Would this ever be possible for digital (prob not HD) AM radio?

[pianoplayer88key]

I was wondering about a couple things. (Btw, HD/IBOC is NOT at all what I expected digital to be.) Assuming we're comparing analog to digital, the power & antenna for the analog and digital transmitter are the same, and the antenna and sensitivity/selectivity/etc specs for the receiver are also identical (and let's assume the analog receiver has very poor selectivity, so a weak signal about 6dB above the noise, or maybe 5kW 100 miles away near the upper end of the band over poor ground, or an IBOC that's too weak to have the hash heard at all on an analog radio, will still be audible +/-200-300kHz or so from its assigned channel)... would the following criteria be possible to achieve?

Digital does NOT interfere AT ALL with analog. For example, you could be at the 614.39V/m contour of the digital station (fCC specifies 614.4V/m max public exposure up to 1300kHz in one of the rules - 1.1310 or somewhere in there IIRC), and be able to detect a weak co-channel analog signal that's at or slightly below atmospheric noise level, whether it's voice, CW, QRSS CW, etc, maybe a F/S of 0.5 microvolts/meter. Also, if there's no analog signal at that location, then a field strength meter that's not digital compatible would either show 0, or whatever atmospheric noise happens to be.
Multiple digital signals could co-exist co-channel with each other, and the user could choose which they wanted to hear. (Maybe this could "clean up" the graveyard channels?)
If it has EVER been possible, assuming NO interference, to detect a trace of an analog carrier at a particular location (for example, barely detect the presence of a QRSS CW carrier, even though it's too weak even for a 60-year seasoned ham whose hearing is still as good as it was when s/he was a teenager to be able to identify it as such - they can only tell it exists, nothing more), you are GUARANTEED to hear it in full quality digital, even with extreme interference tht may be 180dB or more stronger co-channel.
The digital signal could have much better fidelity (and not have the sound you get when you use too low of a bitrate for the frequency response), frequency response, etc, and still take a tiny fraction of the bandwidth compared to analog AM. Also the rolloff off-frequency past the -3dB zone would be quite pronounced. For example, two signals with an analog frequency response to 20kHz, 2kHz RF bandwidth (+/-1kHz), and the same "ID code" (or whatever would be used to separate co-channel programs) could co-exist at 1002 and 1004kHz broadcasting from the same antenna.

From what I can tell, HD / IBOC can't do ANY of that. Is there any chance that any digital system could possibly achieve those criteria? If not exactly, how close could we come to that? Or were my expectations when I first started hearing of the idea of digital radio (even possibly before anyone at iBiquity even knew what IBOC was) a bit too much?

 

[radioskeptic]

“[W]ere my expectations when I first started hearing of the idea of digital radio (even possibly before anyone at iBiquity even knew what IBOC was) a bit too much?”

The short answer is “Yes!” The long answer is, “No, your hopes for digital radio are much more than “a bit” too high. They’re totally unrealistic!”

 

[w9wi]

I was wondering about a couple things. (Btw, HD/IBOC is NOT at all what I expected digital to be.) Assuming we're comparing analog to digital, the power & antenna for the analog and digital transmitter are the same, and the antenna and sensitivity/selectivity/etc specs for the receiver are also identical (and let's assume the analog receiver has very poor selectivity, so a weak signal about 6dB above the noise, or maybe 5kW 100 miles away near the upper end of the band over poor ground, or an IBOC that's too weak to have the hash heard at all on an analog radio, will still be audible +/-200-300kHz or so from its assigned channel)... would the following criteria be possible to achieve?

Never say never, as sometimes digital technology has accomplished amazing things, but I think your scenario is VERY unlikely to be possible.

The 600V/m thing would probably be impossible to measure... shortcomings of any possible *receiver* would render reception of weak signals on nearby frequencies impossible, even if the *transmitter* was 100% clean & radiating absolutely nothing on the weak station's frequency. But I suppose that really wasn't, fundamentally, your point.

I would suggest that with spread-spectrum transmission across a fairly wide band, you *could* accomplish many of your criteria. Basically, the station's frequency constantly changes in a pseudo-random sequence. ("pseudo"-random, because the receiver must be able to generate the same sequence to know where to find the transmitter at any given instant) Spread transmission across 200KHz or more of band, and the amount of time the digital transmitter would spend in any given analog station's channel would be so brief as to be essentially undetectable. You could have a couple of additional digital stations jumping around the same 200KHz of band; at any given instant, the chances of two of them landing on the same frequency at the same time would be tiny. A small amount of error correction would mask the problems. Across this much band, transmission standards could be set that would allow high-fidelity transmission.

