Single frequency repeater (SFR)

A single frequency repeater receives a DMR call on one TDMA slot of a frequency (the inbound slot) and repeats it on the other slot of the same frequency (the outbound slot), 90 ms later. Radios in direct mode hear both copies. Switch: CONFIG_SFR, on with DMR except where a board lists it as unsupported (the HR_C5000 has no CRC_MCU_Control for the idle bursts of section 3 item 4, gotchas 14.6) or not written (see docs/build-config.md). Background and the HR_C6000 findings: docs/spec/gotchas.md 8.11, 8.12 and 14; the host protocol: docs/spec/mmdvm-protocol.md 6.

There are two kinds:

1. Layers

The HR_C6000 glue in drivers/modem/hrc6000/sfr_glue/ is not built; it documents what a driver level SFR did (raw RX RAM reads, RF switching per burst) and holds the extra status bytes' layout.

2. What is built

• MMDVM SFR. The host turns it on with SET_CONFIG's simplex flag clear and SET_FREQ RX = TX (both non-zero), evaluated after either command; the simplex flag set, RX != TX, a retune or the session's end turns it off.
  • Uplink: every burst heard on the inbound slot goes up as DMR_DATA1 (inbound TS1) or DMR_DATA2, framed as the simplex uplink frames them (headers and terminators from their LC, voice with the EMB of its position and the call's embedded LC in B-E); a call heard without its header gets a header made from the embedded LC first, once voice F has given it. DMR_LOST1 / DMR_LOST2 300 ms after the last burst of a call without a terminator. Our own repeat never goes up.
  • Downlink: either slot's DMR_DATA command feeds the outbound slot (both space bytes carry its space, up to 16). Each frame is ACKed or NAKed: 6 RF busy (an inbound call is being repeated; the rest of a stream an RF call cut into), 5 full, 4 a burst type that can't be sent. Idle bursts and repeated terminators are ACKed and dropped. DMR_ABORT, DMR_START and DMR_SHORTLC are ACKed (the radio owns its keying and sends no CACH).
  • Defaults: TS1 in, TS2 out, TS framing (the ETSI direct mode sync of the outbound slot), 1 s hang, 180 s TOT, the color code from SET_CONFIG. hotspot_sfr_set_settings() takes a channel's settings once a UI or codeplug provides them; command 0xD5 overrides the framing at run time.
• Runner behavior (gotchas 14):
  • a burst the modem places on the outbound slot (ts = the outbound slot) is ignored: our echo or another repeater's;
  • own echo: a burst with the kind, position and payload of one we put on air in the last 250 ms is dropped (the HR_C6000 reports what it sent in empty inbound slots), except a voice header or terminator while an inbound call is open: a header run is identical copies (gotchas 14.10);
  • late entry: outside a voice call a lone voice A is held; a B 60 ms after it starts the call;
  • while keyed (or for a burst the modem reports out of step) the checksums are checked here, not taken from the chip: RS(12,9) of headers and terminators, CRC-CCITT of PI, CSBK, MBC and data headers. A header or terminator that fails and matches the call's LC in its first five bytes is rebuilt from the call's LC (sfrLcBurstGuess);
  • keyed aligned to the inbound call (DMR_TIMING_ALIGNED, the outbound TS) when one is being heard, else on its own slot clock (DMR_TIMING_OWN), always impolite (gotchas 1.8); two bursts kept queued in the modem, one engine decision per outbound slot; a gap inside an inbound call (a burst missed) is DMR_BURST_EMPTY: the slot stays empty and is received, never an idle burst, which corrupts the inbound burst decoded next to it (gotchas 14.4, sfr.tx_gaps); a burst heard while such placeholders wait in the modem takes the oldest one's slot (tx_fill), so it still goes out 90 ms later; idle bursts only in the hang and a network stream's gaps; DMR_EV_TX_REJECTED unkeys.
• Idle beacon (optional, off by default): with the radio setting rpt_beacon on, the runner keys its outbound with idle bursts for roam_beacon_len_s every roam_beacon_s while it carries nothing, so roaming radios can measure the site (sfrRunnerSetBeacon, hotspot_sfr_set_beacon; dmr-roaming.md 7). Traffic has priority. The phase is the runner's own (no network time).
• Debug link (section sfr, the channel's repeater role when it runs, else the hotspot's mode, sfr.mode = channel, hotspot or off; sfr.evq_drops the role's modem events lost): sfr.on sfr.keyed sfr.busy sfr.slots sfr.rx sfr.repeated sfr.ignored sfr.echo sfr.bad_crc sfr.tx_bursts sfr.tx_idle sfr.tx_starts sfr.up sfr.up_lost sfr.tx_beacons sfr.tx_gaps sfr.ignored_slot sfr.ignored_cc sfr.last_ignored (why bursts were ignored: reported on the outbound slot, another color code; the last one's color code and data type or voice index in hex), sfr.rx_fec (frames/bits/lost: the repeated voice's AMBE frames as heard, the C0/C1 Golay bits corrected in them and the frames whose C0 failed; the bits go out as heard, so an SDR seeing more errors in the repeat than this means the transmit side adds them), sfr.rx_fec_pos (those bits by the frame's place in the burst: errors piling up in the first frame point at the receiver still settling at the slot's start).

