HF Packet Decode overview¶
The HF Packet Decode feature decodes AX.25 amateur radio packet data transmitted over HF and VHF. It provides a real-time view of decoded frames, signal activity, and connection status for monitoring packet communications on the FLEX-8600. The decoder uses a shim layer over libmodem for physical-layer packet demodulation and integrates with the audio engine for live decode.
Before you start¶
- Ensure AetherSDR is connected to a FLEX-8600 radio
- The radio must be tuned to an active packet frequency (HF or VHF)
How it works¶
HF Packet Decode pulls demodulated audio from the radio's audio stream and passes it through an AX.25 decoder. Decoded frames appear in a scrollable text area as they are received, showing source, destination, and payload information. A signal activity indicator provides real-time visual feedback of packet detection and decode status.
The decoder supports two modem profiles: - HF 300 (300 baud, AFSK): Uses free-running phase-diversity lanes (PLL alpha 0) spread evenly across the symbol period for robust decoding. TX preamble is 80 flags (~2.13 seconds). - VHF 1200 (1200 baud, Bell 202/APRS): Uses a bank of demodulator lanes with multiple space-gain multipliers. Nine lanes use the exact Direwolf A+ space-gain values (MIN_G=0.5, MAX_G=4.0) in a geometric series. Duplicate suppression collapses identical frames seen by multiple lanes into a single emission, similar to Direwolf's multi_modem.c. The TX preamble is 64 flags (~0.43 seconds) for VHF 1200 to reduce dead air and slot time.
Both profiles use an abstract demodulator interface (IAfskDemod) that allows VHF (Direwolf-derived) and HF (libmodem) demod types to coexist in the same lane vector. The HF 300 profile uses LibmodemAfskDemod wrapping the libmodem sinc_corr_afsk_demodulator, while VHF 1200 uses DirewolfAfskDemod wrapping the AetherAFSKDemod.
Duplicate suppression collapses the same frame seen by multiple lanes into one emission, like a multi-decoder TNC (e.g. Dire Wolf).
The dialog is opened from the digital modes area when HF packet decode is active, or from a related menu entry. The dialog provides five tabs: AX.25 (APRS), KISS TNC, Terminal, Mailbox, and D-STAR.
What each control does¶
| Control | Kind | Behavior |
|---|---|---|
| Decoded frames | text_area | Scrollable display of decoded AX.25 frames showing source, destination, and payload information. New in v26.5.2.1. |
| Signal activity | widget | Real-time signal activity indicator showing packet detection and decode status. Provided by PacketActivityWidget. |
Tab navigation¶
The dialog provides five tabs across the top: AX.25, KISS TNC, Terminal, Mailbox, and D-STAR. Click a tab to switch to that page.
- AX.25 (APRS) — Default tab showing decoded APRS frames, a station table, and a transmit area.
- KISS TNC — KISS TNC server interface for external applications.
- Terminal — Connected-mode AX.25 terminal client for keyboard chat.
- Mailbox — Personal Message System (PMS) mailbox for storing and forwarding messages.
- D-STAR — D-STAR modem page for D-STAR digital voice operation (new in v26.7.4).
Status bar behavior¶
The status bar below the tab content shows modem status, gain stage, and packet activity. When the D-STAR tab is active, the status bar is hidden.
The transmit area (with the Transmit button and text input) is visible only on non-D-STAR tabs and only when diagnostics debug mode is enabled.
Transmit button behavior¶
The Transmit button on the AX.25 tab and the Send button on the Terminal tab are marked with TX keying indicators. Clicking either button transmits the entered packet and keys the transmitter.
Tips¶
- The decoded frames area scrolls automatically as new frames are received. Use the scrollbar to review older frames.
- For best decode results, tune the radio to a clear frequency with active packet activity. Typical HF packet frequencies are in the 14.100-14.110 MHz range on 20 meters and corresponding allocations on other bands. For VHF 1200, standard APRS frequencies apply (144.390 MHz in North America, 144.800 MHz in Europe, etc.).
- The VHF 1200 profile adjusts TX preamble automatically based on the selected profile. If using a transverter, the VHF 1200 preamble may need adjustment for T/R switching lead-in time. Set
kVhf1200TxPreambleFlagsin the configuration to tune this value. - The D-STAR tab provides D-STAR digital voice modem functionality. When active, the packet activity status bar and transmit area are hidden.
Troubleshooting¶
- No frames decoded — Verify the radio is connected and tuned to a frequency with active AX.25 packet activity. Check that the audio level is sufficient; the decoder needs a clean signal to demodulate.
- Garbled or partial frames — Weak signals, interference, or incorrect tuning can cause decode errors. Try adjusting the receiver bandwidth or retuning to center the signal within the passband.
- VHF 1200 decode issues — Ensure the correct profile is selected. If using a transverter, verify that the TX preamble provides enough lead-in time for T/R switching.
- D-STAR tab not working — Ensure the radio is tuned to a D-STAR frequency. The D-STAR modem requires a clean digital signal for proper decode.