If the reason AM is dying is because the sound quality is poor, and AM HD is too flawed to "save" the band, could AM stations use frequency modulation on AM? There's a narrow-band FM mode that uses 3 kilohertz of bandwidth and rejects interference like regular FM, in fact NBFM is used on FM subcarriers.
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Could we use frequency modulation in the current AM band?
- Thread starter nd2023
- Start date
It was used in the 1950's in amatuer radio. At some pont it was no longer permitted.
I suspect it would work on AM, but the narrow deviation would not provide very good audio, nor as much immunity from noise as
we might like. Wide band deviation is not likely to work at all without redesign of AM IF sections to handle wider bandwidth.
I suspect it would work on AM, but the narrow deviation would not provide very good audio, nor as much immunity from noise as
we might like. Wide band deviation is not likely to work at all without redesign of AM IF sections to handle wider bandwidth.
Tom Wells said:
It's still legal* but nearly never used. For awhile in the 40s/50s narrow-band FM was touted as a way of avoiding interference issues on cheap radios.
Wideband FM occupies too much space. Using the same standards in use in the FM band, a single station on 970 would occupy from 870-1070KHz. I suspect most IBOC opponents would be asking to bring IBOC back
* "(1) No angle-modulated emission may have a modulation index greater than 1 at the highest modulation frequency. " IIRC the modulation index for FM broadcasts is 5.
This was proposed decades ago, but the narrow bandwidth and long wavelengths associated with the Mediumwave band reduces or eliminates most of the advantages associated with FM, including interference rejection. Indeed, FM is not really used for anything at all below 25 MHz around the world. Europe switched their 27 MHz CB radio band from AM to NBFM in the '80s, but that's about it.
As has been pointed out in many threads here and elsewhere, there is little that is inherent in amplitude modulation that makes its audio quality poor. Rather, you should attribute AMs' poor audio quality to the receiver manufacturers, who insist on building the lowest-cost receivers. On a good receiver, such as the GE (later RCA) Super Radio III (and earlier versions of the Super Radio) in wideband mode, AM sounds as good as most FM, except for AM's electrical interference, which has already been discussed in this thread. The Super Radio is no longer manufactured and mine is no longer functional, but it was not a super-expensive product; you could often find new units on sale on the Web in the $50 area.
I live in a unique territory. Old Appalachian traditions meet Metro city slicker. There is an all-you-can-eat buffet serving what appears to be old time, back country, yee-haw food in grandmother's style. And the pricing of the lunch time special is outstanding.
A big part of the food is one big watered down mess of unknown ingredients. But I can select just enough known and recognized items to get my $6.00 worth.
That is a great picture of today's radio receivers. Years ago... when FM broadcasting was still a novelty found here and there, I studied the characteristics and concepts of the FM process. If you had a receiver built the way the originators of FM broadcasting thought they should be built (think of that as the $27 buffet lunch somewhere down in Atlanta) listening would be so much better. What we have available at retail today are receivers that do not even reach the standards equal to my borderline $6 country lunch.
If it were practical to convert the 540-1690 Khz band to frequency modulation technology, I can already see the receivers that would be come to the market place. It would be something like Toys-R-Us meets Big Lots. Welcome to the lunch with un-defined chicken parts.
A big part of the food is one big watered down mess of unknown ingredients. But I can select just enough known and recognized items to get my $6.00 worth.
That is a great picture of today's radio receivers. Years ago... when FM broadcasting was still a novelty found here and there, I studied the characteristics and concepts of the FM process. If you had a receiver built the way the originators of FM broadcasting thought they should be built (think of that as the $27 buffet lunch somewhere down in Atlanta) listening would be so much better. What we have available at retail today are receivers that do not even reach the standards equal to my borderline $6 country lunch.
If it were practical to convert the 540-1690 Khz band to frequency modulation technology, I can already see the receivers that would be come to the market place. It would be something like Toys-R-Us meets Big Lots. Welcome to the lunch with un-defined chicken parts.
GRC, I think I may know that very restaurant. About ten miles from some state reservoir in the Georgia Mountains. I'm not too familiar with the area, but it seems like it was 20 miles or so from Ellijay on SR 52?