3. What the modem driver must do

The runner assumes these of hal_dmr_modem_t; they are the open work for each radio's driver:

1. Keep receiving, and report bursts, while it transmits: the inbound slot is heard between our bursts. The runner asks for it with tx_listen() after every tx_begin (a plain call never does). Report ts when it knows the slot, and in_step false for a burst whose status it can't trust.
2. tx_begin(DMR_TIMING_ALIGNED, ts): send in the slot opposite to the bursts being received, in step with them (the repeat goes out 90 ms after the burst, gotchas 14.3). DMR_TIMING_OWN with a ts: its own slot clock.
3. dmr_tx_burst_t.sync_override: the outbound sync (MS, BS, or TS1 / TS2 direct mode sync) on every burst.
4. DMR_BURST_IDLE with the payload given (the ETSI idle message) and no CRC of its own (gotchas 14.4); data bursts with host_crc sent as they are; DMR_BURST_EMPTY: nothing sent in that slot (received with tx_listen).
5. DMR_EV_TX_SLOT_STARTED once per burst taken from its queue, DMR_EV_TX_DONE after tx_end has drained it.
6. An SFR channel never switches to repeater (RMO) mode on BS sync (gotchas 14.7).

Status per modem driver:

The board switches the RF per burst: the RF TX interrupt keys the PA, the timeslot interrupt that starts the next slot drops it (the AT1846S goes back to RX about 1.3 ms into the slot, gotchas 3.4). A board with one synthesizer for receive and transmit retunes it per burst as well (gotchas 14.6). The rest of this section is the HR_C6000 bring-up, measured on an AT1846S board. Item 1 was missing in the chip: the slot between our bursts was written "no receive" (0x41 = 0x00), so nothing was heard there and MMDVM SFR repeated only what it heard before keying (HIL 2026-10-10: mode switching and the first repeat on TS2 passed, the rest of the call failed). With tx_listen (hrc6000_dmr_tx_listen) the engine writes "receive" (0x50) for that slot instead, for that transmission only; hrc6000DmrTest test_tx_listen checks it. Two more things were needed on air (gotchas 14.9): the per-burst switch back to RX goes receiver off, RX frequency, RX supply, RX on (it went from DMR TX straight to RX on), and a gap in the inbound call leaves our slot empty (DMR_BURST_EMPTY) instead of an idle burst. On air (HIL 2026-10-12, 06c5bc6): the bench's 3-superframe call goes up whole (18 voice frames and the terminator) and is repeated whole on TS2 (17-18 of 18 voice bursts, the terminator, then the hang's idle bursts). The voice repeat came 210 ms after the inbound burst, not 90 ms, and only 2 of the bench's 8 headers were repeated: the echo rule dropped the identical headers, and the gap left two placeholders queued ahead of the voice (gotchas 14.10). Both fixed in the runner (host tested), to be confirmed on air.

4. Standalone SFR

A channel's role comes from its codeplug bytes (sfrChannelDecode, docs/spec/open-firmware-cps.md 7.3.3: flag 1 bit 1, SFR bytes A and B; the CPS keeps them as read, the Codeplug Editor sets them). codeplug.c hands them to the core as read (channel_t.sfr_flag1 / sfr_a / sfr_b); radio_sfr_channel() turns them into a role and an sfrConfig_t. A role needs one frequency: RX = TX (a user may have no TX frequency). On a two-frequency channel, or in the VFO, the bytes are ignored and the channel is an ordinary one.

The setting is stored in bits 1-2 of the settings record's rpt_beacon byte (settings-store.md), set in Settings > Talk & listen > SFR receive and by debug link setting 30.

CW ID (CONFIG_SFR_CWID, core/radio/radio_cwid.c): the repeater role can identify itself in Morse. The setting sfr_cwid picks the interval: 0 off (default), 1 5 min, 2 10 min, 3 15 min, 4 30 min. It is stored in bits 3-5 of rpt_beacon, set in Settings > Talk & listen > SFR CW ID and by debug link setting 32.

On air the ID came 2 s after the hang, at 20 wpm (60 ms elements), 800 Hz, about 1.8 kHz deviation, with the 150 ms lead and tail, and the next one an interval later after new traffic. The runner's counters carry on over the ID (radio_sfr_start keep_stats). Bench figures: the HR_C7000 repeater's hardware page.

Debug link view dmr: dmr.sfr_cwid (the setting), dmr.sfr_cw (keying now), dmr.sfr_cwid_sent, dmr.sfr_cwid_cut (cut or skipped).

