PhosphorRTTY

PhosphorRTTY decodes Radio Teletype (RTTY) signals in real time using your iPhone or iPad microphone. Hold it near a shortwave radio tuned to an RTTY signal and watch the text appear on screen, character by character, on an authentic phosphor CRT display.

No internet connection required. No external hardware. No account. Just your iPhone or iPad and a radio.


WHAT IT DECODES

RTTY is the oldest digital mode in amateur radio: two audio tones representing mark and space, encoding text via the 5-bit ITA2 (Baudot) alphabet or 7-bit ASCII. PhosphorRTTY handles the standard configurations:

Baud rates: 45.45, 50, 75, 100 Bd (auto-detected or manual)
Shifts: 170, 450, 850 Hz (auto-detected or manual)
ITA2 (Baudot) and ASCII modes
USB signals, and LSB signals with mark and space inverted


AUTOMATIC SIGNAL DETECTION

The app automatically finds and locks onto RTTY signals:

AFC (Automatic Frequency Control) acquires and tracks drifting signals
Auto-baud detection identifies the transmission speed
Auto-shift detection determines the frequency separation
Squelch suppresses noise when no signal is present
Or disable any of these and tune manually for full control.


FOUR TABS

DECODE: the main teleprinter display. Decoded text appears character by character as it is received. The status panel shows baud rate, shift, mark and space frequencies, signal quality and lock state. Clear, pause and resume, start and stop.

SIGNAL: live signal analysis. FFT spectrum display (0 to 4 kHz) with mark and space cursors, scrolling waterfall, mark and space tone indicators, bit period histogram with baud rate markers, and a paper punch tape visualisation of the decoded data stream.

LOG: session history. Every decode session is saved with timestamp, duration, character count and the full decoded text. Copy, share or delete individual sessions, or export them all at once.

CONFIG: all parameters. Audio input gain, decode mode (auto or manual for baud, frequency and shift), ASCII mode, mark and space inversion, display theme, font size and RYRY sequence display options.


IPHONE AND IPAD

On iPad PhosphorRTTY runs natively in every orientation, with larger type and the teleprinter, spectrum, waterfall and punch tape opened out across the full width of the screen.


THE DISPLAY

Two authentic phosphor CRT themes:

Amber (classic teleprinter terminal)
Green (military and commercial terminal)
Both feature horizontal scanlines, edge vignette and phosphor glow effects.
Monospaced font throughout. RYRY test sequences can be dimmed or hidden to keep the display clean.


DSP ENGINE

The signal processing is done entirely on-device using a per-sample I/Q quadrature demodulation pipeline:

Biquad low-pass filtered I/Q channels for each tone
Direct mark and space magnitude comparison for bit decisions
Digital Phase-Locked Loop (DPLL) with per-transition phase correction for timing recovery
512-point FFT for frequency analysis and AFC
Asymmetric envelope detection for robust tone tracking through fading
Tested against real over-the-air recordings from the 20m amateur band. Under 1% error rate on clean signals, 4 to 5% on moderate HF conditions with fixed tuning.


WHERE TO FIND RTTY

Amateur radio: 14.080 to 14.099 MHz (20m band), 7.080 to 7.100 MHz (40m band), 21.080 to 21.100 MHz (15m band). RTTY contests run throughout the year, and during a contest these frequencies are packed with signals.

Maritime and commercial RTTY transmissions can be found across the HF spectrum. A shortwave receiver or SDR with audio output is all you need.


REQUIREMENTS

iPhone or iPad with a microphone
iOS or iPadOS 26.2 or later
A radio receiver tuned to an RTTY signal (shortwave radio, SDR, or WebSDR audio played through a speaker)
No in-app purchases. No subscriptions. No tracking. No network access.