Sound-card interfaces — what you actually need
An interface does three jobs: it carries receive audio to the computer, transmit audio back to the radio, and it keys the transmitter. Everything else is packaging. Understanding those three jobs — and the fourth, quieter one of keeping the two machines electrically apart — tells you which box to buy, or whether you need one at all.
Start here: do you need one?
If your radio has a USB socket that presents itself to the computer as a sound card, you already own an interface and it is better integrated than anything you could bolt on. This covers most rigs sold in the last decade — the FT-991A, IC-7300, IC-705, FT-710, TS-590 and many more.
One cable carries everything, the isolation is handled inside the radio, and the audio never leaves the digital domain until it is already past the point where hum could get in. If this is you, stop reading and go and set your levels instead.
You need an external interface when the radio has no USB sound card — an FT-891, an older Kenwood, most handhelds, anything from before about 2010.
The four jobs
| Job | What it means |
|---|---|
| Receive audio | Radio speaker or line-out into the computer’s input. |
| Transmit audio | Computer output into the radio’s mic or data input. |
| PTT | Something has to key the transmitter at the right moment. |
| Isolation | Keeping the two devices electrically separate. The quiet one. |
Isolation is the one that matters
Connect a radio to a computer with plain audio cables and you have joined two mains appliances by their signal grounds. Two things follow, and both are miserable to diagnose:
- Ground loops. Mains hum superimposed on your audio, in both directions. Your pictures arrive with banding and so do everyone else’s copies of yours.
- RF ingress. The cable acts as an antenna for your own transmitter. Symptoms range from audio distortion that only appears at high power, to the computer locking up mid-transmission.
A properly isolated interface uses small audio transformers and an opto-coupler for PTT, so no continuous copper path exists. This is the single feature worth paying for.
How PTT gets keyed
Three approaches, in rough order of how much they will annoy you:
- CAT. The software tells the radio to transmit over its control port. Precise, no extra wiring, and the same connection reports your frequency back so the logbook fills itself. Use this where you can.
- Serial RTS/DTR. A control line on a serial or USB-serial port drives an opto-coupler. Simple, reliable, works with anything that has a PTT input.
- VOX. The radio — or the interface — keys when it hears audio. No wiring at all, which is its entire appeal. It also clips the start of every transmission slightly and can be triggered by the wrong sound at the wrong moment.
The common options
Named for orientation, not endorsement. All three approaches work; the right one depends on what your radio already has.
- Built-in USB codec. Free, already fitted, best integrated. If your rig has it, this is the answer.
- Digirig. Very small, inexpensive, carries audio plus CAT and PTT over a single USB connection. Suits a modern rig with a serial control port and no sound card.
- SignaLink USB. Larger, self-contained, keys PTT from the transmit audio using its own circuit rather than a serial line. That independence is why it still works with radios that predate CAT entirely.
- Homebrew. Two audio transformers and an opto-coupler on stripboard will do everything an isolated interface needs to do, for the price of a coffee. A genuinely good weekend project.
What you do not need
Open Signal needs no virtual audio cable, no FLDIGI, no Hamlib install and no bridge program. The DSP, the SSTV codecs and the rig control are compiled into a single executable, which removes the layer of plumbing that historically caused most interface problems.
That plumbing is the main practical difference from MMSSTV, where a virtual audio cable is usually part of the arrangement.
Interfaces — FAQ
Do I need an interface if my radio has a USB port?
No. A rig with a built-in USB sound card — an FT-991A, IC-7300, IC-705 and many others — already contains the interface. One USB cable carries receive audio, transmit audio, PTT and usually CAT control, and the isolation is handled inside the radio. Buying an external interface for such a rig solves a problem you do not have.
What does galvanic isolation actually do?
It breaks the electrical connection between radio and computer while still passing the audio, using transformers or opto-couplers. Without it, the two devices share a ground path, and that path carries mains hum into your audio and RF from your antenna into your computer. It matters more than any other feature.
Digirig or SignaLink?
A Digirig is small, inexpensive, and carries audio, CAT and PTT down one USB cable, which suits a rig with a serial CAT port. A SignaLink is larger, keys PTT from the audio itself using its own VOX circuit, and needs no serial port at all — which is why it still works with radios far older than any of this.
Why does my computer misbehave when I transmit?
RF getting in where it should not, almost always through the cable between the radio and the computer. Ferrite chokes on both ends of that cable, a shorter cable, and a properly isolated interface all help. It is a symptom worth chasing, because the same RF path is often what is degrading your transmit audio.
More about Open Signal
No virtual audio cables required
One self-contained executable with the DSP, SSTV codecs, logbook and CAT control compiled in. Point it at your interface and transmit.