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A software decoder that receives the Famicom's VHF RF output (NTSC-J) with a HackRF One and displays it on your PC in real time — full NTSC color decoding plus FM intercarrier audio. C++20 + libhackrf + SDL2. No GNU Radio required.

Live decode of a real Famicom (Super Mario Bros.):

Synthetic color-bar golden test output:

The HackRF tunes 2.0 MHz above the video carrier to keep its DC spike out of the signal (ch1 → 93.25 MHz) and shifts back down in software.

Note: real RF modulators drift. The unit this was developed against outputs its ch1 video carrier at 90.83 MHz (420 kHz below nominal). If you can't get sync, run --spectrum first to find the actual carrier and pass it with --freq. Envelope detection tolerates a residual offset of ±100 kHz or so.

HackRF One (Great Scott Gadgets) is an open-source SDR covering 1 MHz–6 GHz with 8-bit IQ sampling up to 20 MSPS over USB 2.0. This project uses it receive-only at 10 MSPS.

Connection: Famicom RF output (75 Ω RCA, the cable that normally goes to the TV's antenna terminal) into the HackRF's ANTENNA SMA port. A proper RCA→SMA adapter is nicer, but as the photo shows, simply joining the coax center conductor to the SMA pin works fine — the modulator output is strong, so cable losses are a non-issue. Keep the connection wired (no over-the-air radiation) and start with AMP off / moderate LNA gain; watch the clip warning and adjust with l/g keys.

The LEDs on the board: 3V3/1V8/RF = power rails, USB = host connected, RX flashes while famidec is streaming.

brew install hackrf sdl2 cmake pkg-config   # macOS; Linux needs the same libs
cmake -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build -j

Developed and tested on macOS (Apple Silicon); the code has no platform-specific dependencies beyond libhackrf and SDL2.

# 1. Check the spectrum first (find the real carrier position)
./build/famidec --channel 1 --spectrum

# 2. Live display (channel preset, or the measured frequency)
./build/famidec --channel 1
./build/famidec --freq 90.83e6

# Record raw IQ while decoding
./build/famidec --freq 90.83e6 --record cap.cs8

# Decode from a recording (hackrf_transfer .cs8 files work too)
./build/famidec --input file --file cap.cs8 --loop

# Headless: dump decoded frames as PPM (debug / verification)
./build/famidec --input file --file cap.cs8 --dump-frames out_ --frames 30

• q / ESC: quit, l / L: LNA ±8 dB, g / G: VGA ±2 dB, c: color/gray toggle, s: screenshot (BMP), h: help overlay
• v: start/stop IQ recording to famidec_rec_NNN.cs8 (a red REC counter shows while recording); replay later with famidec --input file --file famidec_rec_001.cs8 --loop. Raw IQ at 10 MSPS is ~20 MB/s (~72 GB/hour) — record short clips
• ← / →: tune ±50 kHz, ↑ / ↓: tune ±1 MHz (live retuning)
• r: CRT emulation (barrel distortion + scanlines + vignette)
• o: toggle the top-right HUD
• f: broadcast FM radio mode — retunes to --fm-freq (default 80.0 MHz TOKYO FM) and demodulates wideband FM (±75 kHz, 50 µs de-emphasis); arrows tune in 100 kHz / 1 MHz steps; press f again to return to TV. Reception depends on your antenna — the Famicom RF cable is a poor one
• Top left (big green): retro-TV style channel number (CH1)
• Top right (yellow): sync lock states and decoded FPS (V-SYNC:OK H-SYNC:OK 60.0FPS), video/audio carriers (VHF:90.83MHz AUD:95.33MHz), and measured latency (DELAY V:35ms A:88ms)
• While unlocked the display free-runs and shows snow, like a real TV; when the vsync pulses are lost in noise, frames keep flowing on the line-PLL flywheel

HackRF One (10 MSPS, tuned video carrier +2 MHz)
  → complex DC blocker (removes the tuner DC spike)
  → NCO mixer shifts the video carrier to 0 Hz
  → 4.3 MHz complex LPF (rejects FM audio & adjacent channel)
  → AM detection (envelope, or carrier-PLL synchronous)
  → AGC (sync-tip / back-porch tracking → IRE normalization)
  → sync separation + flywheel line PLL / vsync detection
  → Y/C band split (3.58 MHz BPF, Y = composite − chroma)
  → per-line color burst phase measurement → chroma QAM demod (U/V)
  → YUV→RGB, 640×480 (240p line-doubled)
  → triple buffer → SDL2 display
audio tap (pre-LPF) → −4.5 MHz mix → ÷25 decimating FIR (400 kHz)
  → FM discriminator → 75 µs de-emphasis → ÷8 → 50 kHz → SDL audio

The Famicom is not broadcast-compliant (non-interlaced 240p, chroma phase advancing 120° per line, one short line per frame), so the decoder:

• splits Y/C by frequency band, not a line comb
• detects vsync as a long-pulse region without requiring equalizing pulses
• measures the color burst independently on every line instead of relying on a burst PLL

Threads: USB callback (push-only into an SPSC ring) → DSP thread (demod → frame assembly) → main thread (SDL render). Measured ~13× real-time headroom at 10 MSPS on Apple Silicon.

./build/synth_ntsc            # synthesize color bars → decode → assert RGB
./build/synth_ntsc bars.cs8   # write synthetic IQ as .cs8 (for E2E tests)
ctest --test-dir build

MIT License — see LICENSE.

Receive-only tool. It never transmits with the HackRF One.