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README.md

gr-rx888 example flowgraphs

am_bcb_audio_demo.grc — booth default (with audio)

Tunable AM Broadcast Band receiver. Drag the Tune slider across 540–1700 kHz, drag the Volume slider, hear the station. Waterfall and PSD show what the tuner is on. AM BCB picks up reliably even on a thumbtack antenna — this is the "always works" demo.

rx888.source (32 MS/s real)
  → float_to_complex (imag = 0)
  → freq_shift_cc (-center_freq, retunes live)
  → rational_resampler_ccc (decim=16) → 2 MS/s complex
  ├→ qtgui_waterfall_sink_c + qtgui_freq_sink_c  (±1 MHz around tune)
  └→ rational_resampler_ccc (decim=10) → 200 kHz complex
     → analog.am_demod_cf (audio_decim=4, 3 kHz audio LPF) → 50 kHz real
     → rational_resampler_fff (24/25) → 48 kHz
     → multiply_const_ff (volume slider)
     → audio.sink (ALSA default → pulse plugin → host PulseAudio)

The extra decim stage before am_demod_cf keeps the block's internal optfir.low_pass Parks-McClellan design inside a sane numerical range — going directly from 2 MS/s to 50 kHz audio asks for several thousand filter taps and pm_remez bails.

Use this inside the Docker image via the audio + X11 + USB run line in ../docker/README.md. The container's audio.sink writes to ALSA's default device, which is wired through the pulse plugin to the host's PulseAudio/PipeWire — no contention for /dev/snd. Skip the pulse bind-mount and the audio sink still fails to open at start.

am_bcb_demo.grc — silent tuner variant

Same tuning + waterfall + PSD as the audio version, no audio output. Use when running headless / on a host without ALSA / when you only want the visualization.

hf_waterfall_demo.grc — full-spectrum diagnostic

The whole 0–16 MHz Nyquist span at 32 MS/s real. No tuning, no decimation — useful for spotting the DC spike, image responses, spurious lines, and verifying the hardware is generally alive. Open from GRC manually:

gnuradio-companion /opt/gr-rx888/examples/hf_waterfall_demo.grc

halfband_coefs_fixed.csv — pre-designed half-band taps

A ~493-tap linear-phase half-band low-pass filter (half the taps are zero by construction, center tap = 0.5, cutoff at fs/4). Drop into a GRC variable_file_filter_taps block when you want to cascade cheap decim=2 half-band stages — much cheaper at high sample rates than a single fat rational_resampler with auto-generated taps.

Typical use (e.g. 32 MS/s → 2 MS/s in four halfband decim=2 stages):

... → freq_shift_cc → rational_resampler_ccc[decim=2, taps=halfband]
              → rational_resampler_ccc[decim=2, taps=halfband]
              → rational_resampler_ccc[decim=2, taps=halfband]
              → rational_resampler_ccc[decim=2, taps=halfband]
              → sink

Worth swapping in when running at 135 MS/s where the single-stage decim is a CPU pig; not necessary for the 32 MS/s booth demo.