FM mode now has the same three-layer architecture as PM mode:
- fm_mod/fm_demod for carrier-level FM modulation
- fm_signal_source/fm_downlink_receiver convenience wrappers
- fm_loopback_demo.py verifying round-trip SCO voltage recovery
Includes GRC YAML for all 4 blocks and doc updates across
blocks reference, SCO guide, and signal architecture pages.
Assembles the complete Apollo USB downlink signal from individual blocks:
PCM frames on 1.024 MHz BPSK + crew voice on 1.25 MHz FM, both PM-modulated
onto a single complex carrier. Receives and splits into decoded PCM frames
and recovered voice audio.
Clean: 399/402 frames, 8s voice. At 25 dB SNR: 395/402 frames.
FM-modulates real Apollo 11 onboard audio onto the 1.25 MHz voice
subcarrier (+/-29 kHz deviation) and demodulates it back, achieving
94.1% correlation with the original. Audio source: NASA/Internet
Archive public domain (Collins bidding farewell to Eagle crew).
- sco_mod: 9-channel FM subcarrier oscillator modulator (inverse of sco_demod),
with round-trip tests proving voltage recovery across all channels
- fm_voice_subcarrier_mod: add audio_input parameter to accept external float
streams (e.g., Apollo mission voice recordings) instead of internal test tone
- loopback_demo.py: streaming TX->RX round-trip with CLI for noise/voice/frames
- agc_loopback_demo.py: full Virtual AGC integration via TCP bridge
