Are there any HD AM stations that don't sound like this?

[FTL_Ian]

I really want to like HD on AM but it sounds like the codec is being asked to do way too much with a small bitrate. You can really hear it on words with "s" in them:

http://images.freetalklive.com/audio/WZBK_HD_Hissy.mp3 (recorded from line outs on a Sangean HDT-1X)

I don't know much about the AAC codec, but when compressing MP3s all one needs to do to eliminate this problem is reduce the frequency range of the codec until it goes away. AM stations may be able to solve this by simply compressing up to 14 kHz instead of 15 kHz.

Are there any AM HD stations on the air that actually sound good?

 

[Play Freebird]

I really want to like HD on AM but it sounds like the codec is being asked to do way too much with a small bitrate. You can really hear it on words with "s" in them:

http://images.freetalklive.com/audio/WZBK_HD_Hissy.mp3 (recorded from line outs on a Sangean HDT-1X)

I don't know much about the AAC codec, but when compressing MP3s all one needs to do to eliminate this problem is reduce the frequency range of the codec until it goes away. AM stations may be able to solve this by simply compressing up to 14 kHz instead of 15 kHz.

Are there any AM HD stations on the air that actually sound good?

Sounds like you've "discovered" AM HD Radio! It's hard to understand why some people can't hear this problem (or at least refuse to admit they do), but I' ll bet a low-pass filter ahead of the encoder would help. Many processors have such an option. However, it may be necessary to reduce the upper audio limit to 10 kHz to achieve a noticeable improvement in coding artifacts. AAC is forced to perform artificial reconstruction of the upper frequencies, due to the limited bit rate of AM HD. This may also explain why some of those HD Alliance promo spots refer to IBOC as "Radio with a Boob Job", because it has a phony top end.

Your are correct that MP3 codecs behave in a similar "counterintuitive" way. When I used Zephyrs for remote broadcasts, I would always set the sample rate at 32 kHz, rather than 48. This would cut off the audio at 15 k (there's no reason to carry frequencies that won't make it past the stereo generator) and reduced the need to throw away more of the audio than necessary.

Another thing that drives codecs crazy is clipped audio. I've heard some talk shows on AM HD that had very objectionable artifacts during phone calls, because the board op was running caller audio too hot.

 

[FTL_Ian]

Good thoughts!

I have had the JVC my car for a few months, but I wanted to wait until I purchased a home unit to confirm my suspicions.

 

[JohnnyElectron]

Your Sangean HDT-1X sounds better than my Directed Electronics home unit and car unit, so that's about as good as it's going to get. I'm curious if there's a CQuam station in your area, how well the AM audio is for that?

 

[FTL_Ian]

The only other AM station in my area is WKBK, and I don't think they are doing CQuam.

 

[Mike Walker]

Limiting high frequency response is the best way to avoid artifacts at low bitrates. But what I heard on that recording wasn't codec related artifacts. It's just poor audio. First of all, it's bass-heavy. Second, it IS hissy. But lossy codecs don't cause hiss. At most they can pass along noise that's further up the chain. Third, and I'm shocked none of the "golden ears" here mentioned this, THAT IS MONO! Not a hint of stereo. If you're going mono anyway, why the hell pay for the digital gear? Analog AM (mono or stereo) routinely sounds a helluva lot better than that sample.

The best most artifact free AM HD recording I've heard was of a music selection on WOR, New York. By the way, when you hear artifacts from lossy compression, they generally manifest themselves as "spitty" sibilants, "swishy" highs (as the codec decides moment to moment which of the slices it's made of the audio spectrum to keep, and which to discard), and a weird, hollow, "comb filter" effect, also caused by certain slices of the audio spectrum being momentarily discarded. It can work quite well, to surprisingly low bitrates. And it can suck like a Hoover (the vacuum cleaner, not the former FBI Director. Still....)

 

[FTL_Ian]

Perhaps "spitty" and "swishy" are better terms to describe what I don't like about that clip.

FYI: The station is in stereo but I recorded in mono as stereo is not necessary to reproduce the artifacts.

 

[4dude]

I dont hear any hiss in that clip,I HEAR A LACK OF BASS AND IT SOUNDS LIKE CRAP!!!!!

