Ham Radio Digital Interfaces: SignaLink, Digirig, and Sound Card Setup Compared

To run FT8, JS8Call, PSK31, or any other sound-card digital mode, your computer needs to exchange audio with your radio. You also need a way to key the transmitter. This is what a digital interface does — it bridges the gap between computer audio and radio audio while keeping them electrically isolated.

There are several ways to accomplish this, ranging from $20 homebrew to $200 commercial units. Here's what actually works and what drives people crazy.

What a Digital Interface Does

Three functions:

1. Audio from computer to radio (transmit audio path)
2. Audio from radio to computer (receive audio path)
3. PTT control (telling the radio when to transmit)

Optionally: 4. CAT/CI-V control (computer controls radio frequency, mode, etc.)

The audio paths need isolation (transformers or optocouplers) to prevent ground loops, which cause hum and potentially damage. The PTT path needs to key the radio's transmit line without creating ground problems.

Option 1: SignaLink USB

The SignaLink USB from Tigertronics has been the default recommendation for over a decade. It's a small box that connects via USB (appearing as a sound card to your computer) and connects to your radio's data/accessory port via a radio-specific cable.

What's inside:

• USB sound card (Texas Instruments PCM2902-based)
• Audio isolation transformers
• VOX-based PTT circuit (triggers transmit when it detects outgoing audio)
• Level adjustment pots for TX and RX audio

Pros:

• Proven reliable over millions of units sold
• Works with virtually every radio ever made (just buy the right cable)
• VOX PTT means no CAT connection needed for basic operation
• Good audio isolation eliminates ground loops
• Available everywhere (HRO, DX Engineering, Amazon)

Cons:

• VOX PTT has a slight delay and can false-trigger on noise
• Relatively large physical footprint for portable operations
• $120+ with cable (not cheap for what's inside)
• External device means more cable clutter
• The jumper header inside (for radio-specific cable configuration) confuses beginners

Best for: Home station operators who want a plug-and-play solution and don't mind the size. Works great, never needs thought once configured.

Option 2: Digirig Mobile

The Digirig is a much newer, much smaller device designed by K0TX. It's roughly the size of two sugar cubes. Designed specifically for portable operating (POTA, SOTA) where space matters.

What's inside:

• USB sound card (C-Media CM108-based)
• Audio isolation
• Hardware PTT via serial RTS/DTR line (not VOX)
• Built-in USB-serial for CAT control on some cable configurations

Pros:

• Tiny — fits in a shirt pocket
• Hardware PTT (more reliable than VOX, no delay)
• Integrated CAT on single USB connection (radio-dependent)
• Much cheaper than SignaLink (~$45-55 depending on configuration)
• Excellent for portable/field use

Cons:

• Newer product, less community documentation (though growing fast)
• Cable ecosystem still developing (not every radio has a pre-made cable yet)
• Some radios need custom cable configurations that aren't obvious
• Small form factor means no physical TX/RX level pots (use software mixer)
• USB-C connector can be fragile with repeated field use

Best for: Portable operators, POTA/SOTA enthusiasts, and anyone who wants small and affordable. Also great for QRP rigs that have data ports.

Option 3: Built-In USB Sound Card (Modern Rigs)

Many radios manufactured after 2015 have USB audio built in. Connect a USB cable from radio to computer and the radio appears as a sound card. Some also include CAT control over the same USB connection.

Radios with built-in USB audio include: Icom IC-7300, IC-7610, IC-705; Yaesu FT-991A, FT-710, FTDX101; Kenwood TS-890S, TS-590SG; Flex radios (all models); Elecraft KX2, KX3 (via USB adapter).

Pros:

• One USB cable does everything (audio + CAT + PTT)
• No external interface needed
• Lowest latency (no extra D/A and A/D conversion)
• Cleanest audio path (digital throughout)
• Nothing extra to carry for portable operations

Cons:

• Only works with radios that have this feature
• Some rigs have driver issues on certain OS versions
• If the USB connection glitches, you lose everything simultaneously
• Can't move the interface to a different radio (it's built in)

Best for: Anyone with a modern rig. This is the obvious choice if your radio supports it. No reason to buy an external interface unless you need to share between multiple radios or the built-in USB is unreliable.

Option 4: Homebrew Interface

A digital interface is electrically simple. The minimum viable build:

• Two 600:600 ohm audio isolation transformers (one for TX, one for RX)
• An optocoupler or transistor for PTT switching
• A USB sound card dongle ($8-15)
• Some resistors for level attenuation
• An enclosure

Total parts cost: $15-25.

