PACTOR
shrike · beta · P4 experimental
The PACTOR mode in hfmodem uses shrike for PACTOR-1, PACTOR-2 and PACTOR-3, with partial experimental PACTOR-4 support. Links begin with FSK and can select multicarrier waveforms. Data and replies share a fixed cycle.
| Generation | Waveform | Link structure |
|---|---|---|
| PACTOR-1 | 100 / 200 Bd FSK | Link entry, data packets and control signals |
| PACTOR-2 | Two differential PSK carriers | Coded data and negotiated speed levels |
| PACTOR-3 | 2–18 differential PSK carriers | Six speed levels on a 120 Hz grid |
| PACTOR-4 · experimental | Chirp, spread-spectrum and normal-mode waveforms | Partial receive, transmit and link support; limits below |
Schematic exchange · frequency × time
frequency 0.3 – 2.7 kHz →
time ↓
Schematic PACTOR-1 link entry followed by PACTOR-3 data cycles. PACTOR-3 channels 5 and 12, at 1080 and 1920 Hz, carry headers and control signals at every speed level.
THE CARRIER SET
PACTOR-3 channel occupancy
PACTOR-3 uses up to eighteen independently pulse-shaped 100 Bd carriers on a 120 Hz grid from 480 to 2520 Hz. Each speed level selects a symmetric carrier set, modulation and code rate.
Speed level
DBPSK has two differential phase states; DQPSK has four. Channels 5 and 12 carry the variable header and control signals.
This speed level
Symmetric about 1500 Hz
Every tone map is mirror-symmetric around the 1500 Hz centre, and every one contains channels 5 & 12. SL1 uses those two carriers; SL6 uses all eighteen. The net user rate climbs from 77 bit/s to 2722 bit/s across the six levels.
LINK SETUP
Link entry
A PACTOR-1 connect burst carries the called callsign in a dual-rate FSK frame. The link can remain in PACTOR-1 or negotiate a higher generation after entry. The encoder below shows the address frame for the selected callsign.
The frame, byte by byte
Closed form (per callsign)
T = [0x55] + ascii(call) + 0x0F… to 9 bytes # address section, 100 Bd (9 bytes) raw = T # redundancy section, 200 Bd (6 bytes) red = T[1:7] MARK(bit 1)=1400 Hz · SPACE(bit 0)=1600 Hz LSB-first · continuous phase · no CRC
Dual-rate frame
The connect frame contains nine bytes at 100 Bd followed by six redundancy bytes at 200 Bd, with continuous phase across the rate change. The receiver demodulates the two sections at their respective rates.
Callsign bytes
The address sends uppercase ASCII after the sync byte, padded with 0x0F.
The redundancy section repeats address bytes 1 through 6 at 200 Bd.
PACTOR-1 and PACTOR-2 data
PACTOR-1 carries data with 100 or 200 Bd FSK. A packet contains a header, payload, status and CRC; the receiver replies with a control signal in the fixed cycle.
PACTOR-2 uses two 100 Bd differential PSK carriers at 1400 and 1600 Hz. Its four speed levels select DBPSK or DQPSK and convolutional code rates. The K=9 code is punctured to 2/3 at SL3 and 7/8 at SL4. The payload is interleaved and protected by CRC-16/X25.
| Generation | Frame arrangement | Rate selection |
|---|---|---|
| PACTOR-1 | Header → data → status → CRC | 100 / 200 Bd |
| PACTOR-2 | Frame marker → coded data | SL1–4 |
| PACTOR-3 | Phase reference → headers → coded data | SL1–6 |
PACTOR-2 transmit payloads have independent monitor evidence at SL1–3. Its control-signal keying remains an implementation hypothesis. PACTOR-3 parameters and figures follow below.
THE DATA PACKET
PACTOR-3 frame structure
The short cycle lasts 1.25 s. A data packet contains a phase reference, headers and coded data.
A phase-reference symbol precedes the header block. The variable header carries speed level and cycle on channels 5 and 12; other active carriers send constant headers. The data field carries payload, status and CRC-16/X25. Link acquisition uses a separate burst.
The coding chain
Payload bytes are whitened, convolutionally coded, punctured and permuted onto the active carriers. The receiver uses soft-decision Viterbi decoding and CRC-16/X25. Each speed level selects its interleaving stride.
Phase-reference symbol — Schroeder's quadratic
A phase-reference symbol precedes the data. Carrier k starts at −πk(k+1)/18, a quadratic phase assignment that reduces crest factor. Subsequent symbols encode differential phase changes.
PACTOR-3 control signals
Six 20-bit control words are transmitted with DBPSK on channels 5 and 12. Every pair differs in 12 bits.
pairwise distance (bits)
Header detection
The receiver correlates the variable-header channels against known codewords to identify speed level and cycle.
Sender
Receiver
Data frame →
← Control signal · ACK / repeat
THE ARQ CYCLE
ARQ timing and the host interface
PACTOR uses cycle-locked stop-and-wait ARQ with control signals for acknowledgements, repeats and rate changes. Shrike exposes an SCS PTC serial host interface.
