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DRM radio system

DRM robustness modes

Digital Radio Mondiale defines five COFDM robustness modes, A through E. They matter to digital SSTV readers because DRM-derived tools are used for file and image transmission, but the modes below are not an analog SSTV mode family: they describe a multiplexed digital radio waveform.

Read the core algorithm notesDownload the JSON table
FACSDCMSCPilot cellsOFDM guard interval

DRM sends services through FAC, SDC, and MSC channels, then maps coded cells onto an OFDM time-frequency grid. Mode E is the DRM+ VHF mode; A-D are DRM30 modes for bands below 30 MHz.

A-E robustness mode table

ModeFamilyUseful symbol TuGuard TgSymbol TsTg/TuFrameCarrier spacingBandwidthsTypical path
Mode ADRM3024 ms2.6667 ms26.6667 ms1/915 symbols / 400 ms41.67 Hz4.5 kHz, 5 kHz, 9 kHz, 10 kHz, 18 kHz, 20 kHzGaussian channels with minor fading.
Mode BDRM3021.3333 ms5.3333 ms26.6667 ms1/415 symbols / 400 ms46.88 Hz4.5 kHz, 5 kHz, 9 kHz, 10 kHz, 18 kHz, 20 kHzTime- and frequency-selective channels with longer delay spread.
Mode CDRM3014.6667 ms5.3333 ms20 ms4/1120 symbols / 400 ms68.18 Hz10 kHz, 20 kHzSimilar to Mode B, but with higher Doppler spread.
Mode DDRM309.3333 ms7.3333 ms16.6667 ms11/1424 symbols / 400 ms107.14 Hz10 kHz, 20 kHzSevere delay spread and Doppler spread.
Mode EDRM+2.25 ms0.25 ms2.5 ms1/940 symbols / 100 ms444.44 Hz100 kHzTime- and frequency-selective VHF channels.

Carrier ranges by spectrum occupancy

Spectrum occupancy codes define nominal bandwidth. For DRM30, codes 0-5 map to 4.5, 5, 9, 10, 18, and 20 kHz. For DRM+ Mode E, occupancy 0 maps to 100 kHz.

ModeOcc.BandwidthKminKmaxUnusedCarrier positions
Mode A04.5 kHz2102none101
Mode A15 kHz2114none113
Mode A29 kHz-1021020204
Mode A310 kHz-1141140228
Mode A418 kHz-983140412
Mode A520 kHz-1103500460
Mode B04.5 kHz191none91
Mode B15 kHz1103none103
Mode B29 kHz-91910182
Mode B310 kHz-1031030206
Mode B418 kHz-872790366
Mode B520 kHz-993110410
Mode C310 kHz-69690138
Mode C520 kHz-672130280
Mode D310 kHz-4444088
Mode D520 kHz-431350178
Mode E0100 kHz-106106none213

Service capacity examples

64-QAM, MSC EEP code rate 0.6Occ. 0Occ. 1Occ. 2Occ. 3Occ. 4Occ. 5
Mode A11.3 kbit/s12.8 kbit/s23.6 kbit/s26.6 kbit/s49.1 kbit/s55 kbit/s
Mode B8.7 kbit/s10 kbit/s18.4 kbit/s21 kbit/s38.2 kbit/s43 kbit/s
Mode C---16.6 kbit/s-34.8 kbit/s
Mode D---11 kbit/s-23.4 kbit/s
16-QAM, MSC EEP code rate 0.62Occ. 0Occ. 1Occ. 2Occ. 3Occ. 4Occ. 5
Mode A7.8 kbit/s8.9 kbit/s16.4 kbit/s18.5 kbit/s34.1 kbit/s38.2 kbit/s
Mode B6 kbit/s6.9 kbit/s12.8 kbit/s14.6 kbit/s26.5 kbit/s29.8 kbit/s
Mode C---11.5 kbit/s-24.1 kbit/s
Mode D---7.6 kbit/s-16.3 kbit/s
Mode E constellationMSC EEP code rateBit rate
4-QAM0.2537.3 kbit/s
4-QAM0.33349.7 kbit/s
4-QAM0.459.6 kbit/s
4-QAM0.574.5 kbit/s
16-QAM0.3399.4 kbit/s
16-QAM0.411122.4 kbit/s
16-QAM0.5149.1 kbit/s
16-QAM0.625186.4 kbit/s

EEP input bits per multiplex frame

These Annex J values are useful when checking whether a chosen image or file payload can fit the selected robustness mode, constellation, code rate, and spectrum occupancy.