It would work fine with something like two or three stations. It begins to fall apart quickly as you add more digital stations to the spectrum. With one spread-spectrum station in a 200KHz bandwidth, the chances of that station being in any given analog station's bandpass at any given instant are 1:20. With 20 spread-spectrum stations in the 200KHz bandwidth, the chances of at least one of the twenty being in an analog station's bandpass are essentially certain.

The other issue would be coming up with an antenna that would work efficiently across such a wide bandwidth. 200KHz is 20% of carrier frequency at the center of the AM band. Efficient antennas that provide a reasonable match to the transmitter output across this wide a band are difficult to accomplish. (I suppose a discone would do it, but for 600KHz that would be a rather bizarre antenna & quite unpopular with the neighbors!) IMHO directional operation would be out of the question.

I don't think you're going to be able to accomplish your criteria for reception of an extremely weak signal, at least not in real time. QRSS CW works because the VERY low symbol rate allows the use of a VERY narrow bandwidth filter. The two go together. You can get away with the use of QRSS techniques in amateur radio, because the amount of data being transmitted is miniscule. Often the call letters (no more than six bytes) are all the station wants to transmit. You *could* transmit, for example, the latest Lady GaGa song using QRSS techniques. But you'd be talking about something on the order of 3 megabytes of data; (assuming MP3 compression) it might take a year or two to transmit!

 

[JohnnyElectron]

I only see a system with a dozen synchronous transmitters in the same city providing decent digital coverage - all on AM, same frequency, or all the AM's in town each diplex their neighbors same channel digital only signal?

 

[Tom Wells]

I only see a system with a dozen synchronous transmitters in the same city providing decent digital coverage - all on AM, same frequency, or all the AM's in town each diplex their neighbors same channel digital only signal?

In a system of free competetive capitalism, this won't work. Requires too much cooperation.
Too much blind faith required on the part of the station. We feed our audio into "some port somewhere" and trust that it's gonna end up
on a multiplexed 10 channel digital feeder? Who is responsible for keeping THAT up and going?

The spread spectrum idea works well for a single user system like the US government, but is not an efficient or dependable concept for broadcast due to the problems with synching of decode detection in so many receivers and the increasing possiblity of signals stepping on each other as more signals enter an area.

Unfortunately the NATURE of digital is exactly the opposite of analog, power vs bandwidth distribution.
A fully modulated single note in AM analog is 2 sidebands with 99% of the power, and wee little bit of carrier left.
All the power is in the sidebands, and occurs at SINGLE frequency points, which can be sliced out razor-thin as you like with proper filtering,
which is pretty much how radio teletype worked, except there were two frequencies for the two data states, but it was
a truly digital bit mode and it simply stops and starts like anything digital. It doesn't work, then it does, then it doesn't.
In turning on and off current flow abruptly as is necessarily done in digital modes, the actual power of this modulation type is
not contained in a single or two or three specific frequencies. It is spread out over a very wide range.
That is why the click such as from a light switch in AM reception does not need to be tuned, but can be heard all over the dial, it IS all over the dial. The other clicks which also behave like this are any power switch, any switched-power device, auto ignition, arc discharge
on bad insulators, and all the switching on and off of ibiquity's sidebands, there is lot of wideband noise energy present now in MW.
Digital is no more or no less impacted by noise in reception than in analog, it just hasn't the benefit of intelligence to interpret.

In order for digital to work at all in MW, would require getting the noisemakers silenced.
I really had to work and twist the antenna loop on my Accurian and had a hard time getting HD to lock on AM at 10 -30 miles
of all the Chicago HDs. Even if analog stopped today, digtial only in MW has it's own built in limiting factor, the hiss itself.

There would still be the wide-band power ouput products unless very careful waveform shape control were implemented.
This means more sidebands are needed, (more resolution, bandwidth, or both), or we have to SLOW that data down to keep the
"corners" from being so abrupt as to produce the wideband products.
Without this, the system would eventually raise the noise floor to point where it would outhiss its own ability to detect data states,
and then we're back to how much redundancy is required...if any, then we must forgo some data ( quality) or inject "effects" of data compression.

I recall seeing a proof done at Valpo Tech where (and I forget whether it referred to Nyquist or who) bandwidth vs amount of data was
compared, and it was "proven" to us that it there is no free lunch, if you need 25 khz of bandwidth, it doesn't matter how you slice it up or use it..you need 25khz of bandwidth. You can only fit so much in the bag, regardless of what you put in there or how it may be sliced up.