You hear the ads on the radio:
“I’m in love with radio again, thanks to my new HD Radio.”
“This new HD Radio has allowed me to rediscover radio in a whole new way!”
They’ve been cropping up in cars recently. They’re slowly taking over. But one question remains unanswered: what does HD stand for?
It doesn’t. It’s a brand name. Trademark, whatever you call it. Given that HD in HDTV stands for “High Definition,” people will make the association that the HD in HD Radio stands for the same. You’d be surprised to know that HD radio is, in fact, not necessarily higher quality than regular radio. Is that deception? Surely the FTC has something to say…
Factual Explanation for Engineers
Edison first developed the multiplexer back in the 1800s. It’s a pretty cool concept: I have one telephone wire, but I can send multiple telephone conversations over that same wire, using analog multiplexing. More recently, digital multiplexing has allowed a similar kind of channel expansion (although it differs greatly from analog multiplexing).
For the phone example, the frequencies necessary to discern human voice are between 200 and 3000 Hz (thus the 2.8 KHz bandwidth necessary for a Single Side Band communication in analog radio). If you put one phone conversation on one wire, you occupy only the frequencies from 200 to 3 Khz on the frequency spectrum. Someone, somewhere figured out that you can “step up” that lower frequency to a higher range of frequencies. Thus you can put another telephone conversation between 5 and 8 KHz, and 10 and 13 KHz. There are “guard bands” in between to ensure that there is no cross talk between the conversation (something you may have noticed if you have multiple lines in your house).
So how does this apply to HD Radio?
Basicaly, the mono signal for FM Radio occupies the human range of hearing, between 20 Hz and 20 KHz. To a stereo decoder, this is called the “Sum” channel or L+R channels. The chosen method to broadcast stereo was to then create a “difference” channel or L-R signal (bonus points to he who can create those two signals from the L and R channels using operational amplifiers). This difference signal is stepped up to 50KHz to 70KHz and modulated along with the sum signal onto the FM carrier (which is at a frequency of 91.5 MHz or whatever).
So how does this apply to HD Radio?
The concept from the last time I started a paragraph of the same title can be expanded. Nowadays, FM stations transmit a lot of digital information modulated at higher frequencies. Sophisticated and well designed receivers and demodulate and decode higher and higher frequencies on top. One of the standards adopted by the FCC includes HD radio, which occupies [please lookup the frequencies and write them down here] bandwidth, and allows between 100-150 kbit/s of digital audio bandwidth modulated on top of regular FM Analog audio.
So why does HD Radio Suck?
Anybody who has been through Paul Lehrman’s audio engineering course (Music 65 at Tufts University) has had the pleasure of listening to a comparison between WAV and MP3 encoding. The most telling test is to generate the difference signal of MP3 encoding, which is to show what you “lose” in MP3 encoding a WAV file to 128 kbit. The result is incredible, that is to say, there is a ton of audio information removed when digital compression is used, including much of the brightness of the signal and most stereo components in the mix. It’s pretty much murdering your music.
If a radio station uses their entire 100-150 kbit/s of data for one channel (that is an HD1 simulcast of the same radio as the underlying FM broadcast), the quality is, in fact, slightly higher than FM radio (to my ears). Very cool technology.
However, being slightly better in quality does not make broadcasters more money. Instead, they have the option to split their available bandwidth into multiple sub-channels (HD1, HD2, HD3 etc), an example of Digital Multiplexing. They can sell more ads if they do this! However, this results in a digital HD1 stream that is LOWER in quality than the FM broadcast it is supposed to replace.
This would not be a problem, except for the fact that most car audio players default to listening to the HD1 stream even if a higher quality FM broadcast is available. There is a button in the menu that says “disable HD radio” which I make frequent use of on these channels. Of course on my car, the brilliant engineers make that option reset to “blindly use the HD radio feed even if it sounds like crap” every time you change channel.
Imagine: I tune to my favorite oldies station (back when 103.3 WODS actually broadcasted OlDieS…) and hear the one of my favorite songs blaring over the speakers in great quality. Suddenly, without warning 4 seconds later the station locks into the HD radio feed stream and bumps me to HD1 and the music sounds like crap. I begrudgingly take my eyes off the road to reset it back to the FM audio. A few minutes later, some godawful commercials come on and I flip to 105.7 (where there used to be only HD1 and it sounded fine). When I flip back, there comes the HD1 stream right back at me.
Motion to call HD radio Horribly Dissonant Radio.
These poorly designed receivers, motivated by the companies bankrolling HD Radio, are being forced into cars left and right. Oh well.