At the Winter SWL Fest, DRM digital shortwave worked, except when it didn't.

The Winter SWL Fest is the world’s largest annual gathering of shortwave listeners and other radio listening enthusiasts. While the Fest is a celebration of 75 years of analog shortwave broadcasting, the event is also a showcase for the new media of international broadcasting.

A exhibit of Digital Radio Mondiale, the new technology for digital transmission below 30 MHz, has been held at the Fest since 2003. This exhibit features DRM-capable receivers picking up actual DRM transmissions.

At this year’s SWL Fest, March 8 to 10 near Philadelphia, the DRM exhibit expanded to include other forms of digital radio: 1) wi-fi internet radio appliances and 2) HD Radio (IBOC), the U.S. domestic digital radio system.

The DRM exhibit

No standalone receiver was available in the North American market in time for this year’s DRM display. We used instead 1) a Ten-Tec RX-320D “black box” HF receiver, 2) a Kenwood TS-2000 amateur transceiver, and 3) a Yaesu FT-847 amateur transceiver. All three units were connected to personal computers with DRM software installed. The Kenwood and Yaesu transceivers had been modified to provide the necessary 12 kHz IF output. A large doublet antenna was installed on the roof of the hotel where the SWL Fest was held.

The transceivers generally provided better DRM reception than the Ten-Tec. The RX-320D, however, costs only $400 and thus is one of the least expensive receivers with DRM capability. Its IF audio output appears to be set at a level too high to operate well with the DRM software. We improved this by using an attenuating adapter into the microphone input of the laptop PC. The RX-320D performance was improved even more by using a Soundblaster Extigy external sound card, with the microphone gain turned all the way down.

In addition to the DRM transmissions already in regular operation, we arranged for special transmissions during the time of the Fest. We were especially interested in attempting trans-Atlantic DRM reception –- realizing that transoceanic distances are probably beyond the expected capabilities of DRM shortwave.

Vatican Radio. Vatican Radio added special transmissions at 1300-1400 UTC on 15460 kHz. On 9 March at 1300 UTC, we were able to see the data ID for Vatican Radio on the computer screen, but we could not hear the audio. After a telephone conversation with Vatican Radio engineers, the modulation mode was reduced from 64-QAM to 16-QAM. With this adjustment, the Vatican Radio audio signal became audible. It remained audible for the rest of the transmission on 9 March and throughout the hour on 10 March. Audio degraded to the “echo” sound, but only on a few occasions. At one point during the program, selections of opera were played: opera never sounded so good on shortwave. This was the first sustained trans-Atlantic reception of DRM at the SWL Fest.

VT Communications. VT in the U.K. offered the Fest special DRM transmissions at 0000-0400 UTC on 6155 kHz. These were from Rampisham, at 33 kW. In general, we were not successful in maintaining an audio signal on this transmission, although the data ID was usually visible. At one point, we sustained audio for twenty minutes. It was not possible for VT to reduce the bitrate to improve the chances of sustaining the audio. In contrast to Vatican Radio, the VT signal had to follow a more northerly azimuth to reach our receivers, and the 49 meter band is particularly crowded at this time of the day.

HCJB (Ecuador). HCJB transmitted from Quito, Ecuador, to the fest using only four kilowatts. At first, audio reception was usually unsuccessful. After frequency changes (to 15200 kHz during the day, and 9800 kHz at night) and a lowering of the bitrate, DRM audio reception was consistent. However, the bitrate was 9.8 kbs (or thereabouts), producing a telephone grade audio. But given the modest power output, the DRM reception from HCJB was impressive.

TDF (France). We monitored the TDF DRM transmissions of Radio France International from Montsinéry, French Guiana, from 1200 to 2000 on 17875 kHz. Audio reception was usually successful. The TDF relay of Radio Netherlands on 15425 at 2200-2300 UTC was flawless.

Radio Canada International. RCI transmitted various programs from its Sackville, New Brunswick, site during the day on 9800 kHz. These were usually audible, although there were periods when the audio dropped out. Transmissions from Sackville’s northerly location sometimes suffer from poor propagation. The RCI relay of China Radio International at 0100-0200 UTC on 6080 kHz was completely audible.

DW Sines. We were not able to hear the audio from the Deutsche Welle DRM transmission via Sines, Portugal, on 3995 kHz (beamed to Europe), but we did see the data ID.