The problem with building a decent AM radio receiver now (for me) is IBOC. If you build a completely wideband radio, to 10 or even 12 kHz, you will get IBOC hash from WBBM while trying to listen to the analog. If you build a completely narrowband radio, to 4 kHz or so, you will get rid of the IBOC hash, but you'll miss out on the superb audio that BobOnTheJob can produce on WIFE. There's not much middle ground: a radio built to 7kHz bandwidth would have only marginally better audio on WIFE and noticeably worse audio on WBBM.
A radio with both wide- and narrow-band modes would be the ideal, but I've not seen one of those in decades, perhaps since the last superradio I saw.
The problem with building a decent AM radio receiver now (for me) is IBOC. If you build a completely wideband radio, to 10 or even 12 kHz, you will get IBOC hash from WBBM while trying to listen to the analog. If you build a completely narrowband radio, to 4 kHz or so, you will get rid of the IBOC hash, but you'll miss out on the superb audio that BobOnTheJob can produce on WIFE. There's not much middle ground: a radio built to 7kHz bandwidth would have only marginally better audio on WIFE and noticeably worse audio on WBBM.
A radio with both wide- and narrow-band modes would be the ideal, but I've not seen one of those in decades, perhaps since the last superradio I saw.
The Grundig S350 (available at Radio Shack) has a wide/narrow bandwidth switch, and in wide mode, you will hear the full 10 kHz NRSC response on AM. However, its perpetually inaccurate "analog tuning with digital readout" annoyed me to such an extent that I returned mine.PTBoardOp94 said:
PTBoardOp94 said:
No, I think I have been to the one you are describing once and both operators have probably received their training from the same "cullinary institute".
The longer I think about the comparison, the more "on target" it seems. We could corner the owner of some of these restaurants as ask them: "There is something slightly perverted and borderline disgusting about some of the foods here. Why do you do that? Is that the way yo' Mama cooked? Is that the best you know?" And with a big of anger the owner might reply: If you knew the problem and expense of hiring decent cooks you would buy your food already prepared in an automated factory, too.
And if we corner the owner of a radio station, he too will explain the problems of competition and the advantage of programming cooked in a factory somewhere.
And if we corner the retailers and ask why they don't have better quality, better designed receivers, they would cry on our shoulder about the problems of being dependent on a stinky kitchen somewhere.
About five years ago I set about to buy a receiver, hopefully in a mid-range price, and realized there are basically NO decent receivers on the market. I have been away from broadcasting and as I began to catch up on the industry I found out the FCC has mandated the NRSC mask or something of that nature. Most stations are forbidden from sending a full spectrum audio signal. Why would any manufacturer and/or retailer of "audio signal catching devices" build a model for catching signals that cannot be broadcast?
Goat Rodeo Cowboy said:
IIRC the latest version of NRSC permits fixed audio pre-emphasis with an upper frequency limit of about 9.5 kHz. Obviously that bandwidth cannot be used if the station transmits "HD" as well as analog. The pre-emphasis was intended to compensate for the sideband rolloff of the r-f filters in the receiver.
So if receiver manufacturers wished to, they could design/market an AM broadcast receiver with an audio output that was fairly flat to about 9.5 kHz. That is not the equal of what FM broadcast receivers can do, but it would sound a lot better than the typical, present-day AM receiver with 3 or 4 kHz audio bandwidth.
If the 19 kHz r-f bandwidth of such an "NRSC AM receiver" could be narrowed by a user control to provide 9 kHz r-f bandwidth (4.5 kHz audio bandwidth) for weak signals, and/or to reduce interference from adjacent channels using HD, and from nighttime adjacent channel skywave, then that should noticeably reduce most of the operational problems associated now with AM broadcasting -- except for SCR dimmers, LED traffic lights, lightning crashes, switching power supply radiation, DSL "carriers," arcing insulators on utility poles etc etc, and which affect FM broadcasts much less if at all.
Most likely the trend to narrowband AM radios started with the auto radio OEMs, who wanted to minimize their complaints from auto dealers about the grief of new car buyers when hearing adjacent channel interference if the receiver r-f bandwidth was greater than 7 - 8 kHz (3.5 - 4 kHz audio response).
There is a least one manufacturer of bandwidth-switchable AM receivers. Recently I bought a Tecsun PL-310, which is small, DSP-based receiver (LW/MW/SW/FM stereo). It was cheap, except for the shipping from Hong Kong. But it is switchable to AM bandwidths of 6, 4, 3, 2 and 1 kHz. I doubt that it implements NRSC, though (the IB doesn't say). It has very good sensitivity and selectivity. I can listen to the Chicago Class A AMs with it in the daytime over a 230-mile path, even though summer isn't far away.
