Tutorial HamDRM

Tutorial: HamDRM (digital SSTV pictures)

What it is. HamDRM, used by the programs EasyPal and QSSTV (its "DRM" mode), sends pictures and other files as a DRM-style OFDM signal that fits into a normal SSB channel of 2.3 or 2.5 kHz. Radio amateurs use it for digital slow-scan television: the file is sent several times in segments, so a receiver that misses a segment can fill the gap on the next pass. In OnAir you tune upper sideband around 14.233 MHz, the receiver finds the signal and collects the file, and the picture appears in the HamDRM view.

What you need

• A radio that reaches HF (RX888, Airspy HF+, SDRplay, or an RTL-SDR in direct sampling mode, or an upconverter). The mode asks for 2 Msps and works down to 250 ksps. See Supported radios.
• An HF antenna for the 20 m band (a dipole or a long wire).
• A frequency. The frequency database lists 14.233 MHz (20 m, the digital SSTV calling frequency) as the HamDRM entry, and that is the mode's default. Upper sideband is used.

Before you start

• This is an amateur radio service. Activity depends on the time of day and propagation; 20 m is typically best in the daytime.
• The decoder expects the audio centred on 1600 Hz, so the dial frequency is the SSB suppressed-carrier frequency. OnAir handles that.
• Honest note from the code: the decoder follows the published description of the EasyPal/QSSTV format and has been tested with OnAir's own generator; it has not been compared with real EasyPal or QSSTV recordings. Recordings from users are welcome.

Step by step

1. In the mode list open Amateur: digital and choose HamDRM (second line Digital SSTV, 14.233 MHz).
2. Press Start. The tab is named Decoders and shows only the HamDRM view in this mode.
3. Watch the status line: searching (modes A, B, E around 1600 Hz), then locked on mode X, then receiving: mode A/B/E, 2.3 or 2.5 kHz, 4/16/64-QAM, SNR n dB, tuning ±n Hz.
4. When all segments arrive the picture is shown. With Save every picture ticked, files are written as they were sent into the hamdrm folder of OnAir's data folder.

What you should see

• A narrow, noisy-looking block of about 2.5 kHz in the waterfall. HamDRM sounds like rushing noise with a faint ripple.
• The status line counts frames ok and bad. A picture builds up from "n / m segments" and "missing: ..." tells you which segments you are waiting for (and "waiting for a repeat" when the transmission ended).
• A list Received shows each completed file with the time, the file name and the sender's callsign if it was sent.

Reading the screen

• The file name and callsign, the segment progress bar, and the picture preview.
• Save writes the selected file; Clear empties the list. Pictures also go to the gallery.
• Files that are not pictures show "received (not a picture this program can show)"; they are still saved with Save.
• The status bar lamp HamDRM shows lock.

Options and settings

• Save every picture (keeps all, as sent).
• The test signal (Tx panel and the synthetic source) can generate modes A, B and E, 2.3 or 2.5 kHz and the QAM orders, so you can test without a signal.

Troubleshooting

• Nothing happens. The mode needs a transmission. Check the frequency, the sideband (USB) and that the band is open.
• Locked, but frames bad. Fading or QRM. The sender repeats the file; wait for another pass.
• Wrong mode shown. The decoder tries modes A, B and E; give it a few seconds.
• Picture incomplete. Segments missing; wait for the repeat.

Tips

• Keep the receiver running; a single sending takes a while and may be repeated.
• HamDRM is not the same as broadcast DRM: the occupancy is far narrower and the audio services are replaced by files.

Legal and privacy notes

Amateur radio transmissions are public but intended for amateurs. Receiving is generally legal; licensing and content rules for transmitting apply to your own station only. OnAir's Transmit feature can send HamDRM-style signals only under your own licence (see Transmit).

Under the hood

OFDM with modes A, B and E (the DRM robustness modes with their guard intervals), 2.3 or 2.5 kHz occupancy, 4-/16-/64-QAM, the DRM multilevel decoder and scattered pilots; the file is cut into data groups with a CRC-16 and MOT-style segments, sent repeatedly. The receiver resamples to 12 kHz, correlates the guard interval for mode and timing, finds the frame with the reference symbol and decodes the cells.

Related

Mode page: HamDRM | DRM | General receiver | Transmit | Supported radios