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‘Tricks’ of HD Radio(s) Revealed

Glen Clark Completes His Digital Tour of Three Markets in Four Days

Glen Clark Completes His Digital Tour of Three Markets in Four Days

This is the second of two installments. In the Feb. 1 issue, the author wrote about installing a Kenwood HD Radio and described how it works. He also critiqued AM and FM performance.

NEW CASTLE, Pa. A number of stations, both AM and FM, have been on the air with HD Radio for many months. Ibiquity Digital has made custom receivers, not much different from traditional modulation monitors, available to stations that purchased an HD Radio exciter. But there was no way to tell how HD Radio would perform in the mobile environment with a consumer-grade receiver.

With the recent release of Kenwood HD Radio receivers, broadcasters are free to listen to the signals of their neighbors who have implemented digital technology. For the broadcaster whose station uses HD Radio, he or she is now free to drive to the most important (or most troublesome) parts of their service areas and to observe exactly what a typical listener would observe.

Once you have an HD Radio installed in your vehicle, there are two obvious questions: Who is running HD Radio now near me? How does the sound quality compare to legacy analog? The Kenwood receiver can answer both of those questions easily, but there are two simple tricks that make it easier to answer.

Remember that the receivers were designed to be used by motorists, not as test instruments, so you have to trick it a bit. The electronics within the Kenwoods are excellent, but the user interface was designed to do listening, not exploring.

A useful trick

Oddly, there was no indicator on the front panel of the receiver that I tested (Kenwood KDC-V7022) that indicates when a station is transmitting in the hybrid digital mode. So the intuitive approach of putting the receiver in scan mode while watching to see the digital indicator light up isn’t workable.

But there is a menu on the radio that give three choices: analog only, digital only or automatic. These modes work the same way regardless of whether you are on the AM or FM band.

In “analog only” mode, the receiver does what you would expect. Scan mode will stop on every station and, even if the station is transmitting HD Radio, the receiver will ignore the digital portion.

In “digital only” mode, it will only stop on stations transmitting in digital that have a signal strong enough to decode the digital portion. If no digital signals are present in your location, the receiver will stay in perpetual scan mode, for hours perhaps, unless you interrupt it.

This can create a danger that isn’t obvious. If you set it to scan and get distracted with another matter, traffic or a cell phone call, you can forget it is scanning. When it finally comes across a digital station 20 minutes later, your speakers suddenly will come alive. Depending on where you left the volume level and how easily you are startled, it can surprise you, especially if you are alone in the car.

In “automatic mode,” the receiver will stop scanning on all stations and, if the station is providing a digital signal, will lock to the digital. If no digital signal is present, the receiver will lock to the analog.

The “digital only” mode makes it easy to see who in your area is transmitting in digital. If the receiver stops a scan when set to “digital only” mode, that station is transmitting in digital. Not all stations with HD Radio equipment installed are running it at all times. So this is a more reliable method than relying on the belief that a particular station is transmitting in digital.

Another trick

Once you establish who is transmitting digital at this moment, the next question that everyone wants answered is, “How much better does digital sound than analog?” Regrettably, there are no dedicated front-panel buttons that allow you to switch at will between analog and digital so that you can do an A/B comparison.

The choice of “digital only” or “analog only” is buried in a submenu and is not easy to get to while driving. The feel of the menus is more what you would expect to find on a Hewlett-Packard oscilloscope.

The controls do not lend themselves easily to doing an A/B comparison. But another trick gets you to where you need to be. These techniques were arrived at by trial and error and seem to work in most cases; but they are not guaranteed by the manufacturer.

Once you arrive on a frequency, it takes longer for the receiver to “lock up” on the digital signal than it takes to lock up on the analog signal. For analog, the average time seems to be under one second. For digital, the average time seems to be two to three seconds.

If you are listening to the station of interest, press the button that changes you one channel up the dial and then immediately press the one that changes you one channel down the dial. It isn’t necessary to stay on the new channel long enough to lock up on it. It is only necessary to unlock the receiver from the first channel so that it has to reacquire. The technique is the same on AM or FM.

If you perform the press-up/down sequence, and you get a first tonal balance and, a moment later, you get a second (and better) tonal balance. The first was analog, the second digital.

This gives you the ability to head the digital and analog samples side-by-side. The analog portion will only be for about a second. But this seems to be adequate for making a comparison that your brain believes.

If you want another comparison, just repeat the process. Digital circuits don’t wear out, so you can drive down the road continually performing the press-up/press-down sequence.

This is especially easy if you have the lipstick-sized remote control in your hand. Rather than hunt for the buttons on the front of the receiver in a bouncing car, your finger finds them on the remote control easily without taking your eyes off of the road.

One other point of interest is the “reset” button on the front panel. This essentially is a “hard reboot” of the receiver in case you somehow lock the receiver in a strange mode from which there is no other escape. The reset button allows you to return to the original condition.

The button is small and recessed, so there is little danger of activating it by accident. But it does do some things that you wouldn’t expect. It wipes out all of your station presets. Hopefully, future models will incorporate some non-volatile RAM here so that recovering from a lock-up condition doesn’t require you to reenter your favorite stations.

Music is like art; whatever you find enjoyable is good. There is no “wrong” music to listen to while forming opinions about how HD Radio performs. However, some cuts illustrate the clarity of digital better than others.

Generally, cuts with tambourines or cymbals are poor choices because they create a lot of midrange energy that can mask coloration created in the transmission system. A cut with a solo synthesizer riff, because the synthesizer has exceptional purity, is better at demonstrating the lack of coloration introduced by HD Radio.

