Robustness knob
Guard time versus payload.
Moving from A toward D increases tolerance to multipath and Doppler by spending more of the OFDM symbol on guard time or wider spacing. The cost is lower payload rate.
DRM signal notes
DRM core algorithm notes
DRM is a complete digital radio system, not just an image encoder. A picture-oriented receive succeeds only after the receiver has found the COFDM grid, decoded FAC and SDC, recovered MSC data, and reassembled the advertised data application.
Three channels
FAC, SDC, MSC.
FAC tells a receiver how to enter the multiplex, SDC describes the service, and MSC carries the payload that may eventually become audio, files, or image data.
Image boundary
DRM carries the payload.
An image receive can fail in RF sync, channel coding, packet or carousel repetition, file reconstruction, or image decoding. Keep the layers separate in station notes.
Encode pipeline
- 1. Source and data adaptation: Audio and data streams are converted into DRM service payloads. For image-related use, files or slide objects must fit the selected service capacity and repetition plan.
- 2. Multiplex into MSC, SDC, and FAC: Service payload enters the MSC; service parameters enter the SDC; rapid acquisition and robustness metadata enter the FAC.
- 3. Energy dispersal: A deterministic PRBS scrambler reduces regular bit patterns before channel coding.
- 4. Multilevel channel coding: DRM uses punctured convolutional component codes and maps the protected bit levels into 4-QAM, 16-QAM, or 64-QAM cells.
- 5. Cell interleaving and pilot insertion: MSC cells are interleaved across time and frequency; pilot cells are inserted for timing, frequency, frame, mode, and channel estimation.
- 6. OFDM waveform generation: Cells are placed on carrier indices, converted to time-domain symbols, and prefixed with the cyclic guard interval.
- 7. Image or file reconstruction: Application data such as MOT slideshow or digital image payloads is repeated according to the service plan.
Receive pipeline
- 1. Source and data adaptation: The receiver does not start with an image. It first needs a valid DRM multiplex and service description before it knows whether useful data or slideshow content is present.
- 2. Multiplex into MSC, SDC, and FAC: FAC is acquired first, SDC describes the current services, and MSC cells are decoded according to the announced constellation, protection, and interleaver settings.
- 3. Energy dispersal: After channel decoding, the same dispersal sequence is removed to recover the original multiplex bits.
- 4. Multilevel channel coding: The receiver demaps QAM cells, deinterleaves bits, and uses convolutional decoding to recover the coded channel blocks.
- 5. Cell interleaving and pilot insertion: Pilot cells let the receiver find the OFDM grid, estimate the channel, and equalise the cells before demapping.
- 6. OFDM waveform generation: The receiver removes the OFDM guard interval, performs FFT processing, tracks carrier frequency offset, and rebuilds FAC/SDC/MSC cell streams.
- 7. Image or file reconstruction: Valid MSC data is reassembled into the advertised data application; missing carousel segments can often be filled by later repetitions.
Channel coding map
DRM uses multilevel coding so that the more vulnerable QAM bit levels can be protected more strongly. The component code is a punctured convolutional code derived from a rate 1/6 mother code with constraint length 7. MSC cell interleaving then spreads adjacent payload cells across the time-frequency grid before pilot cells and OFDM symbols carry them over the channel.
| Mode family | Constellation | Overall rates | Component rates |
|---|---|---|---|
| Modes A-D | 16-QAM | 0.5, 0.62 | 1/3 + 2/3; 1/2 + 3/4 |
| Modes A-D | 64-QAM | 0.5, 0.6, 0.71, 0.78 | 1/4 + 1/2 + 3/4; 1/3 + 2/3 + 4/5; 1/2 + 3/4 + 7/8; 2/3 + 4/5 + 8/9 |
| Mode E | 4-QAM | 0.25, 0.33, 0.4, 0.5 | 1/4; 1/3; 2/5; 1/2 |
| Mode E | 16-QAM | 0.33, 0.41, 0.5, 0.62 | 1/6 + 1/2; 1/4 + 4/7; 1/3 + 2/3; 1/2 + 3/4 |
Receiver failure map
No FAC lock
Treat this as a waveform acquisition problem: wrong centre frequency, wrong bandwidth, bad SNR, unresolved Doppler, or insufficient pilot correlation.
FAC locks, no SDC
The receiver may know the robustness mode and occupancy, but not yet have the service description needed to interpret MSC payload.
Data app incomplete
If MSC data is present but the image is missing, log missing carousel objects, packet-mode gaps, decode errors, and whether a later repeat filled the same object.
Practical image-data checklist
| Layer | What to log | Why it matters |
|---|---|---|
| RF/audio | Centre frequency, bandwidth, sideband/audio chain, SNR, drift, and recording format. | Bad acquisition can mimic a broken image decoder. |
| Mode | Robustness mode A-E, spectrum occupancy, interleaver depth, and observed FAC stability. | The same image payload behaves differently under A, B, C, D, or E. |
| Coding | MSC constellation, protection level, SDC status, and any decoder error counters. | Image-data recovery depends on the DRM signal being decoded before the file layer. |
| Application | MOT slideshow, file transfer, HamDRM/EasyPal profile, object names, repeats, and final image hash if available. | This separates transport failure from image-format failure. |
Receive notes
- Mode A gives the highest DRM30 capacity in the same occupancy but assumes a comparatively benign path.
- Mode B is often the practical default when a path has fading or longer delay spread.
- Modes C and D sacrifice capacity for Doppler and delay-spread tolerance and are only defined for 10 kHz and 20 kHz occupancies.
- Mode E is DRM+ for VHF and uses 100 kHz occupancy, so it should not be treated as a narrow HF SSB digital SSTV setting.
- For amateur digital SSTV, confirm the exact EasyPal/HamDRM software profile, centre frequency, audio bandwidth, callsign practice, and file repetition behaviour.