The Best Process for HD Radio Audio
     

Credit: iStockphoto/Eugene Kazimiarovich

One in a series of articles about getting the best performance out of your investment in HD Radio.

In the discussion forum at the amusingly-named GearSlutz.com, there’s a thread from July 2011 with the title, “HD Radio sounds like junk.” The poster admits that the high end is better than analog FM, but complains that it’s “swishy,” like a low-bitrate MP3 file. The first commenter to that question offers, “Possibly this is a fault of the radio station processing?” Indeed, it very well might be.

HD Radio does use a relatively low bitrate. With our present “hybrid” (i.e., analog+digital) systems, AM HD-R typically runs at 40 kilobits per second. A single FM channel in HD-R is typically 96 kbps. But many FM stations multicast, splitting that bitrate across several HD “streams.” The result in both cases is that the final bitrate is comparable to that for a Web stream.

Most of us made this mistake when we started streaming over the Internet: We split the audio from our existing processor to drive the encoder. We then discovered that it sounded awful. The same is true of HD Radio, and the low bitrate is the primary reason.

Omnia’s Frank Foti released an important paper about this several years ago titled “Audio Processing and HD Radio.” It’s available at omniaaudio.com/tech/Audio_Processing&HD-Radio.pdf and is recommended reading.

In that paper, Foti says flatly, “A processor for HD Radio has a completely different set of requirements. The most important issue is in dealing with data reduced audio” (emphasis mine).

Foti’s paper details how clipping, in particular, must be given special treatment. Most top-of-the-line processors, including those from Omnia, actually have two separate peak limiters. A more traditional “clipper” might be used with the analog, but a softer “look-ahead” limiter will be required for a digital stream. Hard clipping is an absolute no-no with any digital “stream” — HD Radio included.

My buddy Jeff Keith with Vorsis (www.vorsis.com) had this to add when I queried him via email: “In any coded environment it’s important to not give the codec something to code that’s (1) difficult, and/or (2) takes bits away from audio that was supposed to be coded.” In his words, codecs are “very good at ‘hearing’ all of the things that are in the audio.” But they can’t differentiate between the good and the bad. Therefore, Jeff says, we must supply the smarts in the processing and “not give it anything bad.”

For the best performance, you’ll buy a processor that’s “digital stream-ready.” But if you’re on a strictly limited budget, you may have to be prepared to make compromises with an older, pre-HD and pre-Web stream processor.

Here are some tips: First, reduce the clipping as much as possible. Next, watch the levels! Don’t ever overdrive the codec’s input. Remember not to use standard FM pre-emphasis. Finally, you may need to slightly roll off the highs after the processor to further ease the load on the codec.

It’s not ideal but you can get good sound on a low-bitrate digital stream like HD Radio even with an older processor. This will buy you some time to save up for that dedicated HD audio processor.

Send your HD Radio tips or column suggestions to rwee@nbmedia.com.

Stephen M. Poole, CBRE-AMD, CBNT, is market chief engineer at Crawford Broadcasting in Birmingham, Ala.


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Comment List:

Stephen's first tip is the real key to good digital audio. Thanks for the great article.
By Michael Payne on 4/27/2013
Special processing is a must for HD radio (and web feed)- you simply don't need as much hard-driving modulation as you used on analog FM or AM, and pre-emphasis isn't required. Cascading codecs issues too. The only thing worse than poor HD is how swishy, phasey, and swirling Sirius SatRad now sounds as they continue to reduce bitrates to add more channels - it is not listenable OTA anymore.
By John P on 4/26/2013
In both HD radio and DAB+ the data compression relies on mimicing the human listening system. Processing produces unnatural sounds which aggrevates the data compression system, this problem becomes worse as the data rate reduces. Sounds like phasing which you get with analog Short Wave. Leaving the unnecessary pre-emphais makes it worse.
By Alanh on 4/26/2013
Interesting article! In Australia DAB+ Digital radio and FM are separate systems with some simulcasting. I pity your mixing of the decompressed HD signal and the FM signal mixed together. Pity you cannot go purely digital HD radio (in FM band) to be able to increase the digital power an stop the mixing above.
By Alanh on 4/26/2013
"The Oasis" on 107.5 in Fort Worth, Texas sounds terrible. It is worst when saxophone music is being played. I hope that The Oasis will clean up its audio so I can listen again because it is too painful to listen to now.
By Peter Wankerman on 4/25/2013
The Omnia Audio paper should be required reading for station owners that have been misled by clueless engineers into avoiding upgrading to HD Radio. The amount of misinformation being promulgated by those that oppose digital radio is incredible. It's good to see a well-written paper that explains the process for obtaining the best possible audio quality from HD-Radio
By Finnis Kronor on 4/25/2013

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