The demo cut doesn’t have to be industrial-strength Euromech. If you have the ability to select the demonstration music that will be played over the air, there are many good mainstream synthesizer cuts, like “White Wedding” by Billy Idol, “Walk Like an Egyptian” by the Bangles and “Sweet Dreams (Are Made of This)” by the Eurythmics.

The first 29 seconds of Van Halen’s “Jump” (before the vocal) are worth listening to. So is the synth riff from 3:28 (when the high hat dies out) to the end. If anyone wonders whether these cuts are challenging, listen to the same cuts direct from the CD and then compare it to the output of an FM analog system.

Jackson Browne’s “Cocaine” is worth listening to critically from start to finish if you ignore the lyrics and focus on the nuances of the instruments.

I should mention that I have had several e-mail responses to the first installment of this article. Several parties expressed doubt about whether the AM demos to which they listened last April at the Las Vegas NAB convention were of “broadcast quality.” For those who have only been only loosely following the evolution of HD Radio, I should make clear that the compression algorithm has been changed since last April. The sonic performance of the present AM system is worlds removed from, and better than, what was in Las Vegas last April.

Hybrid mode is interim step

Sending digital signals is easy. So is sending analog signals. Sending them both in the same slot of spectrum in a way that they don’t interfere with each other is the hard part.

In the race to listen to HD Radio in the field and in manufacturers’ booths at trade shows, it is easy to lose sign of the fact that, several years down the road, HD Radio performance will be markedly improved.

For economic reasons, most stations cannot convert abruptly from analog to full digital, as there are no receivers in the field. Until a majority of listeners have HD Radio-capable receivers, broadcasters have to continue to provide a legacy analog signal that can be heard on the “installed fleet” of analog-only receivers.

So the first step on the migration path to digital for most stations is the “hybrid mode,” the simultaneous transmission of the analog and digital signals.

In the future, when analog receivers are to that day what 8-track tape players are to 2004, there will be no need to continue to transmit the analog signal. Broadcasters will be able to convert to full digital.

At that time, there will be two distinct factors that will make the sonic quality of the HD Radio signal even better. First, there would be no self-interference. That is, there will be no analog signal confusing the digital decoder in weak signal areas, so digital coverage will extend farther.

Second, when the analog signal is taken away as a source of interference to other stations on the channel, the power of the digital signal can be increased. This allows the digital signal to exceed the atmospheric noise where it previously did not. The result, again, is extension of coverage.

I had the opportunity to listen to the Ibiquity AM test transmitter when it was in full-digital mode. This was not planned. It was simply a coincidence that I was driving into the Baltimore area as Ibiquity was running tests. But the Kenwood receiver immediately recognized the full-digital protocol and switched into that mode. The audio quality was impressive.

Some broadcasters have proposed that a few AM stations may want to go to full-digital mode from the very first day. Stations that, because of limited coverage areas, now have no ratings or cash flow and have little to lose would be logical candidates. This may be the kilowatt daytimer on the edge of town that has had many owners, and even more formats, over the last decade.

There is the possibility that a full-digital AM station might capture the same kind of “cult mystique” that FM stations held in 1970. Without expectations for cash flow and profits, there would be opportunity to experiment with new music and new formats.

Additionally, a full-digital AM station might benefit its larger sister station by pulling more HD Radios into the market at an earlier date. If listeners know that there is a station in the market that already transmits the sonically-impressive full-digital signal, it could stimulate interest and digital receiver ownership.

Digital nighttime AM

Listening to AM HD Radio at night sometimes will produce results that, if you aren’t familiar with the system architecture, can be puzzling. There may be places where you intuitively expect to be able to receive a digital signal and the receiver will not go into digital mode.

Conversely, there may be places where you intuitively don’t expect to be able to receive a digital signal, yet the receiver instantly locks up in digital mode. A quick look at the AM spectrum layout can help to make sense of these puzzling results.

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The sketch in the diagram shows the spectral arrangement of the AM digital system. There are six sets of subcarriers called “ensembles,” each 5 kHz wide.

The power within the ensembles decreases closer to the carrier frequency. Ensembles E1 and E6 in the figure are called the “Primary” sidebands. They carry the L+R information. The data in E6 is identical to the data in E1.

In the event that E1 is corrupted at a particular instant by an adjacent-channel station on the low side, the chances are statistically small that E6 will be corrupted at exactly the same instant by an adjacent-channel station on the low side. Because the data in these two ensembles are identical, if either one of them is received correctly, the receiver can continue in digital mode.

Ensembles E2 and E5 in the figure are called the “Secondary” sidebands. They carry the L-R information. The data in E5 is identical to the data in E2.

In the past, AM stations were most concerned about the size of their nighttime interference-free contour. While there is some consideration given to adjacent-channel stations in the calculation of the analog NIF contour, the NIF contour value is determined mostly by the amount of incoming skywave received on the same channel as the station of interest.

When operating at night with HD Radio, stations will now be interested in a more complete picture of what skywave is arriving. One station may have a noisy channel, 10 kHz above its own frequency but a quiet channel 10 kHz below its own frequency. Another station may have two quiet adjacent channels while a third station may have two noisy adjacent channels.

Each of these situations will create a different mosaic of where the AM HD Radio signal can be received at night and where it cannot be received.

Finally, after years of waiting, broadcasters and consumers finally can own a mass-produced radio that receives both AM and FM digital signals. There will be more models released by more manufacturers soon.

Broadcasters now can judge for themselves just what digital brings to the table. The performance that I observed in a mobile environment was impressive.

Realizing that HD Radio performance will improve further when the hybrid signal is replaced in the future by the all-digital signal, the decision of whether to implement HD Radio gets even easier.

Tell us about your own experiences with HD Radio. Write to us at [email protected]