Connect → transfer → disconnect
Control signals → ARQ events
| CS1 / CS2 | ACK · request next |
| CS3 | break-in |
| CS4 | speed up one level |
| CS5 | NAK · speed down |
| CS6 | toggle cycle length |
The mapping from codeword to event is not fixed by the published description.
1.25s
short cycle
1
window (stop & wait)
mem
-ARQ soft combine
THE NUMBERS
Protocol constants
PACTOR-3 physical-layer and timing values follow ITU-R M.1798. Coding tables below describe the shrike implementation.
▸ Physical layer ITU-R M.1798
| Quantity | Value |
|---|---|
| Carriers | independent pulse-shaped DPSK · not OFDM |
| Symbol rate | Bd |
| Tone spacing · channel 0 | Hz · Hz |
| Occupied band · centre | – Hz · Hz |
| Header / control channels | 5 & 12 (1080 / 1920 Hz) |
| Emission designator |
▸ Speed levels ITU-R M.1798
▸ PACTOR-1 connect §2
| Quantity | Value |
|---|---|
| Waveform | single dual-rate FSK frame · continuous phase |
| Rates | 9 B @ 100 Bd (address) + 6 B @ 200 Bd (redundancy) |
| Tones | MARK/bit1 = 1400 Hz · SPACE/bit0 = 1600 Hz · LSB-first |
| Address image | T = [0x55] + ASCII(call) + 0x0F fill → 9 B |
| On-air bytes | Address T, followed by T[1:7] at 200 Bd |
| Frame CRC | none — the receiver validates the recovered characters instead |
| Address field | any callsign; the frame carries no capability field, so a connect does not negotiate a generation |
▸ Coding chain §3
| Quantity | Value |
|---|---|
| Whitening | word[i] ^= crc16_table[i] on info bytes · not in the spec |
| Data conv code · SL1 | K=9 · · rate 1/2 |
| Data conv code · SL2–6 | K=7 · |
| Transmit G0/G1 order | , bits packed LSB-first over the transmit form, terminated, over the whitened field |
| Puncture · SL5 (3/4) | G1= G2= |
| Puncture · SL6 (8/9) | G1= G2= |
| Interleaver | Stride by speed level: . At SL1, channel order matches code order. Implemented SHORT/LONG paths specify full-frame placement and code order. The general interleaver research entry remains open. |
| Decoder | soft-decision Viterbi |
| CRC | CRC-16/X25 (0x1021, init/xor 0xFFFF, reflected, residue 0xF0B8) |
▸ Packet structure & control signals §3
| Quantity | Value |
|---|---|
| Acquisition preamble | ±800 low-PAPR multitone burst used for link acquisition |
| Phase-reference symbol | 1 symbol, no data · channel k keyed at −πk(k+1)/18 (Schroeder) |
| Header block | DQPSK · symbols on every lit tone · constant headers + variable, on 5 & 12. No FEC field: the codewords are published and the decoder matched-filters them |
| Data-field modulation | differential · dibit → phase step |
| Grid rows / carrier | (short cycle) · (long) |
| Control signals CS1–6 | 52D56 · AABA2 · AD45A · 95AC5 · 74339 · 4B4AD (20-bit, d=12) |
| SL header — detection mechanism | matched-filter correlation over the known codewords, thresholded per SL hypothesis |
| Packet on an established link | One phase-reference symbol, the header block and data rows. A trailing run-out symbol carries no data. |
▸ ARQ & cycle timing ITU-R M.1798
| Quantity | Value |
|---|---|
| Scheme | cycle-locked stop-and-wait · window 1 · memory-ARQ |
| Cycle length | short s · long s (longpath / s) |
| Phase-reference pulse | one symbol before each packet and control signal |
| Control-signal window | s |
| Carrier swap | tones swapped per cycle (involution, fixes 5↔12) — frequency diversity |
| Speed adaptation | climb on ACK, drop on NAK (CS4 / CS5) |
| Link-layer FSM | stop-and-wait + gear-shift + graceful disconnect |
| Host interface | SCS PTC serial hostmode |
Complete PACTOR-3 implementation inputs maps the codewords, coding, framing and carrier placement to the public source.
PACTOR-4: experimental, partial support
Shrike decodes selected chirp, changeover and normal frames from recordings. It also has transmit waveforms and an experimental link and host-data path.
Coverage is incomplete. Normal-long SL6–10 decoding is available as offline diagnostics, outside the live receiver. Robust-long transmission is unimplemented.
Recorded replay reaches a CMS prompt and queues B2F login/proposal traffic. No successful on-air PACTOR-4 mail transfer has been demonstrated. The Winlink results below are PACTOR-1 and PACTOR-3.
PACTOR-4 protocol guide covers packet families, coding, permutations and the PRNG. Support and limits.
Observed results
Winlink mail sent and received through WS8EOC over hfmodem’s PACTOR-3. Four messages received through KB5LZK over PACTOR-1, with a normal session close.
PACTOR-2 codec behavior has bench evidence. Reliability across gateways and confirmed link shutdown are separate checks.
Implementation evidence records the conditions and coverage.