Proposal - NBFM+HD in the AM Band

[kfbkfb]

Considering the interference in the AM Band
(storms, computers, HD carriers etc.),
I propose allowing Narrow Band FM in the AM Band.

Since the NBFM signal would be a closed system
(different than the current FM Stereo signal),
a powerful analog noise reduction system such
Dolby C could be included in the NBFM standard.

Due to the improved interference rejection of FM
as compared to AM, perhaps HD carriers could be
added to the NBFM signal and wouldn't interfere with
other NBFM signals.

NBFM and AM signals could coexist in the AM band.

Kirk Bayne

 

[KeithE4]

The minimum bandwidth of an FM signal with +/- 5 kHz deviation and a maximum audio frequency of 10 kHz would be 30 kHz. The formula, called Carson's Rule, is 2 * (deviation + max_audio_freq), so 2 * (5 + 10) = 30. This wouldn't fit into a standard AM channel even without HD added. Since the audio frequencies are greater than the deviation, I don't know how well this would work, if at all.

NBFM that is used by two-way radio and hams has an upper frequency of 3 kHz (telephone quality) and a deviation of +/- 5 kHz - or even 2.5 kHz in some cases. OK for those services, bad for broadcasting. The audio quality is horrible but it gets the message across, which is all it's designed to do.

 

[Play Freebird]

The minimum bandwidth of an FM signal with +/- 5 kHz deviation and a maximum audio frequency of 10 kHz would be 30 kHz. The formula, called Carson's Rule, is 2 * (deviation + max_audio_freq), so 2 * (5 + 10) = 30. This wouldn't fit into a standard AM channel even without HD added. Since the audio frequencies are greater than the deviation, I don't know how well this would work, if at all.

Correct. When audio frequencies exceed the deviation, then the modulation index drops below unity and the signal-to-noise advantage of FM rapidly deteriorates.

For example, conventional FM "SCA" subcarriers (often used for ethnic programming and background music) don't provide the same degree of quieting as a wideband monaural FM signal, because they have low modulation indices. If you've ever listened to an SCA in a weak signal area, you'll notice that the signal to noise ratio is much lower on the subchannel than the main channel.

Here's a technical paper that discusses modulation index in far more detail:

http://www.scribd.com/bagherifar/d/6217907-Silicon-Labs-Fm-Tutorial

 

[Ai4i]

I propose allowing Narrow Band FM in the AM Band.

The Harris AM stereo system has come and gone.

 

[Darth_vader]

"(often used these days for ethnic programming and radio reading services)"

Fixed. Muzak actually shut down their last few SCA channels in 2007.

 

[JohnnyElectron]

ai4i, and I might add, the Harris AM stereo system (from CKLW) sounded spectacular on day pattern! WSM sounded great too.

 

[Tom Wells]

'Way back in 1990 and 1991 when I ran a pirate (paid my fine and been "good" since), I built the transmitter
and included "special" coupling from the modulated stage back to oscillator that caused a controlled amount
of frequency deviation along with the regular AM modulation.

This was on 7/415 mhz.
It was not noticably detrimental to the sound I heard on regular AM detection.

Don't remember which pirate directory mentioned this feature, but whoever wrote it noted that
decoded in FM, it was the best sounding audio they had ever heard on a SW signal.

Unfortunately the only airchecks I made were from AM detection.

 

[satech]

This was proposed way back in the '80s, as "Noise-Free Radio" (NFR). The problem is that the combination of low RF frequencies and narrow transmission bandwidth still leaves the FM signal prone to interference, so "noise-free" was by no means a guarantee.

And considering how much complaint there was about the Harris AM Stereo system's slight amount of distortion on conventional AM mono receivers, that ensured that any attempt to use NBFM on the AM band would never be approved, at least back then when people actually cared about AM!

 

[satech]

'Way back in 1990 and 1991 when I ran a pirate (paid my fine and been "good" since), I built the transmitter
and included "special" coupling from the modulated stage back to oscillator that caused a controlled amount
of frequency deviation along with the regular AM modulation.

The stillborn Belar AM Stereo system worked much the same way, except it used the FM component to carry the L-R stereo difference channel, which was modulated with +/-1.25 kHz deviation and 100 uS pre-emphasis.

 

[pianoplayer88key]

Wasn't there a type of AM Stereo that used independent sideband encoding/decoding? Also how in general does C-Quam work, and what makes it incompatible with IBOC? And is there some way to eliminate platform motion in situations where you have co-channel interference (whether caused by the station's own out-of-phase skywave or another station being a few Hz off freuency) while still allowing for stereo reception even on very weak signals (like a 50kW around 1100 kHz at 300-350 miles over 10-15 mS/m ground as heard on a pocket AM radio)?

 

[satech]

Wasn't there a type of AM Stereo that used independent sideband encoding/decoding?

Yes, that was the Kahn/Hazeltine ISB system.

Also how in general does C-Quam work, and what makes it incompatible with IBOC?

C-Quam stands for Compatible Quadrature AM. IBOC also uses quadrature modulation (they call it "QAM"), so the two cannot be transmitted simultaneously.

And is there some way to eliminate platform motion in situations where you have co-channel interference (whether caused by the station's own out-of-phase skywave or another station being a few Hz off freuency) while still allowing for stereo reception even on very weak signals (like a 50kW around 1100 kHz at 300-350 miles over 10-15 mS/m ground as heard on a pocket AM radio)?

No. Motorola's newer C-Quam decoders, as well as the DSP-based C-Quam decoding in most HD Radios, will gradually blend to mono when the signal is weak and/or when platform motion is detected.