PhosphorDSC — Decoder Test Results

Accuracy testing of the PhosphorDSC decoder against real over-the-air recordings of HF DSC traffic. The testing CLI (DSCDecode) shares every DSP source file with the iOS app via symlinks — same code, same output.

Decoder stack

Sample rate: 8 kHz (resampled from source if needed)
FFT: 512-point Hann-windowed, ~15.6 Hz/bin
Tone detection: I/Q quadrature demodulation with biquad low-pass filtering (102 Hz cutoff at 170 Hz shift); asymmetric decay-average envelope tracking
Bit decision: Y / B magnitude comparison with hysteretic squelch (open > 0.002, close > 0.001)
Clock recovery: DPLL with per-edge phase nudge, α = 0.05
Character code: ITU-R M.493 ten-bit error-detecting code, seven information bits plus a three-bit count of B elements
Synchronisation: phasing-sequence search over the received bit history, locked on three phasing characters including at least one RX, per M.493 Annex 1 § 3.3; polarity auto-detected via a parallel bit-inverted lane
Time diversity: each character taken from its DX copy, or its RX repeat four characters later when the DX copy fails
Integrity: error-check character (even vertical parity over the information characters) verified on every call
AFC: parabolic sub-bin peak interpolation (3-point fit), centre-frequency tracking with snap-on-acquire and ±3 Hz/frame clamp in steady-state, 1 Hz retune threshold; 170 Hz shift snapped to spec (±85 Hz around centre)
Search band: 800–2500 Hz, spanning both common audio-tuning conventions (see Scope below)

Real-world recordings

Tested against the publicly-available HF DSC recordings on the Signal Identification Wiki GMDSS DSC page. These are over-the-air captures — no cleanup, no SNR preconditioning.

Recording	Audio centre	Calls decoded	Content
GMDSS_1	~975 Hz	1, ECC OK	Safety test call, ship 215322000 to coast station 004634060
GMDSS_2	~1709 Hz	1, ECC OK	Safety test acknowledgement, coast station 002191000 to ship 230145000 (inverted polarity)
GMDSS_3	~2245 Hz	1, ECC OK	Safety call, ship to ship, working on 12360.0 kHz
Dsc_examples	~1675 Hz	2, ECC OK	Routine call and its acknowledgement between two coast stations, working on 8291.0 kHz

Decoded output — GMDSS_1

--- TEST CALL | ECC OK --- FORMAT INDIVIDUAL TO 004634060 COAST STATION CATEGORY SAFETY FROM 215322000 SHIP TC1 TEST TC2 NO INFORMATION END ACK RQ SYMBOLS 120 120 000 046 034 006 000 108 021 053 022 000 000 118 126 126 126 126 126 126 126 117 085

Decoded output — Dsc_examples

--- ROUTINE CALL | ECC OK --- FORMAT INDIVIDUAL TO 002320001 COAST STATION CATEGORY ROUTINE FROM 005030001 COAST STATION TC1 J3E TP TC2 NO INFORMATION RX FREQ 8291.0 kHz TX FREQ 8291.0 kHz END ACK RQ SYMBOLS 120 120 000 023 020 000 010 100 000 050 030 000 010 109 126 008 029 010 008 029 010 117 085 --- ROUTINE ACK | ECC OK --- FORMAT INDIVIDUAL TO 005030001 COAST STATION CATEGORY ROUTINE FROM 002320001 COAST STATION TC1 J3E TP TC2 NO INFORMATION RX FREQ 8291.0 kHz TX FREQ 8291.0 kHz END ACK BQ SYMBOLS 120 120 000 050 030 000 010 100 000 023 020 000 010 109 126 008 029 010 008 029 010 122 090

Download Test Files

These WAV files are synthesised DSC calls built to ITU-R M.493 — 200-bit dot pattern, phasing, DX/RX time diversity and a computed error-check character. Every one decodes with its error check passing. They can be played from any audio source (laptop, second phone) near the iPhone’s microphone to test the app without a shortwave receiver. All MMSIs are fictitious.

File	Description	Size
dsc_01_distress_alert_sinking_off_fastnet.wav	Distress alert: sinking, 51°23'N 009°36'W, 14:25 UTC	~150 KB
dsc_02_distress_relay_all_ships.wav	All-ships distress relay from a coast station: flooding, no position	~150 KB
dsc_03_urgency_all_ships.wav	All-ships urgency call, working on 8291.0 kHz	~150 KB
dsc_04_safety_geographic_area.wav	Safety call to a geographic area, 55°N 015°W, 10° × 20°	~150 KB
dsc_05_routine_individual_call.wav	Routine individual call to a coast station, 8414.5 kHz	~150 KB
dsc_06_safety_test_call.wav	Safety test call	~150 KB
dsc_07_safety_test_call_ack.wav	Safety test call acknowledgement	~150 KB
dsc_08_routine_sdr_tuning_1000_hz.wav	Routine individual call with tones near 1000 Hz (SDR tuning)	~150 KB

Scope

HF and MF DSC only. VHF channel 70 DSC runs at 1200 Bd behind an FM receiver and is not decoded. Two audio-tuning conventions are supported:

Maritime-receiver convention: USB dialled 1.7 kHz below the channel, placing the tones at 1615 Hz (Y) and 1785 Hz (B), the 1700 Hz centre ITU-R M.493 specifies.
SDR / WebSDR convention: USB dialled 1.0 kHz below the channel, placing the tones near 915 and 1085 Hz.

AFC search band is 800–2500 Hz to span both, and the decoder runs normal and polarity-inverted lanes in parallel so LSB and USB captures both decode without any manual toggle.