For listeners unfamiliar with the hybrid digital HD radio terminology...

[iyiyi]

This is not correct, on several counts. The ATSC DTV signal most certainly has a carrier associated with it, and at 4300 feet from my local DTV stations, I can testify that it is absolutely possible to overload an amplifier, creating images and intermodulation products and front-end receiver overload.

One of my locals is on RF 45. When it's on, and when I'm using an amplifier at my rooftop antenna, a signal in a neighboring market on RF 44 will not decode under any circumstances. Turn off my local 45 and 44 comes in like a local. Remove the amp and use a sufficiently directional antenna, and 44 decodes just fine.

With DTV, the important thing isn't raw signal strength at the receiver - it's the signal-to-noise ratio. Many inexpensive amps (and some expensive ones) have such high noise figures that they actually do more damage to the signal than good. If you're at a great distance from the desired signals, a low-noise amp is likely to help. If you're close enough to an antenna farm that an FM trap is needed, you likely have enough RF coming in that an amp is of questionable value, especially "cranked."

It is true that a powerful nearby FM signal can also overload an amplifier, and that the use of a trap can reduce this. But it's also perfectly possible to use a combo antenna to receive FM and VHF DTV. I use a CM Quantum 1110 for just that purpose, and it works just as well (at least on the signals remaining on VHF, which are fewer) in the digital era as it did in the analog days. I don't run an amp on the VHF antenna, and I trap my local FMs at the receiver end (using Microwave Filter in-line traps) when I'm using that antenna for TV.

I find, at my location close to a tower farm, that the most critical factor is directionality. Being able to get a reasonable null on the locals - FM, analog TV or DTV - makes an enormous difference across the rest of the dial, which is why one of my summer projects will be replacing the CM 4248 corner reflector I'm using for UHF with a 7' parabolic.

There is no carrier on an ATSC DTV signal. There IS however, plenty of RF in your area. Certainly enough to overload many, if not most, mast mounted preamps. This RF is the digital modulation. Although ATSC emits no carrier, the digital modulation of the ATSC signal is most definitely RF and can swamp a nearby RF amp.

Second paragraph (45's effect on 44 and your solution) is 100% correct.

Third paragraph is 100% correct. However, many- if not most -people aren't subject to those RF conditions.

Fourth paragraph is 100% correct.

Fifth paragraph is also 100% correct. My experience tells me that installing the Channel Master 4251 will not quite yield the results you are hoping for. It is a monster to install, requires a very sturdy mounting structure, looks pretentious and/or goofy, and looking at TV Fool's signal rosette, it doesn't seem to provide any extra signals to justify the expense and goon work involved. AFIK, 49 in Buffalo provides the same programming as 44, plus 49 has a local newscast. I suggest a Winegard HD 9095P, HD 9032 or an AntennaCraft MXU-59. For RF amp I suggest the Antennacraft 10G201 (good overload performance), 10G202 (high gain) or the 10G212. The 10G212 has a remote gain control that can knock off 10dB in overload situations and also a remote switchable FM trap. The 10G212 unfortunately has a NF of 4, compared to the 202's 2.6 and the 201's 2.5 NF.

I currently run a MXU-59 at 30 feet AGL with the 10G201 amp. I have a new 10G202 amp that I will eventually get off of my ass and install. Maybe I'll be able to get a fourth ION channel, an NBC shop at home channel and a Daystar channel when I do. Whoopee! It will be interesting to see how the 10G202 will work with trop this summer, but the 10G201 is currently doing all right. Stick with a corner reflector yagi for good directivity. Sometimes less is more.

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[fybush]

There is no carrier on an ATSC DTV signal.

I refer you to pages 133 and 148-150 of the NAB Engineering Handbook, 10th edition. The carrier in ATSC DTV takes the form of a pilot frequency, nominally 309.440 kHz above the lower channel boundary.

Fifth paragraph is also 100% correct. My experience tells me that installing the Channel Master 4251 will not quite yield the results you are hoping for. It is a monster to install, requires a very sturdy mounting structure, looks pretentious and/or goofy, and looking at TV Fool's signal rosette, it doesn't seem to provide any extra signals to justify the expense and goon work involved.

All true, to an extent - but a renovation a few years ago provided me with the ability to construct just such a sturdy mounting structure, and I'm fortunate to have neighbors who aren't bothered by some shiny aluminum on the roof. Oh, and I'm also fortunate enough to have a 4251 sitting disassembled in my basement waiting to go up, for free.

I've been fortunate here in having the ability to do some empirical testing courtesy of a fellow DXer at almost exactly the same distance from the TV towers. He has had much more success with his 4251 than I've had with the 4248, enough to convince me to at least give it a whirl and see what it does.

I like the idea of the 10G212 - I will have to give that a try.

 

[iyiyi]

309.440kHz is not a VHF or UHF carrier frequency. It is the nominal frequency of the location of a 2 microsecond "synch pulse". There is no carrier transmitted on an ATSC signal. A good analogy would be to call the 19kHz pilot of a stereo FM station the "carrier". Your pal with the 4251 may live the same distance from 45 as you. Might want to do a TV Fool rosette of his and your locations. It may be possible that your azimuth looks through the antenna farm where his antenna faces away from it when viewing the same station. The 4248 is a little suburban pop gun of an antenna. I suggest that you try a larger corner reflector yagi

 

[w9wi]

309.440kHz is not a VHF or UHF carrier frequency. It is the nominal frequency of the location of a 2 microsecond "synch pulse". There is no carrier transmitted on an ATSC signal. A good analogy would be to call the 19kHz pilot of a stereo FM station the "carrier".