I wonder if the hiss is coming from your receiver......

 

[Eli Polonsky]

Your Sangean HDT-1X sounds better than my Directed Electronics home unit and car unit, so that's about as good as it's going to get. I'm curious if there's a CQuam station in your area, how well the AM audio is for that?

I have a Sangean HDT-1, and there is still one station in C-Quam AM stereo here, WJIB 740 Cambridge, MA.

The Sangean does actually demodulate C-Quam AM stereo, but unfortunately, only in narrowband mode. The receiver does not seem to have any AM wideband circuit for analog. You get separation, but with the same muddy, muffled fidelity of any modern conventional analog AM mono tuner. I doubt the frequency response of the analog AM circuit in the HDT-1 even reaches 5k. It sounds more like it rolls off at 2k.

The three local AM HD stations that I can get here in the Boston area, WBZ 1030, WMKI 1260 and WXKS-AM 1430 sound quite good on the HDT-1. The high end response is relatively clean, considering the process involved. It's almost as good as FM HD.

But, to hear WJIB 740 in wonderful, full-frequency, wideband C-Quam AM stereo, I have to crank up my mid-1980's Sony STR-AV490 wideband AM stereo/FM stereo receiver, which I have plugged into the aux inputs of my big old powerful Sansui receiver from around the same vintage. Anyone who hasn't heard that before doesn't believe that analog AM can sound that good.

 

[JohnnyElectron]

Can anyone verify if they improved the analog AM bandwidth issues on the Sangean HDT-1X over the HDT-1 model, or not??

 

[Mike Walker]

By the way, if it was originally stereo, and you want us to judge the sound quality, you should present it in stereo! Contrary to your claim, artifacts can vary tremendously betwen stereo and mono, due to psychoacoustic masking, dependent upon codec and bitrate. Sometimes artifacts that are obvious in stereo disappear in mono. Anything error in phase relationship between the channels (such as could occur with an analog tape recorder whose heads are slightly out of alignment) will either (randomly) be cancelled, or boosted when switching to mono. This random phase cancellation/augmentation can create it's own comb-filter effects.

In other words, whether analog or digital, you can't judge the audio quality of a stereo broadcast when listening in mono!

 

[FTL_Ian]

Thanks for the expertise, but this clip perfectly reproduces the artifacts I have a problem with.

 

[OKCRadioGuy]

This is what happens with the suits and the government get together and have a bastard child. It's called AM HD radio, the biggest joke on the industry EVER. The sooner we DEMAND something that works instead (specifically an out-of-band solution!) the better off we'll all be.

 

[hipporadio]

OK, I usually don’t “take the bait” and indulge in sometimes-dubious “air check” files here. but I’ll bite... and concede that audio bandwidth in WZBK’s AM-HD signal is a dramatic improvement over the more-typical “mud” tossed from the speakers of a Jeep OEM radio. “FM-like”—IT IS NOT! ...Unless you might remember the days of the Optimod 8000 on some tiny station where the tower is barely-taller than the local grain elevator. In fact, that is what the WZBK AM-HD audio reminds me of: emasculated bass and a fraudulently-abundant-but gritty high-end. Did you notice the harshness of the male announcer’s voice – ANOTHER “signature” of the old 8000 I thought we were finally over by the early-80s!

And for God’s sake PROCESS THE AUDIO, Saga! This station sounds like a pre-pubescent 16-year-old—DESPERATELY-awaiting the onset of male-maturity! I understand... They target the usual subjects drawn to the former TranStar “AM Only” format; but allow the audience to hear the program! WZBK is attempting even-more-fragile 32kbps STEREO AM IBOC with a whopping 1kw-day/146w-night power into a ONE-FIFTH-WAVE “stub” in an area notorious for having about the WORST GROUND CONDUCTIVITY in the country! Heck, I have a “hobby”—called a Part 15 AM—grounded in a salt-water tidal marsh that has bigger Begonias [and possibly BETTER COVERAGE in analog] than Keene’s 1220 has in HD STEREO!

And THIS is what an AM station can expect from its $100,000+ “investment”? —and a listener from his hard-to-find $200+ HD radio? HD radio enthusiasts are dwelling in a more fantasy-filled realm than I earlier imagined!