Receive CODEC2 Voice Messages
Some satellites send short recorded voice messages down as data rather than as FM audio: the speech is compressed with the Codec2 vocoder to a few hundred bits per second and packed into ordinary telemetry frames. SkyRoof decodes these messages back to audio with its built-in decoder, and each message appears in the Telemetry panel, ready to play.
At the moment HADES-SA is the satellite that sends them, in Codec2 700C mode, interleaved with its telemetry frames and SSDV image packets on the same downlink. Nothing extra has to be turned on: if you are decoding HADES-SA telemetry, the voice messages are decoded too.
Receiving a Message
The steps are the same as for decoding telemetry — open the Telemetry panel, select the satellite and its telemetry transmitter, and make sure the SDR is running and tuned to it.
Each message appears as a node in the tree, under the pass node, labeled with the time it was first heard, how many sub-frames arrived, and how long the recording plays:
04:15:02 Voice 27 sub-frames, 10.8 s
The node updates in place as the message comes in. A gap of a few seconds with no sub-frames ends the message; the next sub-frame after that starts a new one.
Select the node to see the details, and click anywhere in the detail pane to play the message. You can play it while it is still arriving — what plays is always what has been received so far.
The detail pane shows:
- Sat and Tx — the satellite and the transmitter the message came from;
- Duration — the length of the reconstructed recording;
- Sub-frames — how many were received, and how many the message spans. There is no "of N": nothing in the transmission says how long a message is, so the second number is the span of what arrived, not the length of what was sent;
- Numbered — the range of sub-frame numbers received;
- Gaps —
none, or how many sub-frames are missing. Missing sub-frames are played as silence, so a message with holes still plays at its right length and the words around the holes stay in place; - Status —
receiving...orcomplete; - Saved — the file it was written to, once it has been saved.
A message the pass ended in the middle of is normal rather than exceptional, and there is no way to tell how much of it was missed — the downlink does not say.
About the Audio Quality
Codec2 700C compresses speech to 700 bits per second, which is roughly a hundredth of the rate of a telephone call. Even a message received without a single lost sub-frame sounds robotic and is often hard to make out. That is the vocoder's own limit at this bit rate, not a fault in the reception or in SkyRoof — a perfect decode still sounds like this. Comparing several receptions of the same message often helps more than trying to clean up one of them.
Saved Messages
Each finalized message is saved automatically as a WAV file, with a JSON sidecar holding its metadata, in the Codec2Voice subfolder of the data folder. Voice sub-frames carry no checksum of any kind, so a message of fewer than three sub-frames is not written to disk — too little to tell a real transmission from a misread frame. Such a message is still shown in the tree and still plays.
Right-click a message in the tree to:
- Play — play the message, the same as clicking in the detail pane;
- Save As... — save the WAV file to a location of your choice;
- Open in Player — open the automatically saved WAV file in the program that Windows associates with WAV files. This command is available only after the message has been finalized and saved, so it stays grayed out while a message is still being received.