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PhantomGram — Paint the Airwaves

Spectrum painting for amateur radio. Encode any image as audio and transmit it through your radio — the picture appears directly on the receiver's SDR waterfall display.

No special decoder needed at the other end. The image IS the spectrum.

HOW IT WORKS

Spectrum painting exploits the way SDR waterfall displays work. A waterfall shows radio frequency on one axis and time on the other, with colour representing signal strength. PhantomGram converts each row of your image into a carefully crafted audio signal where pixel brightness maps to tone amplitude and pixel position maps to frequency. When transmitted via SSB radio, the receiver's waterfall reassembles the picture automatically.

TRANSMIT

Select an image from your photo library or camera, or use the built-in test patterns (bars, gradient, checkerboard, concentric circles). Choose a resolution preset — from QUICK (80×64, under 4 seconds) to SLOW (640×256, maximum detail). Adjust the frequency range with the dual-knob slider. The image is encoded automatically, a TX preview shows the picture as the receiver's waterfall will draw it, and the status line shows the send time. Tap SEND to transmit, and watch the cursor and timer follow the image down.

RECEIVE

Tap START and point your iPhone's microphone at a radio speaker or connect via audio cable. The live FFT spectrum shows incoming signal energy across the frequency band. Below it, a continuous waterfall display reconstructs the image in real time as it arrives. When the sender's header tone is heard, the status line says so. Adjust gain to brighten weak signals. Save decoded images to the built-in log for later viewing and sharing.

FEATURES

• Encode images as spectrum painting audio via inverse FFT with random phase
• Adjustable frequency range (0–10,000 Hz) with sensible defaults for SSB radio (200–2,800 Hz)
• Four resolution presets: QUICK, STANDARD, DETAIL, SLOW
• Real-time receive decode with live FFT spectrum display
• SDR-style waterfall colourmap (black → blue → cyan → green → yellow → orange → red)
• Optional header tone, sent before the image and detected on receive
• Status lines for encoding, sending and receiving
• Received image log with save and share
• Phosphor CRT aesthetic — green or amber display themes
• Zero external dependencies — pure Swift and Accelerate framework DSP
• iPhone speaker and microphone — no additional hardware required

TYPICAL USE

Connect your iPhone's headphone output to your radio's microphone input (or simply hold the phone near the mic). Tune your radio to an appropriate frequency, key the transmitter, and tap SEND. Anyone monitoring that frequency with an SDR receiver will see your image appear on their waterfall display.

For receiving, tune your radio to the transmission frequency, connect the radio's speaker output to your iPhone (or use acoustic coupling), and tap START.

FREQUENCY GUIDANCE

• STANDARD (200–2,800 Hz) — fits within normal SSB audio passband