PhosphorFeld

PhosphorFeld decodes Hellschreiber (Feld-Hell) signals in real time using your iPhone's microphone. Point your phone at any shortwave radio tuned to 14.063 MHz and watch the letters paint themselves onto your screen, pixel by pixel, exactly as Rudolf Hell intended when he invented the system in 1929. Hellschreiber is the oldest digital mode still in active use. Unlike RTTY or FT8, which transmit character codes, Hell transmits the actual pixel shapes of each character as an on-off keyed audio tone. There is no character set, no encoding table, no error correction. The signal paints its own letters. Your eye does the decoding. It is the only digital mode where the decoder is the human brain. DECODING Real-time Feld-Hell reception via microphone or line input. The scrolling glyph strip displays the received signal as a greyscale bitmap — the traditional "fuzzy" presentation that lets your eye-brain system extract readable text from noise, without any hard thresholding or decision-making by the software. Adjustable gain, contrast, and squelch controls are accessible directly on the decode screen. SIGNAL ANALYSIS Live FFT spectrum display with tone frequency cursor. Scrolling waterfall spectrogram. Real-time envelope trace showing the on-off keying pattern of the Hell signal. Automatic tone detection and tracking (AFC) acquires the strongest tone in the Hell bandwidth and follows frequency drift. SESSION LOGGING Every decode session is saved with full-resolution glyph strip data stored as a compressed PNG image. Browse previous sessions with thumbnail previews. Open any session to view the complete decoded strip at full resolution with vertical scrolling. Export sessions as PNG images via the share sheet. DISPLAY Greyscale glyph strip rendering with pixels taller than wide, matching the aspect ratio of traditional Hellschreiber receivers. Three glyph height settings. Multi-line wrap with automatic scroll — columns paint left to right, wrap to new lines, and scroll up like a teleprinter paper feed. Amber and green phosphor CRT themes for the interface chrome. CONFIGURATION Automatic or manual tone frequency selection (500–3000 Hz). Adjustable squelch threshold to blank noise during silence. Signal polarity inversion for LSB/USB sideband selection. Glyph height and phosphor colour theme selection. COMMON HELLSCHREIBER FREQUENCIES 14.063 MHz (20m), 7.035 MHz (40m), 3.580 MHz (80m), 10.137 MHz (30m). Tune any shortwave receiver to USB, point your iPhone at the speaker, and press START. Or just reply a previously recorded signal. TECHNICAL Standard Feld-Hell: 122.5 baud pixel rate, OOK modulation, 980 Hz default tone. I/Q quadrature demodulation with biquad low-pass filtering. Peak-hold pixel sampling. Column-level squelch. 512-point FFT for spectrum analysis and AFC. All DSP runs entirely on-device at 8 kHz sample rate. Zero external dependencies. PRIVACY Microphone access required for signal decoding. No network access. No analytics. No tracking. No third-party SDKs. No account required. Session data stored locally only. PhosphorFeld is part of the Phosphor series of radio signal decoders for iPhone.