ModeParameterOcc. 0Occ. 1Occ. 2Occ. 3Occ. 4Occ. 5
Mode A64-QAM Rall 0.53,7574,2487,8788,85716,37418,336
Mode A64-QAM Rall 0.64,5095,0969,45010,62819,64621,998
Mode A64-QAM Rall 0.715,3226,01811,15712,54723,19325,976
Mode A64-QAM Rall 0.785,8986,66412,36413,90825,70428,788
Mode A16-QAM Rall 0.52,5052,8325,2505,90410,91412,222
Mode A16-QAM Rall 0.623,1313,5406,5657,38113,64515,280
Mode B64-QAM Rall 0.52,8803,3126,1336,99112,72714,304
Mode B64-QAM Rall 0.63,4563,9727,3618,39015,27217,162
Mode B64-QAM Rall 0.714,0804,6928,6889,90018,02620,264
Mode B64-QAM Rall 0.784,5205,1969,63010,98019,98022,456
Mode B16-QAM Rall 0.51,9202,2084,0894,6628,4849,534
Mode B16-QAM Rall 0.622,4002,7605,1115,82610,60611,920
Mode C64-QAM Rall 0.5---5,514-11,581
Mode C64-QAM Rall 0.6---6,615-13,898
Mode C64-QAM Rall 0.71---7,808-16,406
Mode C64-QAM Rall 0.78---8,654-18,188
Mode C16-QAM Rall 0.5---3,675-7,722
Mode C16-QAM Rall 0.62---4,595-9,651
Mode D64-QAM Rall 0.5---3,660-7,800
Mode D64-QAM Rall 0.6---4,391-9,359
Mode D64-QAM Rall 0.71---5,185-11,050
Mode D64-QAM Rall 0.78---5,746-12,242
Mode D16-QAM Rall 0.5---2,439-5,199
Mode D16-QAM Rall 0.62---3,050-6,500
Mode E16-QAM Rall 0.339,938-----
Mode E16-QAM Rall 0.4112,243-----
Mode E16-QAM Rall 0.514,907-----
Mode E16-QAM Rall 0.6218,635-----
Mode E4-QAM Rall 0.253,727-----
Mode E4-QAM Rall 0.334,969-----
Mode E4-QAM Rall 0.45,962-----
Mode E4-QAM Rall 0.57,454-----

FAC, SDC, and MSC channel roles

ChannelRoleModes A-DMode E
MSC
Main Service Channel		Carries the service payload: audio frames and data streams such as file or slide data.64-QAM or 16-QAM; EEP and UEP are available for the MSC.16-QAM or 4-QAM; EEP and UEP are available for the MSC.
SDC
Service Description Channel		Describes the services and the parameters needed to decode them.16-QAM or 4-QAM with code rate 0.5.4-QAM with code rate 0.5 or 0.25.
FAC
Fast Access Channel		Carries rapid acquisition data: mode, spectrum occupancy, interleaver depth, service flags, and related controls.4-QAM with code rate 0.6; 72 input bits and 65 OFDM cells per FAC block.4-QAM with code rate 0.25; 116 input bits and 244 OFDM cells per FAC block.

Mode choice at the receiver

Mode A

Best capacity choice when the path is comparatively clean, such as stable local or regional MF/HF coverage.

Small guard interval and narrow carrier spacing maximise useful rate, but give less margin for long multipath and Doppler than B/C/D.

Mode B

General HF/MF robustness mode when fading and moderate multipath are expected.

Longer guard interval than A improves delay-spread tolerance, with lower service capacity.

Mode C

Difficult skywave or moving-path cases where Doppler spread matters more than raw throughput.

Wider carrier spacing improves Doppler tolerance, while capacity drops and only 10/20 kHz occupancies are defined.

Mode D

The most rugged DRM30 robustness mode, selected when long multipath and motion make A/B/C too fragile.

Very long guard ratio consumes capacity; useful when preserving a low-rate stream matters more than image or audio throughput.

Mode E

VHF DRM+ operation on a 100 kHz raster with the same DRM multiplex/signalling model and a much shorter frame.

Higher bandwidth and shorter OFDM timing suit VHF broadcast planning, not narrow HF SSB digital SSTV audio channels.

Digital SSTV boundary

In this reference, Mode A through Mode E mean the ETSI DRM robustness modes. Amateur digital SSTV tools may use DRM-derived profiles in narrower audio channels, so always check the exact software profile before transmitting.

Source notes