Below is a link to an MP3 file starting with 6 kHz BW, and switching down through 4, 3, 2 and 1 kHz bandwidths every 2-3 seconds. The link is a bit of a hassle to use, but for anyone interested...
http://www.filefactory.com/file/b186585/n/AM_Clips_at_5_Audio_Bandwidths_mp3
RF
PS: I have no connection whatsoever with Tecsun.
PPS: The PL-310 also has a readout of signal strength in "dBu," and an indication of audio S/N. The dBu reading is not accurately showing dBµV/m, though, that's for sure.
K
kenglish
Guest
What we need is to resurrect the Denon TU-680NAB "NAB Supertuner", add a circuit that detects HD Radio and automatically sets the filters to null it out, and sell them to all us folks who still want a good radio.
And, how about a Car-radio version, too?
And, how about a Car-radio version, too?
kenglish said:
Apparently this is "the marketplace at work". If they built such a radio and brought it to market, and sell them to all us folks who still want a good radio... how many would they actually be able to sell?
It is not unusual for me to begin searching for some nifty electronic device that I am so sure has to be available only to find that apparently there is me and maybe 193 other people in the whole US who want one.
kenglish said:
GRC says that number is 194. I bet it's closer to 197.
Nick said:
The short answer is that it would be a disaster because of FM's susceptibility to multipath distortion, which the 530-1600 kHz band would create via multiple skywave paths, skywave/groundwave interference, and re-radiation from structures. In the RF domain, multipath is linear distortion, which the FM detector transforms into nonlinear distortion by the very nature of FM detection. AM, on the other hand, is an intrinsically linear modulation process that is sometimes received on a truly linear detector (a product detector or so-called "synchronous" detector) and sometimes on a nonlinear detector (an envelope detector). If an AM radio uses a linear detector, then multipath creates only linear distortion (comb filtering and other frequency response artifacts) but not the nasty-sounding added spectral components that we associate with "multipath distortion" on FM.
Bob Orban
rorban said:
I had thought that multiplication (as in amplitude modulation) was considered to be an inherently nonlinear process. I thought that the proof of this was that if you multiply any time-domain signal by itself, you obtain a quadratic function, which is not linear (the graph of y=x^2 is a parabola). I now take it that I was wrong. Comments?
DanStrassberg said:
If you look at the modulation sidebands (which contain all of the information), AM modulation is linear because scaling and superposition both hold. Scaling: when you apply twice the modulation amplitude, the amplitude of the resulting sidebands goes up by 2x but nothing else changes. Superposition: If you modulate with a sum of frequency x and frequency y, the sidebands produced by this modulation are the same sidebands that you would get if you modulated first with frequency x and then frequency y and then added these sidebands together.
"What about negative overmodulation?", you might ask. Carrier pinch-off is a limitation of the technology of most high-powered transmitters where they no longer produce ideal AM. In ideal AM, instead of pinching off, the carrier would just flip polarity and the modulation would continue to be linear. In the limiting case, there is no carrier at all and you have pure double-sideband suppressed carrier modulation. In all cases of ideal AM, the carrier is just a constant term added to the modulating signal before this signal is multiplied by the carrier.
Compare this with FM sidebands, which are described by Bessel functions and where neither scaling nor superposition hold. In an ideal transmission system, the FM detector's nonlinearity is the exact inverse of the modulator's nonlinearity, so you get the modulated signal back without distortion. However, any linear distortion in the FM transmission path (including multipath and limited IF bandwidth) will cause nonlinear distortion at the FM detector's output because the loss of some of the information in the FM signal means that the FM detector can no longer exactly cancel the non linearity in the modulator.
Bob Orban
Still, if you amplitude modulate a purely sinusoidal RF carrier with a purely sinusoidal audio signal (and do not exceed--or even quite equal 100% modulation on negative peaks), the frequency spectrum of the modulated RF carrier includes components at the sum and difference frequencies. No such components exist in the spectra of the separate RF and audio signals. Doesn't this suggest that a nonlinear operation has taken place? Also, the most straightforward (note that I said straightforward--no intention to pass judgment on practicality in real-world situations) purely analog technique for mixing the audio and RF signals to produce a modulated RF carrier is the use of a transconductance multiplier, which the baseband analog folks consider to be the quintessential NONLINEAR active-circuit element. No question that FM is--if you will--MORE nonlinear than AM, but is AM PURELY a linear operation?
