Computer and sound-card interfaces

What it is

A computer interface lets a computer talk to a radio for digital modes such as FT8, PSK31 or RTTY. The software makes and decodes the signal in the computer's sound card, so the radio only has to supply audio and be told when to transmit. Three jobs, always: receive audio, transmit audio, and a transmit key.

Three ways to connect

Method	What it is	Notes
Radio with built-in USB	one cable gives the computer a sound device and a control port	simplest; no extra box
External interface box	a unit with its own sound device, isolation and transmit key	works with radios that have no USB audio
Computer's own sound card	audio cables from line in and out to the radio, with an isolating interface	common; needs care with levels

On the radio, choose the data or accessory port where one exists: its audio is flat and skips the microphone's shaping, so tones go out unchanged. A microphone jack can work, but the radio's own speech processing and equalizer must then be off.

Setting levels

• Receive: adjust the radio's audio-out or the computer's input so the software's level meter sits in the middle, neither tiny nor clipping.
• Transmit: raise the software's output until the radio just starts to show ALC, then back it off until the ALC is quiet. A digital signal should use almost no ALC; ALC action distorts it. Set output power with the radio's power control, not by driving the audio harder.
• Too much drive makes a wide, dirty signal. Other stations see it as a spread of extra tones.

Keying the transmitter

The radio needs to be told to transmit. Options: a CAT command (see CAT control and rig software), a serial control line such as RTS or DTR, or an audio-actuated VOX. A serial or CAT key is the most predictable; VOX can clip the start of a transmission or trigger on a stray sound.

Timed modes like FT8 also need an accurate computer clock, usually within about a second, so use a time-sync tool or network time.

Noise and ground loops

Connecting a computer to a radio creates an extra path for current and noise. A ground loop shows up as hum or buzz, and computer and power-supply noise can leak into receive audio. Isolation transformers in the audio lines break ground loops, and ferrite chokes on the USB and audio cables help against RF. See Common-mode current and ground loops and Filters and ferrites for RFI.

If a transmission makes the computer reset or freeze, RF is entering a cable. Shorten, shield or choke it, and keep the computer and its cables away from the antenna's feed line.

Related

• CAT control and rig softwareControlling the radio from a computer.
• Digital mode softwareWSJT-X, fldigi and friends.
• Digital modes overviewThe landscape of data modes and what each is for.
• FT8 and FT4Weak-signal 15-second and 7.5-second modes.
• Common-mode current and ground loopsThe hidden path.
• Mic gain, compression and ALCGetting a clean SSB signal.