Assessment of DRM reception. Our DRM listening was mostly successful. However, audio dropped out on enough occasions that the non-enthusiast shortwave listener would likely be frustrated.

Very careful frequency management will be necessary to make DRM work on shortwave circuits over medium to long distances. This might be aided by remote monitoring stations that automatically receive the DRM transmission and send signal information back to the transmitter via the internet. If the audio signal drops out, the transmitter would adjust the QAM and/or bitrate levels until audio is achieved. A frequency change might even implemented. This could automatically trigger a frequency change in a “smart” receiver, or at least place a text message on the receiver display announcing that a frequency change is imminent.

While we were listening to HD (IBOC) radios that were part of the exhibit, we noticed that when the station dropped below a certain signal level, the radio would revert to the analog mode, thus retaining the station’s audio. This is an attractive feature. I am not advocating an IBOC system for shortwave, but if the DRM transmitter and receiver can automatically switch to analog under the worst reception conditions, this might maintain audio where it would otherwise be lost.

Will DRM be useful for long haul shortwave? The sole remaining advantage of shortwave in this modern multimedia age is that shortwave can deliver a signal under adverse conditions. Shortwave can overcome jamming better than satellites can overcome jamming, and better than websites can circumvent blockages. Shortwave can deliver a signal into a distant target country when a nearby relay is not available.

DRM is more sensitive to reductions in signal strength, and to the presence of interference. As such, DRM could eliminate the last remaining advantage of shortwave. Analog shortwave will always be needed as the failsafe for international communication.

Perhaps DRM makes better sense for local and regional applications, such as longwave, medium wave, and short-hop shortwave broadcasts. Radio New Zealand International is already an example, with its feeder transmissions to radio stations in the Pacific region. Radio Canada International might experiment with a transmitter in southern Ontario, using 2 or 3 MHz bands at night, and 5 or 6 MHz by day, for DRM early adopters in North America.

It would be good to see more use of text by DRM broadcasters, given that the text content often survives even when audio is lost. International broadcasters could fairly easily feed their news scripts into the DRM text transmission facility.

But the power and bandwidth required for DRM is probably overkill for text transmission. International broadcasters might also experiment with amateur radio’s PSK-31 mode. Using only a narrow sliver of spectrum, PSK-31 text messages are usually receivable even under appalling reception conditions. Small receivers could be developed that could, for example, be affixed to hotel windows. They could receive news to be read in real time or stored for later reading.

We would also like to try side-by-side tests of DRM and analog transmissions, similar to the promotional audio samples showing the benefits of DRM. For example, is DRM that is reduced below 10 kilobits per second an improvement over analog using the same amount of electricity?

In these side-by-side comparisons, the analog side should include reduced carrier single sideband transmissions. Using receivers with synchronous detection, this type of sideband transmission would eliminate selective fading and reduce interference. Analog receivers with synchronous detection might be easier to manufacture, and might have lower battery consumption, than those than can receive DRM.

Internet radio

In our digital radio display, we also displayed two wi-fi internet radios. One was the Slim Devices Squeezebox, which would not work with the hotel’s wireless system. The other, the Acoustic Energy Wi-Fi Internet Radio, worked very well. On the AE, we listened to Vatican Radio at the same time we were listening to the same station via DRM shortwave.

Listeners at the Fest were impressed with how easily the AE could receive audio streams from VOA New Now, BBC World Service and the domestic BBC channels, Radio Prague, and many of the other 10,000 radio stations available through its menu. Now that internet radio can be listened to on a device that looks, feels, and operates like a radio, and is portable at least within range of a wi-fi node, it may have potential as a replacement for shortwave radio for the reception of foreign broadcasts. These devices could be especially interesting, and usable in cars, where “city wide” wi-fi will be available.

To be sure, internet radio requires a broadband connection, in a country where your favorite station is not blocked by the authorities. But a person who can afford the first generation of DRM receivers is more likely than the general population to afford a broadband connection. And if a station is blocked via the internet, then chances are its DRM signal would also be jammed.

In a time of local crisis, the internet can fail locally, due to overuse or to sabotage. In a time global crisis, it can fail globally, for the same reasons. The world must return to shortwave for information.

We shortwave listeners who are experimenting DRM want to continue to help push DRM to its greatest capability. However, because DRM shortwave will probably be easier to jam than analog shortwave, we continue to need analog shortwave for the proverbial rainy day.

Posted: 22 Mar 2007