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-   -   FmXtra (https://www.avsforum.com/forum/154-hd-radio/718117-fmxtra.html)

goobenet 09-01-2006 08:34 AM

I got a chance to hear FMXtra from rochester, mn the other day... Man is that cool. Instead of traditional HD where you need another transmitter, new antenna, dummy load, and new processing, FMXtra just uses the subcarrier bands. It essentially "streams" a 90kbit aacplus datastream on the subcarrier channel of your analog signal.

This is how digital radio SHOULD be done. No licensing, just a PC that exports a subcarrier stream. But alas, they're having the same problem as IBUZ, no receivers available to the public yet, but hey, it's a competitor, and cheaper. I can tell that this could definately be a IBOC killer.

Bob Smith 09-01-2006 09:44 AM

I'm afraid this is a non-starter. When used as a subcarrier, multipath would kill any subcarriers in short order. This is the main reason FM sounds so bad most of the time, the stereo difference signal is a 38 KHz subcarrier that really gets bashed with multipath.

COFDM as used in IBOC is inherently immune to multipath since the slot time is longer than the multipath.

Receiving the subcarrier digital signal would require an even more pristine signal than that required for good quality FM reception. The additional subcarriers are only given a small percentage of the total deviation by the FCC so the SNR of these carriers would also be much lower than that of the main FM signal.

scowl 09-01-2006 09:45 AM

Quote:
Originally Posted by goobenet
It essentially "streams" a 90kbit aacplus datastream on the subcarrier channel of your analog signal.
It's actually called "FMeXtra" with the little e in there.

It's limited to 64 kbps for stations broadcasting in analog stereo and 128 kbps if the station wants to broadcast in mono instead. With AAC+ that will get you one good digital audio channel or two mediocre ones. Stations also have to get rid of all other subcarrier services like DirectBand.

FMeXtra is compatible with HD radio -- stations can broadcast both at the same time.

scowl 09-01-2006 09:50 AM

Quote:
Originally Posted by Bob Smith
I'm afraid this is a non-starter. When used as a subcarrier, multipath would kill any subcarriers in short order. This is the main reason FM sounds so bad most of the time, the stereo difference signal is a 38 KHz subcarrier that really gets bashed with multipath.

COFDM as used in IBOC is inherently immune to multipath since the slot time is longer than the multipath.
FMeXtra also transmits in COFDM. It just keeps all of its carriers in the subcarrier bandwidth. I haven't read about how well it performs though.

Bob Smith 09-01-2006 11:32 AM

Yes, but it is a SUBCARRIER, not separate carriers, as with IBOC. There is a big difference. With multipath, the result to an FM subcarrier is harmonic and intermodulation (non-non linear) distortion, which is a big killer to COFDM. Multipath to a COFDM carrier causes liner amplitude distortion, which COFDM is able to cope with. A subcarrier is not the same thing as a carrier.

scowl 09-01-2006 12:51 PM

Damn, I haven't thought in analog electronics terms in so long it's hard to picture these things. Please tell me if I got this right...

The COFDM carriers in FMeXtra are really part of the audio (or modulation) of the main FM carrier (a single carrier) while in HD radio, each carrier is an independent radio signal. When multipath occurs with FMeXtra, the reflected signal interferes with all of the COFDM carriers (subcarriers) in the main carrier at the same time. When COFDM carriers are separate radio signals, the reflection of a carrier in multipath can only interfere with itself, not other carriers.

I'm glad I don't have to think in analog any more!

Bob Smith 09-01-2006 02:05 PM

You've got it 100% right!

Please don't give up completely on analog, there are so few of us analog dinosaurs around anymore!

Bob

scowl 09-01-2006 04:41 PM

Quote:
Originally Posted by Bob Smith
You've got it 100% right!
Good. My head hurts. I'm going back to the land of ones and zeros....

goobenet 09-01-2006 05:53 PM

Drink more beer, everything tends to make more sense that way. :)

Multipath be damned, this sounds pretty good for a subcarrier.

scowl 09-05-2006 09:25 AM

In my case, namely the RF pinball machine I work in here, I don't think FMeXtra would work for many stations. I can barely receive have of them in stereo yet HD radio pulls them without problems.

mattdp 09-06-2006 03:27 PM

FM eXtra in little ol' Rochester, MN (my city)?


What station is broadcasting FM Extra?

Man... I'm in over joyed that we would actually be a "beta tester" type market for FM eXtra. Rochester's radio and TV kinda stinks because we are such a little market. I mean, we can't even get our market area's digital CBS and ABC stations, and we've got to HD Stations (the classical station and the NPR "downright liberal" talk station), and all radio besides 88.7 (89.3 The Current) and KROC-AM 1340 just aren't worth listening to [in my opinion, anyway]

scowl 09-06-2006 04:46 PM

I stumbed upon upon an interview with Derek Kumar of Digital Radio Express, the owner of FMeXtra. To fight multipath, they "incorporate an adaptive RF channel equalizer based on waveform properties". That sounds vaguely what they've been doing in recent ATSC receivers (not that I understand analog electronics, but the wording is simlilar).

It sounds like they're breaking some SCA peak power rules to get the the maximum data rate and they're working on a "superinjection" system that would improve performance by breaking some more rules, but nothing like what HD radio is getting away with. The craziest thing is that he's counting on FM stations to start switching their analog stations to mono so FMeXtra will have enough room to transmit 128 Kbps. I can't see that happening.