Why a user may want both slots: the inbound copy comes straight from a talker in range, the repeat from one out of range. A user whose inbound slot is TS2 has TS1 as its outbound slot, so two users on the same frequency with different inbound slots still hear each other: they are users of two different repeater set-ups, which can't share one frequency.

No CACH: the repeater keys only its own slot (it listens on the other one), so there is no continuous BS outbound to carry it. Users find the slots from the outbound slot's sync (TS framing) and the hang's idle bursts.

On air (SDR bench, 2026-10-19), the numbers on the radios' own hardware pages:

• The user's call goes out on slot A with the TS1 direct mode sync; the repeater repeats it on slot B with the TS2 sync, every voice burst 90.2 ms after the original (72 of 72), then the hang's idle bursts. The repeat carried 2 of the 3 headers and no terminator (the users' fade time ends the call), and its voice frames needed Golay corrections where the direct copy was clean: an HR_C7000 repeater's receiver takes bit errors while it switches per burst (gotchas 14.13). Both open.
• An HR_C6000 user played a bench call (TS1 sync) whole, no frame lost. It never reported the repeated copy during the call (rx_sfr_dup 0): the chip decodes the voice of the slot it locked onto, although the driver writes "receive" for every slot (rx_lock_slot(0)); its reports for the other slot are stale status (gotchas 14.12). With sfr_rx 1 or 2 it falls over to the other copy only between calls or after a loss. A user out of range of the talker locks to the repeat; one whose talker fades mid-call hears a gap until the receiver's lost-signal restart. Hearing both slots during a call needs the raw RX RAM path (section 3).
• An HR_C7000 user reports both copies. Its driver gives the voice-access flag to every chip slot with a call on it; with the flag on one slot only, every burst read the voice RAM of the last one decoded and the user played the same burst for the whole call (gotchas 14.15). Fixed in the driver; on air (2026-10-25, test_sfr_chain_rfoundry_user, 12 calls) the HR_C7000 listener's voice.rx_repeats was 0 on every call.

Not built: the HR_C6000 driver level parts (raw RX RAM classification, sfrRawClassify; the register switches of command 0xD5); a setting on the radio for the role (the Codeplug Editor sets it). On the radio, Tools > Single-freq repeater shows the channel's role, slots and the repeater's state, and a repeater channel's Home screen says "SFR repeater" or "Repeating" where a DMR channel shows its talk target.

A two-frequency repeater made of two radios is the split duplex extension (split-duplex.md, CONFIG_HOTSPOT_SPLIT), not the SFR: it registers in mmdvm.c and hotspot.c ahead of the SFR (a priority extension), so while a split role is active the SFR stays off, and without a role the SFR works exactly as described here.

5. Tests

• Host: tests/host/sfrTest.c (the logic), tests/host/hotspotSfrTest.c (MMDVM SFR through mmdvm.c and hotspot.c against the simulated modem: trigger and suffix, status bytes, an RF call repeated and sent up, a network stream, RF priority, echo, late entry, DMR_LOST1, command 0xD5, DMR_ABORT, back to simplex), tests/host/dmrCallTest.c test_sfr_user (both copies played once, the talker fading out, TX slot, sync and timing, ERDM color code), tests/host/radioCoreTest.c test_sfr_roles (the repeater role from the codeplug bytes repeats a call on TS2, PTT refused, hotspot mode and a scan take over and give it back, the keying refused, TX inhibit, two-frequency channel refused; the user role's TX slot and sync), test_sfr_cwid (the ID schedule and callsign, an ID after a call in FM on the channel's frequency, the role back after it, a retune cutting it, no callsign); tests/host/roamTest.c (the sfr_cwid bits stored and clamped).
• HIL: tests/hil/test_hotspot_sfr.py (MMDVM SFR on and off through MMDVM on the radio's channel, no traffic sent; marked radio_tx because a call heard would be repeated). On-air repeat tests need the driver work of section 3 and the SDR bench (tools/bench/mmdvm_hotspot.py --sfr). tests/hil/test_sfr_standalone.py runs the standalone roles from codeplug channels: the repeater's inbound quality (sfr.rx_fec, sfr.rx_fec_pos), TX inhibit, the CW ID and the user's receive modes against the bench talker, and test_sfr_chain_rfoundry_user, a whole chain of RadioFoundry radios (talker, repeater, listener) judged by what the listener played (voice.rx_in, voice.rx_peak, voice.rx_repeats).

6. Known issues by radio

Standalone SFR is experimental. This section names radios on purpose (an owner-approved exception to the generic-docs rule, tools/generic_allow.txt); keep each radio's details on its hardware page. Unless noted: SDR bench, a UHF test frequency, CC 2, the bench's MMDVM talker sending a 4-superframe group call (72 voice frames). "Bits" are AMBE bits the receiving radio's Golay decoders corrected (sfr.rx_fec, by frame place sfr.rx_fec_pos).