Tutorial DRM

Tutorial: DRM (DRM30 and DRM+)

What it is. Digital Radio Mondiale (DRM) is a digital radio standard for long, medium and short wave (DRM30, below 30 MHz) and for the VHF bands (DRM+, robustness mode E, a 100 kHz channel). It gives near-FM quality over long distances, plus text, a slideshow and data services. In OnAir you tune a DRM signal, the receiver finds the robustness mode and spectrum occupancy by itself, and you listen to the audio service, read the station information and see the pictures and Journaline pages it sends.

What you need

• A radio that reaches the band. Shortwave and medium wave need an HF-capable radio (RX888, Airspy HF+, SDRplay, or an RTL-SDR in direct sampling mode) or an upconverter. A HackRF needs an upconverter or a long antenna below 30 MHz. The mode asks for 2 Msps; it works down to 48 ksps. DRM+ in VHF needs a radio rate of at least 100 ksps. See Supported radios.
• An antenna for the band: a long wire or an active loop for shortwave, a VHF antenna for DRM+.
• A frequency. The default is 9.5 MHz (DRM30, 31 m band). The tuner has Bands buttons that tune to a starting frequency: SW 41 m 7.325 MHz, SW 31 m 9.5 MHz, SW 25 m 11.65 MHz, SW 19 m 15.1 MHz, VHF I 47.1 MHz, VHF II 87.6 MHz (next to FM stations) and VHF III 174.1 MHz (between the DAB blocks). In VHF the arrows step by the 100 kHz DRM+ raster. Whether a DRM transmission is on the air in your region at the time is something to check against a broadcaster schedule: the shipped data has no station list.

Before you start

• Shortwave propagation changes with the time of day and the season. Try the 41 m and 31 m bands in the evening and the 19 m band in the daytime.
• Gain: keep the HF front end out of overload. Strong AM stations next to a DRM signal can raise the noise floor; the level shown in the Receiver tab is in dBFS of the 48 kHz (or 192 kHz for mode E) channel.
• The receiver finds the robustness mode (A to D, or E) and the occupancy by itself.

Step by step

1. In the mode list open Broadcast radio and choose DRM (second line SW, MW, VHF).
2. Press a Bands button or type a frequency, and press Start.
3. Watch the status: the receiver goes from searching to acquiring and then decoding. Meters: SNR dB, QUALITY %, AUDIO OK %, LEVEL dBFS.
4. Open the Radio tab: the audio service plays; the tab shows the service name, the Text message and the Signal summary, and Time from the signal (DRM broadcasts a clock). Pick another service in the SERVICES list if the multiplex has more than one.
5. Data services (slideshow pictures, broadcast website text, EPG, TPEG messages) appear in their own panes when a data service is selected.

What you should see

• A flat block as wide as the occupancy: 4.5, 5, 9, 10, 18, 20 kHz (and 100 kHz for mode E) shown in the Receiver tab.
• SNR of about 10 dB or more gives clean audio for the usual QAM modes; below that the audio frames fail. The AUDIO OK % meter turns green above 95 %.

Reading the screen

Receiver tab sections: Receiver (state, status, level, quality), Signal (robustness mode, occupancy, interleaver long 2 s or short 400 ms, MSC / SDC QAM mapping, protection levels, frame identity, reconfiguration, time, SNR from the pilot MER, carrier offset, sample clock offset, Doppler spread, delay spread, strongest path, guard correlation), Measurements, Counters (FAC, SDC, multiplex frames, audio frames, text segments: ok and bad), Audio (decoder state and detail), Services, Journaline (when sent), Not supported.

Options and settings

• The test signal can generate modes A to E, any occupancy, 64-QAM, 16-QAM or 4-QAM, protection levels, the long or short interleaver, a melody, tone, silence or noise-like audio, a text message, and the short-wave channel models of Annex B.1 (AWGN, Rice with delay, US consortium, CCIR poor, channel 5, channel 6) so you can see how the receiver copes with fading.

Audio codec facts

• DRM audio is AAC (xHE-AAC in newer services). The Audio section of the Receiver tab shows the decoder state and details. Codec background: Voice codecs.
• AAC with SBR or parametric stereo: only the AAC core plays (the audio library has no SBR for 960-sample frames), so the sound is band-limited, and mono for parametric-stereo services.
• xHE-AAC: the frames are received, but whether sound comes out depends on the audio library's USAC decoder; the decoder and detail lines in the Audio section say what happened.

Troubleshooting

• Never leaves "searching". The band is empty at this time, or the antenna is poor, or the radio does not reach HF. Try another frequency and time of day.
• Locks, audio frames bad. SNR too low or fading. Wait; try a different antenna or position.
• No sound but frames received. The service uses a codec the audio library cannot decode (see above). The state line says so.
• The picture or website does not appear. The data service is not selected, or it uses an application OnAir does not decode (stream mode data, single packet mode, alternative frequency applications and some others are not decoded).
• DRM+ not found. A DRM+ channel needs 100 ksps or more; DRM+ with unequal error protection is untested.

Tips

• Checked with generated signals and impairments; recordings of real signals are welcome (see Signal samples).
• Journaline text pages are in the Receiver tab, section Journaline.
• DRM+ and FM stations can share the same part of band II: use the VHF II button and watch for the 100 kHz block.

Legal and privacy notes

Listening to broadcasts is legal in most countries. Some broadcasters use conditional access; OnAir does not decode it.

Under the hood

DRM (ETSI ES 201 980, DRM+ in v4.x): COFDM with robustness modes A to D (below 30 MHz) and E (above 30 MHz), 64-/16-/4-QAM, three channels (FAC for service selection, SDC for service description, MSC for the main data), multilevel coding and a long or short time interleaver. Audio is AAC (and xHE-AAC); data uses MOT, Journaline (ETSI TS 102 979), EPG and TPEG.

Related

Mode page: DRM | HamDRM | DAB and DAB+ | General receiver | Supported radios | Troubleshooting