Scott is correct on this.

See page 5 of http://www.8vsb.com/doc/harris-8vsb.pdf.

The first “helper” signal is the ATSC pilot. Just before modulation, a small DC shift is applied to
the 8-level baseband signal (which was previously centered about zero volts with no DC
component). This causes a small residual carrier to appear at the zero frequency (unmodulated
carrier) point of the resulting modulated spectrum. This is the ATSC pilot. This gives the RF PLL
circuits in the DTV receiver something to lock onto that is independent of the transmitted data.
Although similar in nature, the ATSC pilot is much smaller than the NTSC visual carrier,
consuming only 0.3 dB or 7 percent of the transmitted power.

_________________________________________________

I guess you could argue that the ATSC pilot is not a "carrier" in the sense of the carrier on an AM radio signal, or an analog TV transmission. The ATSC pilot is far too weak to perform that function. You could argue that its function is indeed closer to that of the 19KHz pilot in FM stereo, in that it is not used to directly demodulate the signal, but to lock receiver circuitry that does the demodulation.

However, the term "carrier" is universally used for these suppressed carriers. (even when they're suppressed completely!, as is commonly the case in amateur radio) That, and the 19KHz FM pilot is not on the same frequency as the suppressed carrier would occupy. The DTV pilot *is* on the carrier frequency.

I can assure you the pilot is continuous. I've listened to the pilots on a few dozen ATSC stations in several markets (using a general coverage VHF/UHF receiver with BFO) and it is in fact a continuous, "dead air" signal.

 

[iyiyi]

"A continuous "dead air" signal". Priceless!

I was wrong by comparing the ATSC "synch pulse" to the 19kHz FM stereo pilot. The FM pilot is a continuous signal. The correct analogy is to compare the ATSC pulse to the 3.58mHz color burst located on the back porch of a NTSC horizontal synch pulse. There is no carrier sent in the chroma of NTSC. An 8 to 10 cycle sample of 3.58 mHz synchronizes an oscillator that provides a carrier to demodulate the chroma signal. The 19kHz FM stereo pilot is continuous and is easy to simply double to create the necessary 38kHz carrier to demod the L-R info.

ATSC is "old school" technology. The "pilot pulse" was originally used to synch both frequency and phase of the PLL. Problems with that scheme caused them to use the "pilot pulse" just for phase correlation. Now a "pilotless" ATSC scheme has been proposed using the field synch pulses in the ATSC data stream to lock the PLL.

This current ATSC "pilot" is 2 microseconds in length, not continuous like the 19kHz FM pilot signal. There is no carrier sent with an ATSC signal; just like there is no carrier sent with a NTSC chroma signal.

To bring the thread back OT, there is absolutely no "pilot" (or carrier) sent with MW or FM HD radio.

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[w9wi]

This current ATSC "pilot" is 2 microseconds in length, not continuous like the 19kHz FM pilot signal. There is no carrier sent with an ATSC signal; just like there is no carrier sent with a NTSC chroma signal.

Can you point me to a document showing that? Everything I've read (including the linked document, written by an engineer at a company that makes transmitters) indicates the ATSC pilot is in fact continuous. What's the repitition rate? (can't spell that word today.....) Everything I hear when I listen to these signals indicates it's continuous.

To bring the thread back OT, there is absolutely no "pilot" (or carrier) sent with MW or FM HD radio.

HD Radio uses a COFDM system which, arguably, consists of a large number of carriers. (I don't recall offhand how many HD Radio uses, but numbers in the thousands are common in other uses of COFDM)

But yes, you certainly wouldn't recognize any of the COFDM carriers as a "carrier" in the sense of a regular analog AM/FM transmission.

 

[iyiyi]

I believe the ATSC pilot occurs at the beginning of each new data field, just before modulation begins.

I was wrong again. Hybrid HD radio DOES use carriers. Not for the digital signal itself, but to cancel interference that the digital data would cause to the host analog signal.

I agree that it is a real bitch finding technical info on ATSC TV and HD radio. It is an arcane technology that is very difficult for a common man like myself to comprehend.

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[fybush]

I agree that it is a real bitch finding technical info on ATSC TV and HD radio. It is an arcane technology that is very difficult for a common man like myself to comprehend.

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Even if you don't want to drop 150 bucks on a copy of the NAB Engineering Handbook (understandable!), there is a remarkably large chunk of the book available for free preview via Google Books, including nearly all of the very comprehensive technical description of ATSC DTV that begins on page 133. A Google search of "ATSC DTV carrier" should get you within shouting distance of the relevant pages. It's a very worthwhile read.

 

[jhardis]

HD Radio uses a COFDM system which, arguably, consists of a large number of carriers. (I don't recall offhand how many HD Radio uses, but numbers in the thousands are common in other uses of COFDM)

But yes, you certainly wouldn't recognize any of the COFDM carriers as a "carrier" in the sense of a regular analog AM/FM transmission.

IBOC radio has 382 subcarriers in "hybrid" mode, and up to 534 subcarriers in "extended hybrid" mode.

However, there still is a "carrier" in the traditional sense. This frequency domain description of the signal is all well and good, but in reality the signal is still created by time domain modulation, albeit at high frequencies (100's of kHz), and albeit with sufficient amplitude to leave little or no power at the carrier frequency. In NRSC-5, in 1011 (for FM) and 1012 (for AM), look at the chapters entitled, "OFDM Signal Generation."

- Jonathan