Connecting any radio for SSTV
Every rig-specific setup guide on the internet is a variation on the same three decisions: where the audio goes, how the transmitter gets keyed, and how drive is set. Menu numbers differ, the decisions do not. Understand them once and you can set up a radio nobody has written a guide for — which is most of them.
Decision one: the audio path
Sound has to get from the radio to the computer and back. There are three arrangements, in descending order of how pleasant they are to live with:
- Built-in USB codec. The rig appears to the computer as a sound card. One cable, isolation handled internally, nothing to buy. Any rig from roughly the last decade.
- External interface. A box between the two providing isolated audio in both directions plus PTT. Required for anything without a USB codec.
- Bare audio cables. Works, and joins two mains appliances by their signal grounds — which invites hum and RF trouble. Fine for receive-only, poor for transmit.
Which you need is covered properly in sound-card interfaces.
The mode trap
This deserves its own heading because it accounts for an enormous share of “my radio is broken” reports, and it is not a fault.
Most modern transceivers have separate transmit audio sources per mode. In plain SSB or plain FM the audio comes from the microphone. In the DATA variants — DATA-USB, DATA-FM, PKT, whatever the manufacturer calls it — it comes from the sound card.
Use the wrong one and PTT keys, the power meter moves, and you transmit an unmodulated carrier while your computer audio goes nowhere. Nothing indicates a problem locally. You find out when someone tells you.
- HF: use the DATA variant of USB, not plain USB.
- VHF/UHF FM: use the DATA variant of FM — and check the separate menu setting that chooses which port the DATA audio comes from. This is the one that catches people, because HF works first time and VHF silently does not.
The FT-991A guide works this through with real menu numbers; other rigs have equivalents under different names.
Decision two: how PTT is keyed
| Method | Needs | Trade-off |
|---|---|---|
| CAT | Control port | Best — also reports frequency for the log |
| Serial RTS/DTR | Serial line + opto | Simple and reliable, no frequency readout |
| VOX | Nothing | Clips transmission starts, keys on stray sound |
Use CAT if the radio has it. Beyond keying the transmitter, it lets the software read your VFO — which is how a logbook records the right frequency without you typing it. Open Signal has CAT for Kenwood, Elecraft, Yaesu and Icom compiled in, so there is no hamlib to install alongside.
On Linux, CAT and serial PTT both need your user to be in the dialout group — the single most common reason PTT does nothing there. See SSTV on Linux.
Decision three: drive
The whole of audio levels and ALC is about this, so briefly:
- Start low.
- Raise until power output stops climbing.
- Back off until ALC barely moves.
- On FM, aim for about 3 kHz deviation instead.
- Set it once. Leave it.
And remember the duty cycle: SSTV transmits continuously for minutes. Many radios are rated for full output only in intermittent service, so consider running the long modes at reduced power.
A checklist for any rig
- Audio in and out established, isolated if possible.
- DATA variant of the mode selected — and the right port chosen on FM.
- PTT method chosen and tested into a dummy load.
- Drive set so ALC barely moves.
- Operating-system audio enhancements all disabled.
- Power reduced for the long modes.
Six items. They are the same six on an FT-991A, an IC-7300 and a twenty-year-old transceiver with an interface bolted to it.
Radio setup — FAQ
What do I actually need to connect a radio for SSTV?
A path for receive audio into the computer, a path for transmit audio back, and a way to key the transmitter. On a rig with a built-in USB sound card, one cable does all three. On anything older you need an interface to provide them.
My radio keys up but transmits no audio. Why?
Almost always the wrong mode. Many rigs take transmit audio from the microphone in their plain voice modes and only from the sound card in their DATA modes — so PTT works, power shows, and nothing is modulated. Switch to the DATA variant of the mode you want.
Should I use VOX or CAT for PTT?
CAT where you can. It is precise, needs no extra wiring, and the same connection reports your frequency back so the logbook fills itself correctly. VOX needs no configuration at all, which is its appeal, but it clips the start of transmissions and can key on the wrong sound.
Does the rig matter much?
Less than the three decisions. A twenty-year-old radio with a properly isolated interface and correct levels will produce a better picture than a modern one set up carelessly. The rig determines how convenient the setup is, not how good the signal is.
More about Open Signal
CAT control for four manufacturers, built in
Kenwood, Elecraft, Yaesu and Icom — compiled into the same executable as the DSP and the SSTV codecs. No hamlib, no bridge programs, no virtual audio cables.