More Carriers in an FM Signal

[kfbkfb]

Given that the newest audio codecs can provide high audio fidelity at low data rates,
my idea is to add more carriers in the FM signal, above the upper sideband of the L-R
difference signal.

The digital audio carriers would have error correction, that way, if the FM signal is
weak and/or multipath ridden (unsuitable for regular Stereo FM), the digital audio
could still be decoded (unaffected by the poor FM signal quality).

This technique could be used with (very) low power FM translators in addition to
full power FM signals.


Kirk Bayne

 

[PTBoardOp94]

You mean like HD Radio?

 

[Will_H_69_9]

Given that the newest audio codecs can provide high audio fidelity at low data rates,
my idea is to add more carriers in the FM signal, above the upper sideband of the L-R
difference signal.

The digital audio carriers would have error correction, that way, if the FM signal is
weak and/or multipath ridden (unsuitable for regular Stereo FM), the digital audio
could still be decoded (unaffected by the poor FM signal quality).

This technique could be used with (very) low power FM translators in addition to
full power FM signals.


Kirk Bayne

yeah, that's exactly what HD Radio is.

 

[kfbkfb]

No, no additional transmitter hardware would be needed.

All of the new signals would be IN the FM carrier, not an add on signal
such as the added sidebands for HD radio, my idea is just an extension
of the method used to add Stereo to FM, more (sub)carriers in the
frequency range above the L-R signal.


Kirk Bayne

 

[Kelly A]

No, no additional transmitter hardware would be needed.

All of the new signals would be IN the FM carrier, not an add on signal
such as the added sidebands for HD radio, my idea is just an extension
of the method used to add Stereo to FM, more (sub)carriers in the
frequency range above the L-R signal.

That's exactly what the early versions of IBOC/HD radio did. Separate transmitter for the digital side bands. In some installations, the additional carriers were 'high level combined' into a single antenna with the analog FM carrier, or transmitted from a second antenna, usually on the same tower. A few stations ran not only a second transmitter, but a second antenna.

Down the road, transmitter manufacturers took a page from DTV transmitter companies, and started building broader-band amplifier stages that could modulate multiple carriers from a single transmitter/amplifier stage.

Keep in mind too, that the FCC requires stations running FM stereo to maintain the full slate of traditional analog carrier space, even when running IBOC/HD-radio. That includes maintaining spacing for 67 and 92kHz subcarriers.

 

[TheBigA]

No, no additional transmitter hardware would be needed.

However, new receivers would be needed, no?

That's the biggest obstacle to HD right now.

 

[Double A]

FMeXtra Tried This and Flopped

I don't believe HD radio ever attempted to use subcarriers. HD radio uses sidebands. Subcarriers are modulated directly onto the FM frequency whereas sidebands are on (slightly) different frequencies above and below the main FM carrier.

Although HD radio doesn't do this, the idea of using subcarriers for digital audio has been tried before with FMeXtra: https://en.wikipedia.org/wiki/FMeXtra , putting a digital subcarrier above the stereo, and if a station is using it, RDS carrier. However, if a station wanted to keep its stereo and RDS subcarriers, bitrate was limited to 40 kbps. That's doable with modern codecs, but doesn't really allow for multiple decent-quality music subchannels. If a station wanted to drop all existing subcarriers and go mono, they could squeeze out 156 kbps with FMeXtra. Not bad, but most music stations wouldn't want to go mono for that. Besides, HD radio offers 96 - 120 kbps while keeping a station's stereo and RDS subcarriers intact.

Before I sound too much like an HD radio fanboy, HD certainly has faults that FMeXtra doesn't have. HD radio causes additional interference to adjacent stations, and completely wipes out first-adjacents, something that FMeXtra does not do. Though we probably wouldn't have seen as many subchannels with FMeXtra, there certainly would be less congestion on the FM band if the US picked it. Additionally, FMeXtra does not charge royalty fees like iBiquity (now Xperi) does for HD radio. And, since it's a subcarrier and not a sideband, the technical upgrades to FMeXtra are much much cheaper and less intensive than to HD radio.

As for codecs, FMeXtra uses some more advanced and specialized codecs than HD radio to make the most of its limited bitrate. FMeXtra supports HE-AACv1, HE-AACv2, and AMR-WB+ while HD radio only supports a (slightly modified) version of HE-AACv1. Though FMeXtra is legal in the US (since it's just a subcarrier) and a few stations still use it, the FCC picking HD radio as America's digital station probably sealed its fate.

I'm certainly not saying this is impossible to do again, but all new receiver hardware would be needed and HD radio basically serves the same purpose.

 

[Kelly A]

I don't believe HD radio ever attempted to use subcarriers. HD radio uses sidebands. Subcarriers are modulated directly onto the FM frequency whereas sidebands are on (slightly) different frequencies above and below the main FM carrier.