Bob Smith 09-07-2006 10:53 AM

It appears they're putting band-aids upon band-aids to make this work. Incorporating an adaptive RF channel equalizer, means they're working before the FM demodulator to try and fix the channdl (probably in the IF strip). The ironic thing is, using this technology would probably equalize the RF channel for the analog stereo difference subcarrier also.

This also means it isn't a simple adapter that hangs on the baseband output of the FM discriminator. It would require 'hooks' back into the RF channel, and a completely re-designed RF channel as well.

This puts the burden of expense at the receiver to equalize the distorted RF carrier due to multipath.

COFDM effectively places the multipath fighting encoding at the transmitter, meaning the receiver only requires a relatively simple receiver to decode the signal. Processing is performed at baseband rather than the more expensive alternative of doing it at RF.

I still don't believe FMeXTRA is going to make it out of the gate.

In a (my) perfect world, the whole FM channel would be replaced with COFDM, and a 500 Kb/s would replace the whole thing. With 500 Kb/s, you could get quality as high as would ever be needed, and much better than the existing FM system.

Bob Smith

scowl 09-07-2006 11:29 AM

Quote:
Originally Posted by Bob Smith
It appears they're putting band-aids upon band-aids to make this work. Incorporating an adaptive RF channel equalizer, means they're working before the FM demodulator to try and fix the channdl (probably in the IF strip). The ironic thing is, using this technology would probably equalize the RF channel for the analog stereo difference subcarrier also.
Maybe that's why he wants stations to switch to FM mono -- analog stereo would sound better than ever if they could reduce the multipath problems. :)

goobenet 09-08-2006 02:24 PM

Quote:
Originally Posted by mattdp
FM eXtra in little ol' Rochester, MN (my city)?


What station is broadcasting FM Extra?

Man... I'm in over joyed that we would actually be a "beta tester" type market for FM eXtra. Rochester's radio and TV kinda stinks because we are such a little market. I mean, we can't even get our market area's digital CBS and ABC stations, and we've got to HD Stations (the classical station and the NPR "downright liberal" talk station), and all radio besides 88.7 (89.3 The Current) and KROC-AM 1340 just aren't worth listening to [in my opinion, anyway]
Of course, who else has money to play with? MPR OF COURSE! The classical station has it running... The guy who installed it was able to pick up the carrier 200(!!!) miles away! Clear as a bell he says.

mattdp 09-15-2006 07:58 AM

What the..... that's insane!!!!!!

Do you know what kind of antenna/tuner he was using? I've heard of people with old modded 70's tuners and APS-13's on high towers can push 200 miles on analog FM.(http://antennaperformance.com/), but digital FM from 200 miles?

KLSE is almost an ideal candidate for FMe, as they broadcast at 94kw atop the KNXR tower (97.5/100kw/1500ft - easy listening).
Go FMeXtra!!!!!!!!!!!!!!!!

Robert2413 09-15-2006 08:58 PM

Quote:
Originally Posted by Bob Smith
It appears they're putting band-aids upon band-aids to make this work. Incorporating an adaptive RF channel equalizer, means they're working before the FM demodulator to try and fix the channdl (probably in the IF strip). The ironic thing is, using this technology would probably equalize the RF channel for the analog stereo difference subcarrier also.

Bob Smith
Prior to the FM demoduator, multipath is equivalent to a linear filter and, in theory, one could create an inverse filter to cancel it out. The filter would have to be in the RF or IF domain before any limiting. But there are a lot of challenges to this sort of "blind deconvolution," particularly if the multipath creates extremely deep notches.

In any event, COFDM is a much more robust modulation technique in the RF domain compared to FM.

goobenet 09-16-2006 11:13 AM

KNXR is the station doing FMeXtra. Not quite sure what tuner they were using, but it was mobile, probably a prototype, made by JVC(?) like the one MPR has installed in their company vehicle. Still think it's pretty damn cool. The way they should've done HD in the first place, then progress to a whole transmitter setup.

Bob Smith 09-18-2006 02:32 PM

Quote:
Originally Posted by Robert2413
Prior to the FM demoduator, multipath is equivalent to a linear filter and, in theory, one could create an inverse filter to cancel it out. The filter would have to be in the RF or IF domain before any limiting. But there are a lot of challenges to this sort of "blind deconvolution," particularly if the multipath creates extremely deep notches.

In any event, COFDM is a much more robust modulation technique in the RF domain compared to FM.
Yes, I agree, COFDM is the only thing that makes sense with a dispersive path. I can't imagine it would be cost effective to do the required RF equalization. I doubt it would even be feasible in a mobile environment where most of the population listens to broadcast radio. With COFDM you get most of this for free.

It would be much worse than direct single channel QPSK because of the non-linear effects of limiting and FM demodulation of the subcarrier. Once the signal drops below FM threshold, there would be no way of recovering it. The data stream would be simply missing during the 2*Pi clicks, you can't RF equalize something below the noise floor.

It may be an easier thing to implement with the existing FM system, but not even close to an optimum system design to replace FM.

I'm not a big fan of IBoC due to the low bit-rates achieved, but if COFDM could be applied to all of the 400 KHz of bandwidth that is available, a very robust system could be devised.

Bob Smith

scowl 09-18-2006 03:52 PM

The non-hybrid HD Radio system delivers something like 160 Kbps.

If FMeXtra had more bandwidth, it could fight the multipath somewhat by sending the data twice so the receiver would have two chances at getting the right data but I don't think that would be a popular option.


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