning.

 

[fybush]

I knew about the lo-band noise but didn't realize it impacted decoding that much. DTV reception still seems to be somewhat magical and needs a sorcerer more than Joe the TV tech.

It's 1951 all over again - we're learning almost from scratch about how much power is needed at the transmitter end, where to put the antenna at the receiving end, what kind of receiver to use and what the effects of propagation are.

Kind of exciting, if you're in the right mindset about it...

I've never practiced DXing TV so didn't realize the skip was all that severe. I've never been able to receive analog stations 120 miles south (with transmitters at 9000 ft ASL) let alone 1,100 miles away.

E-skip is a different beast - it works best at distances of about 900-1300 miles, and usually not at all on closer signals. And because of the angle at which the skip reflections come in, it can be disrupted by mountainous terrain, or so I've been given to understand. It's certainly not as common out your way as it is back east.

Sorry I missed your pass through Tempe at our famous hot dog place. Could have been a great training session. ;D Maybe one day I'll pass through my old digs near Long Pond.

The door's always open - just look for the big antenna on the roof!!