You answered your own question. FMextra tried this already. It worked, sort of. Ultimately Ibquity's HD-Radio was chosen, so FMextra was officially deemed DOA.

Given the slow growth and lack of consumer interest in even HD Radio, why would anyone want to revisit changing the way listeners listen to FM? That ship sailed 30 years ago.

 

[frankberry]

Another problem with putting a digital stream on a subcarrier is the crosstalk from the main L+R audio.
Even a tiny bit of harmonic distortion within the main L+R audio will cause "splatter" on the subcarrier(s).
This could cause digital audio dropouts.

 

[kfbkfb]

Yes, FMeXtra is/was the same concept, but FMeXtra is only
about more carriers, not adding specialized error correction
to deal with corrupted (harmonic distortion, weak signal,
severe multipath etc.) FM signals.


Kirk Bayne

 

[anotherguyTN]

To most listeners HD radio has been useless because of the lack of radios and/or the lack of stations doing anything with it in some areas, except possibly to feed translators. So why would this idea be any better?

 

[oldradiotapes]

like iBiquity (now Xperi)

What happened to the deal with DTS?

 

[TheBigA]

What happened to the deal with DTS?

Nothing. Xperi purchased DTS.

https://www.xperi.com/markets/home-solutions/

 

[Groove1670]

You answered your own question. FMextra tried this already. It worked, sort of. Ultimately Ibquity's HD-Radio was chosen, so FMextra was officially deemed DOA.

Given the slow growth and lack of consumer interest in even HD Radio, why would anyone want to revisit changing the way listeners listen to FM? That ship sailed 30 years ago.

The net gain has been pairing up analog translators with HD. No one really cares about HD audio quality. There might be a select few that might search out a dedicated specialty channel.

 

[Groove1670]

Another problem with putting a digital stream on a subcarrier is the crosstalk from the main L+R audio.
Even a tiny bit of harmonic distortion within the main L+R audio will cause "splatter" on the subcarrier(s).
This could cause digital audio dropouts.

Worked with a FM that had a paging subcarrier and elevator music. The main channel was AC at the time. Nothing like hearing Neil Diamond's Heartlight when you're pager went off. We used the pagers to reach sales people. Had a few regular customers, but not many.

 

[TexasTom]

There's yet another problem with this idea, which is degraded signal-to-noise ratio (SNR) at higher subcarrier frequencies. This is the reason why FM stereo is noisier than FM mono -- and it is even worse for subcarriers that are above the stereo subcarrier.

The effect of the degraded SNR for the proposed digital subcarriers would be sharply limited range. Your digital subcarriers would work just fine as long as you're close enough to the station's transmitter for full quieting. But as you got further from the transmitter and the SNR starts to degrade, your digital subcarriers would first become unreliable and then drop out completely. My suspicion is that you'd lose the digital coverage well inside a station's 60 dbuv protected coverage area.

 

[KTN Corp]

Before I sound too much like an HD radio fanboy, HD certainly has faults that FMeXtra doesn't have. HD radio causes additional interference to adjacent stations, and completely wipes out first-adjacents, something that FMeXtra does not do.

That's only on Hybrid Digital mode. The recent tests in digital-only mode does not cause interference to first and other adjacent stations.

It's really pointless to switch standards--like DAB--at this point. HD Radio has finally achieved significant penetration after a long adoption period (and periods of doubt about viability)! The only obstacle left is to go all-digital, like TV, and FM-only, which will take an Act of Congress. The time is right to switch from obsolete analog with all the HD receivers in use.

 

[davideduardo]

That's only on Hybrid Digital mode. The recent tests in digital-only mode does not cause interference to first and other adjacent stations.

It's really pointless to switch standards--like DAB--at this point. HD Radio has finally achieved significant penetration after a long adoption period (and periods of doubt about viability)! The only obstacle left is to go all-digital, like TV, and FM-only, which will take an Act of Congress. The time is right to switch from obsolete analog with all the HD receivers in use.

There is really no "significant" penetration if you go by percentages.

HD-capable radios are a real minority of all receivers. They represent essentially 0% of home and work radios and in-home and at-work is still more than half of all listening. Less than 30% of vehicles have HD, meaning about 200,000,000 do not have HD radios. Those without newer top of the line cars with HD tend to be Hispanics, Blacks, Native Americans... so you can see how prejudicial such a move would be ; taking away free radio for those less able to pay subscription service would be politically untenable.

And don't be deceived by WZZZ-HD2 or the like showing in the ratings. Most all the "HD" attributed showings are actually for standard FM translators for HD channels. And even when a "real" HD sub shows, which is very rare, it is getting a 0.1 because for subscribers just one meter once a week will get you a consolation prize of 0.1 share points.

So, if nearly no fixed location radios and only about a third of in-vehicle ones are HD equipped, that means that only around 15% of all usable radios have HD. What station would desire to lose 85% of its audience?