Air band AM receive and broadcast FM receive

Status: written, not yet tested on a radio (2026-10-22). What the radios RadioFoundry supports can do on the aeronautical VHF band (AM, 108-137 MHz) and FM broadcast (65-108 MHz), and how RadioFoundry does it on any radio that has the hardware. Both are receive only and need no menu: the frequency decides. An FM channel or the VFO in the air band is received in AM; one in the broadcast band plays on the broadcast receiver.

1. Which radios

A radio has air band receive when its build has CONFIG_AM_RX and its board sets CAP_AM_RX and rx_am, and broadcast receive when it has CONFIG_BCAST_FM and sets CAP_BCAST_FM and board.bcast. Which radios do is in feature-matrix.md (am_rx, bcast_fm), generated from the boards. A radio's own facts (pins, audio path, open questions, bring-up checks) are on its hardware and bring-up pages.

2. Chips

3. Air band AM

3.1 Representation: derived from the frequency, codeplug unchanged

The open-firmware CPS codeplug has no AM modulation, and RadioFoundry keeps the standard format as long as possible. So AM is not stored: an FM channel or the VFO tuned inside 108-137 MHz is received in AM on a radio that can. This is how most radios with an air band behave, it needs no CPS change, and qdmr can program air channels today (as FM channels in that range). The band is an ITU allocation, the same everywhere, so the range is a core constant (RADIO_AM_LO_HZ / RADIO_AM_HI_HZ in core/radio/radio_am.h), not a board value. A DMR channel in the range is received in AM too: tune() sets view.ch.mode to FM in the air band and the broadcast range (listen_band() in radio_sm.c), the VFO's own mode with it, and RREQ_MODE refuses DMR there. The codeplug keeps the stored mode. Against a stored AM flag: it would need the native codeplug first, and an AM channel outside the air band has no use on these radios.

The Codeplug Editor follows the same rule (listenBand() in site/static/js/codeplug/model.js, with the air band and 65-108 MHz for broadcast): an RX frequency typed in either band turns on RX Only, the channel list shows the mode the radio uses, and the Problems list warns about a DMR channel there or a TX frequency there without RX Only.

3.2 Behavior

3.3 Board contract (hal_radio.h)

rx_am(hz, bw) tunes and receives AM; the board leaves AM at the next rx() or set_mode(). tx() is never called while AM is on. NULL = no AM. In AM status() gives:

• noise on the squelch's scale (larger = weaker; the core opens below 64 at level 1 down to 40 at level 9), from whichever reading of its receiver tells a carrier from an empty channel in AM. Only the board knows its receiver's readings, their scale and the gain it runs at, so the board makes the value.
• rssi_dbm, the signal level with any gain the receiver took off added back.
• tone_ok: anything (the core takes it as true).

On an FD6818 (fd6818_set_am, fd6818_am_status, fd6818_am_noise):

The gain loop runs at the core's status poll (RADIO_POLL_MS). These are the air band settings an FD6818 radio's own firmware uses (its hardware page has them); the loop's tick there isn't known. rx() and set_mode() end AM.

Why the squelch stays on noise_sq: the chip's own squelch (programming guide, squelch section) needs both RSSI 0x67 over 0x78 and noise_sq under 0x4F. In AM the gain loop holds 0x67 near its target whether or not a signal is there, so only noise_sq tells them apart: on the bench it read 59 to 72 on empty channels and fell as a carrier rose (about 61, 53, 45 and 31 for carriers 5 dB apart, 14 to 19 for a strong one). Level 1 opens where the chip's own squelch does (under 0x4F's noise_sq_th_in).

4. Broadcast FM

4.1 A second receiver, chosen by the frequency

Broadcast FM is wideband FM (about 200 kHz) on a separate chip, so it is a separate receiver (hal_bcast.h, board.bcast, CAP_BCAST_FM), not a channel mode. As with the air band, nothing is stored: an FM channel or the VFO tuned inside the broadcast receiver's range (board.bcast->lo_khz to hi_khz, 65-108 MHz on the RDA5807M) plays on it. The helpers are in core/radio/radio_bcast.h. The channel receiver stays on its last frequency, muted.

4.2 RDA5807M (drivers/bcast/rda5807m)

From the public datasheet (RDA Microelectronics, rev 1.1; register summary checked against done.land).

The driver tunes 65-76 and 76-108 MHz on a 50 kHz grid, keeps shadows so TUNE and SEEK are never written twice, and follows the chip's STC. The board supplies the bus; drivers/bus/i2c_gpio is a bit-banged master (open-drain SDA, push-pull SCL, no clock stretching).

4.3 Behavior

4.4 Broadcast step

Settings › Talk & listen › FM broadcast step, radio_settings_t.bcast_step, stored in settings_t.rs_bcast_step (page 1; bcast_step + 1, 0 = the default). The row shows only on a radio with the receiver (UI_FEAT_BCAST).

Either way any frequency can be typed in: 90.2 is tuned and heard as entered.

4.5 Board glue

The HR_C7000 boards share one composition, platform/hrc7000/bcast_hrc.c (hrc_bcast_init): the RDA5807M driver on a bit-banged I2C bus (open-drain SDA) on the board's two pins, an optional enable pin and level, and the board's audio path callback. A board fills in hrc_bcast_cfg_t and sets board.bcast = &rda5807m_hal; its pins and audio path are on its hardware page.

The board decides how the broadcast reaches the speaker. The core keeps the channel receiver's audio off while the broadcast plays, so the two never ask for the speaker at once (hal_bcast.h audio).

5. Build switches

The board must also set the capability bits; without them the code does nothing at run time. Per-board defaults: feature-matrix.md.

6. Tests