Editor’s Note: The following column originally appeared at RBR + TVBR on April 10, 2008. Given the scattershot rollout of HD Radio multicast offerings on a national level, Hadfield’s words remain highly relevant. Hadfield presently serves as Director of Engineering at iHeartMedia’s Seattle-Tacoma operations. Since he penned this column, Hadfield has also been Director of Engineering for Portland, Ore.-based Alpha Media.
FM HD Radio programming considerations for your HD3 channel
By Marty Hadfield
You have probably heard it said that in the programming battles with Sirius XM, one significantly competitive edge HD Radio has going for it is the comparatively better fidelity over satellite programming.
While that may be considered to be true for your HD1 and HD2 channels, it would be a stretch of the imagination to apply that thought to your HD3 channel.
The reason? Every time another HD channel is added to your initial HD1 on-air bit stream, the available program fidelity is reduced. By the time you are adding HD3, while keeping the fidelity relatively high on HD1 and HD2 channels, the quality available for HD3 approaches that of a poorly equalized telco loop.
The typical programming response to this situation is to head toward a talk-based format, thinking that voice sounds better than music under low bandwidth conditions. However, there are important differences between an old analog telco loop and your HD3 that makes a talk format perhaps one of the worst choices.
The telco loop example usually just limits the lower and upper frequency response, making music sound poor but speech carried through as reasonably acceptable.
In contrast, with virtually all digital encoding, the digitizing process eliminates small slices of the fidelity of the original program content to produce a form of reduced bit rate encoding. With most music programming there is so much content in the original source material that the overall process is designed to “mask” a great deal of the sliced out pieces. On the other hand, with the spoken word, our hearing is easily annoyed by those missing slices of voice fidelity.
This is particularly disturbing to the human ear when coupled with other audible side-effects of low bit rate audio, such as your announcers sounding like they are underwater with swirling/gurgling sounds. If their speech goes on and on, it can be really distracting. If you have an opportunity to listen to satellite-fed programming sometime, you will probably hear all of these effects.
One other cautionary note about HD3 – while most music formats may sound pretty good, oddly enough, Rock formats often sound quite bad at HD3 transmission rates.
[As of April 2008], I have only heard one spoken word format that I believe sounds reasonably acceptable on HD3. Some non-commercial stations are running World News and other BBC World Service programming on their HD3 channel. The resulting programming sounds reminiscent of listening to distant broadcasts on Shortwave, which is where most of us outside of Europe would have heard the BBC.
Under most conditions, each of your HD channels should use audio processing appropriate for both the type of program content and the available bit rate. There are many audio pre-processors available specifically for use on FM HD Radio. These are available through your usual list of broadcast equipment vendors.
If you have a really sharp engineer that is well versed in the art of preconditioning audio for reduced bit rate transmission, they can probably knock together a custom processing package.