Technical Topics, June 1999
Digital Shortwave Update
The quest for a digital shortwave broadcast standard grinds steadily forward. The Digital Radio Mundial (DRM) steering group met recently. The VOA was there and Dr. Kim Andrew Elliott presented a synopsis on Communications World on April 24, 1999. Here is what was said. (The following is an approximate transcript from the RealAudio archive file.)
Elliott: Don Messer is Chief of the VOA Office of Engineering, Spectrum Management Division. He was one of VOA’s delegates at a DRM steering group meeting, held in Las Vegas just before the annual convention of the National Association of Broadcasters. Don told me that some progress was made at that meeting towards the development of a digital shortwave standard.
Messer: Oh yeah Kim, there was quite a bit of this. Not so much at the meeting itself, where some decisions were made, but during the past couple of months we have completed some laboratory tests on different system components to get to digital shortwave as fast as possible. And we made a big decision about a month ago on what that system should look like. In other words, as a result of these laboratory tests, those were discussed at the steering board meeting and approved. You have to understand that this is a process where you do the work and then the steering board has to approve it. At the Las Vegas meeting itself, basically we rubber stamped things that were decided about a month ago. In addition we set up a schedule for testing of the new system.
Elliott: So the system you are envisioning, is it moving towards one that is dedicated or one that combines analog and digital on the same or adjacent channels?
Messer: The final consumer product digital radio will have both the ability to receive standard double sideband AM and whatever the digital system is that turns out to be. That means you would still be able to tune in on analog programs if you choose to. The digital system itself is most likely to be a pure digital system contained within the ordinary 10 kHz channels. That’s one of the big things we are trying to do–to make sure we can get better signals, higher quality, higher reliability still within the same 10 kHz channels. The results of our laboratory tests indicate that the best way to do that is to have a single dedicated digital channel 10 kHz wide not encumbered with anything else. This is not like the development going on in the US of In Band On Channel (IBOC) systems.
Elliott: You mentioned receivers–are receiver manufacturers still involved in the DRM meetings? Are they still on board?
Messer: Yes, they are. I didn’t say this before but the DRM as a consortium has about 30 or 40 members of which about 6 or 8 are receiver or transmitter manufacturers. The main receiver manufacturers are Sony, Bosch, and Sangean. There are one or two others, but these three are doing most of the work. The Fraunhofer Institute in Germany is doing a lot of work on the source coding. So there is quite a bit of consumer awareness and understanding of what it takes to build a consumer product.
Elliott: With 10 kHz then dedicated to digital and no need to share any of that with analog, it sounds like there may be room for data transmissions as well as audio.
Messer: Yes, there are two types of data transmission we are considering. One is to support programming–what is it you are listening to, is it music, is it the VOA or BBC, things like that. Then you look to see if you have enough bits left over, can you do something else with it? You have to understand that we only have 10 kHz and we are trying to make the audio as good as possible within the 10 kHz bandwidth. That’s a top priority. The system is also being designed for medium wave transmissions–groundwave as well as skywave.
Elliott: Will the audio quality be FM quality, CD quality, how good?
Messer: Well CD quality–no, FM stereo quality–no, FM mono quality–probably. Its probably going to be around the quality of mono FM. It will certainly be better in the medium wave band (what we call AM broadcast in the USA) than current double sideband AM. In shortwave transmissions it is my guess that we may have to devote additional bits to error correction and error prevention and that will subtract from the total that we can send through. So quality will probably be less than we can do on the medium wave. But on the other hand, and this is extremely important for shortwave broadcasters, the signals will be noise free and fading free in the intended target areas.
Elliott: Can we expect some tests coming up to compare digital performance with standard analog performance under typical conditions of fading, adjacent channel interference, and co-channel interference?
Messer: Yes, Kim, we can expect comprehensive field tests by the end of this year both at shortwave and medium wave frequencies with as much variety as possible to test out multi-hop, daytime and nighttime situations, and various power levels. So I think that by the end of this year or certainly by this time next year, we will have completed an extensive number of field tests to sort of nail things down.
Elliott: I know there are some experimentalist shortwave DXers out there who like to try the latest high end receivers and techniques. Will they have a chance to participate in these tests possibly by installing a card in their computer or something like that?
Messer: There is a possibility of that, but it’s not in the plan right now. I haven’t mentioned this before but I am the head of the DRM system evaluation group and I think I will suggest this as a possibility. I can’t make a promise because it is hard to find hardware to do these tests but I will bring it up to the system evaluation group.
Elliott: Regarding jamming do you think digital shortwave will help overcome jamming?
Messer: I don’t think it matters. These are not signals that are difficult to jam. The jammer usually has the power advantage being closer to the target area. All the evidence we have so far shows that these digital signals require less power under normal conditions. So if you want to get into a jam, anti-jam war you can always crank up the power. I expect if some body wants to jam they will still be able to do it. This is not a panacea for that.
Elliott: I was thinking of countries that might restrict the sale of shortwave radios with only the digital codes transmissions they want their people to hear.
Messer: Yes that kind of selective access is certainly possible.
Elliott: And it becomes more possible with digital transmissions than with analog?
Messer: Yes.
Until next time, stay tuned.