Question on AM HD

[W2JUV_AL]

An article in the current issue of Popular Communications on HD Radio (AM) states the power level of the digital sidebands to be 6dB below the analog carrier. The NRSC mask shows the digital sidebands to be 25dB (or more) below the unmodulated analog carrier. Whose right?

 

[ve3jf]

An article in the current issue of Popular Communications on HD Radio (AM) states the power level of the digital sidebands to be 6dB below the analog carrier. The NRSC mask shows the digital sidebands to be 25dB (or more) below the unmodulated analog carrier. Whose right?

Neither one. The digital power in the AM system is about 12 dB below the analog carrier. For more details about the spectrum and power levels of the AM IBOC signal, see http://topazdesigns.com/iboc/AM-IBOC-Parameters.html

 

[Play Freebird]

This raises an interesting question: What is the ITU Emission Designator for a AM station (or for that matter, an FM station) transmitting digital carriers?

In particular, what would be considered the "necessary" or "occupied" bandwidth? Keep in mind the generally accepted definition of occupied bandwidth is the range containing 99% of the total integrated power of the transmitted spectrum.

With FM IBOC (as the rules are currently written), the total power of both the lower and upper digital sidebands cannot exceed 1% of the station's nominal power. This takes advantage of a "loophole", in that the remaining 99% is the analog FM signal which already fits within the assigned 200 kHz channel, so it can be argued (from a legal standpoint) that the addition of the digital signal does not increase a station's bandwidth.

However, if FM digital sideband power is increased 10 dB as proposed, those digital carriers must be included in the total integrated power, causing occupied bandwidth to double to approximately 400 kHz. Would this violate any international treaties?

 

[CW]

This raises an interesting question: What is the ITU Emission Designator for a AM station (or for that matter, an FM station) transmitting digital carriers?

In particular, what would be considered the "necessary" or "occupied" bandwidth? Keep in mind the generally accepted definition of occupied bandwidth is the range containing 99% of the total integrated power of the transmitted spectrum.

With FM IBOC (as the rules are currently written), the total power of both the lower and upper digital sidebands cannot exceed 1% of the station's nominal power. This takes advantage of a "loophole", in that the remaining 99% is the analog FM signal which already fits within the assigned 200 kHz channel, so it can be argued (from a legal standpoint) that the addition of the digital signal does not increase a station's bandwidth.

However, if FM digital sideband power is increased 10 dB as proposed, those digital carriers must be included in the total integrated power, causing occupied bandwidth to double to approximately 400 kHz. Would this violate any international treaties?

The accepted mask for AM or FM broadcast is done by the country's licensing body...in our wonderful case, the Freakin' FCC (Peter TV rules!)...The IBOC mask is designed to fit within the channel allocation...raising the power level on FM would not cause it to go outside the mask...it would merely raise the upper and lower digital sidebands 10db....BUT they would still have to fit within the 200kHz channel mask......( if you look at the FM IBOC mask, it is basically a straight up and down wall on the sides at the channel limits...AM is another matter becuase some of the digital info rides under the AM analog carrier and are limited to +/- 15 kHz as noted above..and the digital sidebands are noise on analog AM; raising its power level would raise the noise floor of the analog signal; thus degrading it further; as if 5 kHz audio isnt enough...on FM, the digital is outside the analog mask iirc....thus raising its level should NOT, in theory, cause any problems with the primary analog...MAY cause more noise on 1st adj but then thats not a worry since they arent allocated that close, right? <sarcastic mode off?>)
As for the ITU designators, I am not sure...would have to look that up......(really hadnt thought about it anyway...since IBOC is a hybrid system)

 

[Play Freebird]

The IBOC mask is designed to fit within the channel allocation...raising the power level on FM would not cause it to go outside the mask...it would merely raise the upper and lower digital sidebands 10db....BUT they would still have to fit within the 200kHz channel mask......( if you look at the FM IBOC mask, it is basically a straight up and down wall on the sides at the channel limits.

Here's the FCC rule governing emissions of FM stations:


§ 73.317 FM transmission system requirements.

(b) Any emission appearing on a frequency removed from the carrier by between 120 kHz and 240 kHz inclusive must be attenuated at least 25 dB below the level of the unmodulated carrier. Compliance with this requirement will be deemed to show the occupied bandwidth to be 240 kHz or less.

But if digital power is increased from -20 dB to -10 dB, relative to the FM carrier, each of of the two digital sidebands would be attenuated only 13 dB, assuming a literal interpretation of the rule (energy within the entire "inclusive" span from 120 to 240 kHz removed from the carrier must be measured.) This clearly exceeds the legal limit by 12 dB.

A spectrum analyzer with resolution bandwidth set to 1 kHz will, of course, indicate a much lower digital power level in comparison with an unmodulated FM carrier, because only a fraction of the total sideband power gets through the bandpass filter as the analyzer sweeps. But how many FM receivers have 1 kHz filters? The person trying to listen to a first-adjacent station is probably using a receiver with an IF bandwidth of 150 kHz or more, which allows almost all of the digital sideband energy through.

So when proponents of the 10 dB power increase claim "it easily meets the emission mask requirements", we must look a little deeper to see that it really doesn't.

 

[Savage]

Big freakin' shock. It's another example of how IBOC proponents - notably Glynn Walden, the Alliance and the NAB in connivance with the FCC, stack the deck and concoct non-real-world technical benchmarks so they can assert bogus claims that HD Radio "doesn't interfere."

It's like how they claim that HD-AM's digital noise is -28dbc by referencing only one of 25 COFDM digital carriers. Unfortunately your typical AM receiver doesn't obligingly sort one out of the bunch, it detects all 25 in the sideband and duly delivers it to your speaker as deafening noise.

Lies, lies, lies. If the pharamceutical industry used the testing standards IBOC-pushers do, half of the world would be dying of drug-induced cancer and infections.

 

[audiophile.]

This raises an interesting question: What is the ITU Emission Designator for a AM station (or for that matter, an FM station) transmitting digital carriers?

In particular, what would be considered the "necessary" or "occupied" bandwidth? Keep in mind the generally accepted definition of occupied bandwidth is the range containing 99% of the total integrated power of the transmitted spectrum.

With FM IBOC (as the rules are currently written), the total power of both the lower and upper digital sidebands cannot exceed 1% of the station's nominal power. This takes advantage of a "loophole", in that the remaining 99% is the analog FM signal which already fits within the assigned 200 kHz channel, so it can be argued (from a legal standpoint) that the addition of the digital signal does not increase a station's bandwidth.

However, if FM digital sideband power is increased 10 dB as proposed, those digital carriers must be included in the total integrated power, causing occupied bandwidth to double to approximately 400 kHz. Would this violate any international treaties?

I'm pretty sure Canada and Mexico would object...

 

[JohnnyElectron]

audiophile wrote: "...I'm pretty sure Canada and Mexico would object..."

And I wrote: I don't know why they- Canada - don't object. Anybody with a half-way decent (or not so decent I.F. filtered) radio within 90 miles of the US/Canada border can tell you how your well-liked Canadian AM stations are obliterated by our IBOC noise 20KHz away -

oh yeah, I forgot, I'm 50 miles away from my station in Canada and I'm not supposed to be listening that far away - right?

Actually, the only way I can still hear my station in Canada is using my HD radio, as it has it's own HD 'self-interference' filter in the rig, so I don't hear 90% of the AM hash from the IBOC AM station 20KHz away, and CAN actually hear my AM station again. FM - at current IBOC levels - is not as bad of a problem yet.