Pros:

• Cheapest possible solution
• Educational (you learn what the commercial boxes actually do)
• Customizable for weird radios or special situations
• Can be purpose-built for a specific radio's connector and levels

Cons:

• Requires soldering skills
• Debugging is on you
• No support if something doesn't work
• Build quality affects reliability
• A badly-built interface can introduce noise or hum

Best for: Hams who enjoy building things, educators, and those on a strict budget who already have soldering skills.

PTT Methods Compared

VOX (Signal-Operated)

The interface detects audio coming from the computer and keys PTT automatically. SignaLink uses this. Simple, no software configuration needed.

Downside: slight delay before TX starts (the VOX circuit needs to detect audio), and possible false triggering from system sounds. Always disable Windows notification sounds on your digital-mode computer.

Hardware PTT (RTS/DTR)

The software (WSJT-X, JS8Call, etc.) asserts a serial port line to key the radio. Faster, more reliable, no false triggers.

Requires correct software configuration — you must tell WSJT-X which COM port and which line (RTS or DTR) controls PTT. Getting this wrong is the #1 cause of "my interface won't transmit" problems.

CAT PTT

Software sends a transmit command via CAT (CI-V, serial, USB). The radio receives the command and keys itself.

Advantage: no separate PTT wiring needed. Disadvantage: slightly slower (command processing time) and if CAT communication has issues, PTT is unreliable. Some contesters avoid CAT PTT because of occasional missed transitions under rapid TX/RX cycling.

Audio Level Settings: The #1 Gotcha

More digital mode problems come from wrong audio levels than anything else. Both too high and too low cause issues:

TX audio too high: Overdrive the radio's audio input, causing ALC action, splatter, and distorted signal. Your FT8 signal looks wide and dirty on other people's waterfalls. The radio's ALC meter should show zero or minimal deflection during digital mode transmit.

TX audio too low: Weak signal, poor decode range. Not using your radio's available power.

RX audio too high: Overloads the sound card's A/D converter, causing clipping. Signals look distorted in the waterfall. Decoding suffers.

RX audio too low: Signals don't rise above the noise floor in software. Fewer decodes.

The sweet spot for most setups:

• WSJT-X power slider at about 25-50%
• Radio's ALC meter showing zero deflection
• Radio output power at desired level (set via radio's power knob)
• Waterfall noise floor around -30 to -20 dB in WSJT-X

Cable Compatibility Matrix

This is where beginners get stuck. Each radio uses different connector types and pinouts for data/audio:

• Icom 6-pin mini-DIN: IC-7300, IC-7100, IC-718, many older Icoms
• Yaesu 6-pin mini-DIN: FT-991, FT-891, FT-450D (different pinout from Icom!)
• Kenwood 6-pin mini-DIN: TS-590, TS-480 (yet another pinout)
• 13-pin DIN: Icom IC-7600, IC-7610 ACC port
• 3.5mm jacks: Many QRP rigs, some older radios
• RJ-45 (modular): Yaesu FT-817/818, some mobile rigs

Do NOT assume a cable from one radio works with another, even if the connector physically fits. The pinouts are different. A Yaesu data cable on an Icom radio can short audio lines or apply voltage where it shouldn't be.

Always verify the pinout matches your specific radio model before connecting.

My Recommendation by Use Case

Home station, modern rig: Use the built-in USB. It's free (you already have the radio) and it works better than any external solution.

Home station, older rig: SignaLink USB. Proven, reliable, well-documented for virtually every radio.

Portable/POTA: Digirig Mobile if your radio has a compatible cable available. Otherwise SignaLink works but it's bulky.

Multiple radios, switching between them: SignaLink with a cable for each radio. Unplug one cable, plug in the other. Or build dedicated interfaces for each radio.

Budget build: Homebrew interface. If you can solder and understand the circuit, there's no reason to spend $120 when $20 in parts does the same job.

Troubleshooting Quick Reference

• No TX audio: Check cable, check Windows sound output device selection, check WSJT-X audio settings
• TX but no PTT: Wrong COM port or RTS/DTR setting in software; or VOX threshold too high
• Hum on audio: Ground loop — verify isolation transformers are present and working
• ALC slamming on digital TX: Audio level too high from computer. Reduce in software, not with radio's mic gain
• Decodes working but signal reports poor: Likely ALC/overdrive issue causing dirty signal
• Everything worked yesterday, nothing today: Windows Update changed the default audio device. Check Sound Settings.