It has been more than half a century since I was an EE undergrad, but I vaguely recall being told that AM could not be implemented using purely linear circuit techniques. For sure it can't be implemented using purely passive circuit elements (in Prof Guillemin's terminology, LLFPB--lumped, linear, finite, passive, and bilateral). In practical terms, that came down to resistors, capacitors, and inductors without ferromagnetic cores. Electrolytic capacitors (and ultracapacitors, which didn't exist back then), should also be eliminated.
It has been more than half a century since I was an EE undergrad, but I vaguely recall being told that AM could not be implemented using purely linear circuit techniques. For sure it can't be implemented using purely passive circuit elements (in Prof Guillemin's terminology, LLFPB--lumped, linear, finite, passive, and bilateral). In practical terms, that came down to resistors, capacitors, and inductors without ferromagnetic cores. Electrolytic capacitors (and ultracapacitors, which didn't exist back then), should also be eliminated.
It's been a few years for me, but I recall that the term linear is used here because we attempt to make the
important information to be mixed intersect upon the >most linear< portion of the sine wave.
Which is why in superhets you want a strong local oscillator signal to mix with and get highest transconductance conversion gain.
When signals go beyond the "linear" portion of the curve we hear distortions, whether "real" or only created due to overload/poor
ratio of mixing.
PLL radios mix with a square wave for conversion detection, and for me, it destroys the sound of the radio,
because I hear this as a non-linearity.
My part 15 uses audio on the grid of a 6SN7, where I inject the 1620 AM osc, then take the RF modulated output.
It is very clean, And I think of it as "linear" because there's no iron, but I know that linear is only a relative term as used in radio
response.
Single ended tirode tube amps in Class A are biased to most linear point of their current/load relationship but at the upper and lower ends of current range the output does get non-linear.
It is correct that passive components alone can not create AM.
important information to be mixed intersect upon the >most linear< portion of the sine wave.
Which is why in superhets you want a strong local oscillator signal to mix with and get highest transconductance conversion gain.
When signals go beyond the "linear" portion of the curve we hear distortions, whether "real" or only created due to overload/poor
ratio of mixing.
PLL radios mix with a square wave for conversion detection, and for me, it destroys the sound of the radio,
because I hear this as a non-linearity.
My part 15 uses audio on the grid of a 6SN7, where I inject the 1620 AM osc, then take the RF modulated output.
It is very clean, And I think of it as "linear" because there's no iron, but I know that linear is only a relative term as used in radio
response.
Single ended tirode tube amps in Class A are biased to most linear point of their current/load relationship but at the upper and lower ends of current range the output does get non-linear.
It is correct that passive components alone can not create AM.
It would be a disaster using FM from 530-1710 for other reasons than multipath. If you go back to Armstrong's original work on FM and solving the "static problem" he discovered that bandwidth/signal to noise were proportionate. The wider the bandwidth, the lower the S/N. This is not to be confused with deviation. Deviation is the "loudness" of your FM signal, instead of the amplitude being varied, that remains fixed and the frequency is varied... the more variation, the "louder" the signal. Bandwidth is just that, how much can be packed into a given channel audio wise. He found that narrowband FM was inherently noisy, as well as the small deviation that could fit in the AM band would have actually been worse than AM. You could only deviate +/- 10khz MAX on the AM band to fit in the current channels (forget IBOC for a moment here). And, your channel could only be that wide, vs. 75khz dev and 100khz channels for VHF-FM.
Now factor in the capture effect of FM and you would have one rollercoaster ride after sundown!! Imagine you're listening to WOR and Cuba comes rolling in and "shuts it off". At least with AM, the signals can mix and your hearing and brain can still pick out the audio you want to hear out of the noise.
The bottom line is, is that if it could have been done then, it would have been.
Now factor in the capture effect of FM and you would have one rollercoaster ride after sundown!! Imagine you're listening to WOR and Cuba comes rolling in and "shuts it off". At least with AM, the signals can mix and your hearing and brain can still pick out the audio you want to hear out of the noise.
The bottom line is, is that if it could have been done then, it would have been.
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