AM HD Radio audio response: 4.4 kHz!?

[JohnnyElectron]

WJR during the day transmits in HD-AM Stereo - BUT the stereo separation is a mere fraction of what their C-Quam analog AM stereo once was. Not the 'full sound' either - the biggest difference is their TOH sounder - they have a big echo chamber with the VO man: "Now broadcasting in HD - High Definition radio (sic)" - and it sounds so whimpy I have to laugh - plus laugh that HD doesn't stand for anything either, except maybe Hybrid Digital. Radio Disney Detroit appears to be in stereo, but I can only get a lock for about 5 seconds at a time, so it's very hard to tell, but the bass end is overly heavy.

Barry,
I think that iBiquity did the two-step AM HD backwards - I would have put the core signal in the 0-10KHz phase and had the enhanced AM info in the 10KHz+ 'sideboards', as I would think that the radio bandwidth on crappy signals would automatically cut down the AM bandwidth, thereby squeezing the CORE HD out of the picture instead of the enhanced? Am I correct in my logic, or are they unable to cram 20K of digital into the 0-10KHz phase modulated signal?

 

[SUPERCASTER]

Is AM HD always at 36KB? XM SatRad does a hell of a lot more at 32KB then HD does at 36K. It also seems like digital would be a whole lot more robust than what it is - I'm surprised it doesn't 'go as far as you would think' - if you have a clean analog, you should get HD, but you don't - at least not on my two radios.

The AM system actually has two streams: a 20 kb/s "core" stream, carrying mono audio, and a 16 kb's "enhanced" stream, which can be combined with the core stream to provide stereo audio. The core stream is carried by the primary digital sidebands (the ones at 10-15 kHz from the carrier, causing all the havoc to adjacent channel stations), while the enhanced stream is carried by the other two sidebands, in the 0-10 kHz region. According to the test results produced by iBiquity, the enhanced stream is less robust than the core stream, so the stereo service area should be smaller than the mono service area. In most published reports that show actually measured coverage, however, they are very cagey about this difference, and never mention whether the decoded audio is stereo or not. Curiously, none of the anecdotal observations I've seen recently mention whether stereo audio is being received. It's not even clear whether all AM HD stations are actually transmitting a stereo signal. Comments?

Barry

Thanks once again for cutting through all the HD "mis/dis-information".

 

[clouseau]

Thanks once again for cutting through all the HD "mis/dis-information".

Please tell us specifically what "mis/dis-information" you think the post you responded to, "cut through".

It states widely known information about the AM HD system and asks a few questions. It seems to be pretty accurate. What "mis/dis-information" does it cut through?

Clouseau

 

[ve3jf]

Barry,
I think that iBiquity did the two-step AM HD backwards - I would have put the core signal in the 0-10KHz phase and had the enhanced AM info in the 10KHz+ 'sideboards', as I would think that the radio bandwidth on crappy signals would automatically cut down the AM bandwidth, thereby squeezing the CORE HD out of the picture instead of the enhanced? Am I correct in my logic, or are they unable to cram 20K of digital into the 0-10KHz phase modulated signal?

I can't speak for the designers, but I think the throughput available is certainly one big issue - it's very limited in the 0-10 kHz region due to the need to minimize interference to analog receivers. For one thing, both the secondary and tertiary sidebands must be in conjugate pairs (i.e., upper and lower sidebands are mirror images of each other, and carry the same info), which cuts their potential throughput in half. This trick makes them a constant-envelope signal that in theory doesn't produce any output from an AM detector, but it doesn't work perfectly (because of pattern bandwidth limitations in antennas, asymmetry in receiver IF response, etc.), so they also have to be at relatively low levels. The primary sidebands don't have these encumbrances, so they can be run at higher power, and they can also carry different info on the upper and lower sideband (in practice, this is used for time diversity). So, it makes sense to use the primary sidebands for the core stream... for daytime operation. At night, however, this strategy backfires, since those sidebands are at the mercy of strong skywave interference. Maybe they should've come up with a "nighttime mode" that drops the primary sidebands and uses the other two for a core mono stream... but they didn't, and we're stuck with a mess instead.

Barry

 

[CW]

Barry,
I think that iBiquity did the two-step AM HD backwards - I would have put the core signal in the 0-10KHz phase and had the enhanced AM info in the 10KHz+ 'sideboards', as I would think that the radio bandwidth on crappy signals would automatically cut down the AM bandwidth, thereby squeezing the CORE HD out of the picture instead of the enhanced? Am I correct in my logic, or are they unable to cram 20K of digital into the 0-10KHz phase modulated signal?

I can't speak for the designers, but I think the throughput available is certainly one big issue - it's very limited in the 0-10 kHz region due to the need to minimize interference to analog receivers. For one thing, both the secondary and tertiary sidebands must be in conjugate pairs (i.e., upper and lower sidebands are mirror images of each other, and carry the same info), which cuts their potential throughput in half. This trick makes them a constant-envelope signal that in theory doesn't produce any output from an AM detector, but it doesn't work perfectly (because of pattern bandwidth limitations in antennas, asymmetry in receiver IF response, etc.), so they also have to be at relatively low levels. The primary sidebands don't have these encumbrances, so they can be run at higher power, and they can also carry different info on the upper and lower sideband (in practice, this is used for time diversity). So, it makes sense to use the primary sidebands for the core stream... for daytime operation. At night, however, this strategy backfires, since those sidebands are at the mercy of strong skywave interference. Maybe they should've come up with a "nighttime mode" that drops the primary sidebands and uses the other two for a core mono stream... but they didn't, and we're stuck with a mess instead.

Barry

NO, what SHOULD have happened is the AMAX standards for rcvrs years ago should have been LAW with noise blankers, limiters added and perhaps even DSP...then AM Stereo would still be around and the IBUZ hash would never have happened.....but instead, instead of fixing the problem, the broadcasters are degrading their products to match the defective receiver design....I have a Sony XRA33 AM stereo with selectable wide/narrow filter...when I want AM HiFI, it sounds GREAT IF the station is still doing 10kHz b.w audio....before the 10kHz mask came out, I used to listen to AM stereo more than FM stereo...because it sounded fuller to me....(KKBQ simulcast in Houston was one..) and it was nice listening to WLS in stereo driving around Texas at night....of course these days forget that!! Too many other signals on 890!

 

[ropertex]

This is HORRIBLE performance for a “new technology” AM tuner—even in “analog mode”... But, alas—it seems commonplace!

The spec writers seem in many cases to be using the analog numbers. 4.4 Khz is quite good for analog AM (which is recently limiting theoretical analog response to 5 khz to provide more robust digital signal)

Many home stereos and car radios have 2.5 or 3 Khz analog AM fidelity...telephone quality.

The HD bashers need to actually buy an HD radio from a store with a return policy, and listen to it in their home.

40 years ago, those who told everyone how bad color TV was were NOT people who actually owned one.