Digital mode interface
| Digital mode interface | |
|---|---|
| Also called | Sound-card interface, data interface, rig interface |
| What it does | Connects a transceiver's audio and controls to a computer for digital modes |
| Signals carried | Receive audio, transmit audio, PTT and usually CAT control |
| Built-in version | USB audio codec and virtual serial port in many modern radios, over one cable |
| Control | |
| PTT methods | VOX, RTS or DTR serial line, CAT command |
| CAT | Computer control of frequency, mode and transmit |
| Transmit level | Set so the ALC meter reads zero |
| Typical modes | FT8, FT4, PSK31, RTTY, JS8Call, SSTV, Winlink |
A digital mode interface connects a radio to a computer so that software can send and receive digital modes such as FT8, RTTY, PSK31 and Winlink. Most of these modes are generated and decoded by software as audio tones. The computer listens to the radio's receive audio through a sound card, and on transmit it produces audio that the radio sends as a single-sideband signal. The interface carries those two audio paths, keys the transmitter, and often lets the software control the radio's frequency and mode.
Built-in USB interfaces
Many HF transceivers and VHF/UHF multimode radios made in the last decade or so have the interface built in. A single USB cable carries audio both ways, appearing to the computer as a sound device, often named something like "USB Audio CODEC," along with one or more virtual serial (COM) ports for control. The serial ports may need a driver from the radio's manufacturer.
In the software, choose the radio's sound device for input and output, not the computer's own speakers and microphone. On the radio, select the data mode or the menu setting that takes transmit audio from USB; this usually bypasses the microphone, speech processing and equalizer.
External interfaces
Older radios, and many handheld and mobile radios, need an external interface connected to the radio's data or accessory jack, or to the microphone and speaker jacks. There are two main kinds:
- Interfaces with their own sound card, such as the SignaLink USB, which connect to the computer by USB. Many key the radio with a built-in circuit that switches to transmit whenever audio arrives from the computer.
- Simpler interfaces that use the computer's own sound card, with audio cables plus a serial or USB connection to key the radio.
Some radio makers sell interfaces for their own radios. Yaesu's SCU-17 has a built-in sound card and carries receive and transmit audio, PTT, FSK keying for RTTY and CAT control over a single USB cable, for Yaesu radios without USB of their own, such as the FT-891 and FT-450D.
Some interfaces are built for particular jobs:
- Compact USB interfaces such as the Digirig combine a sound card with a serial port for CAT control and PTT in a box small enough for portable operating.
- Bluetooth TNCs such as the Mobilinkd connect a handheld or mobile radio to a phone or tablet app for APRS and packet.
- Hardware TNCs such as the Kantronics KPC-3+ contain their own modem and handle the packet protocol themselves, so the computer needs only a serial or USB connection.
- Software modems such as Direwolf do a TNC's job in software, turning any sound-card interface into a packet station.
Good external interfaces use isolation transformers in the audio lines. They break ground loops, which cause hum, and help keep RF and stray currents out of the computer.
CAT control
CAT (computer aided transceiver) control uses the radio's command set over a serial link to read and set the frequency, mode and split settings and to switch the radio to transmit. Each manufacturer has its own commands, such as Icom's CI-V, and software libraries such as Hamlib translate them, so a program only needs to know the radio model. The serial port, baud rate and other settings in the software must match those in the radio's menu. CAT also lets logging software record the frequency and mode of each contact automatically. Usually only one program can open a serial port at a time, so sharing a radio between several programs needs a program or feature designed for it.
Keying the transmitter
| Method | How it works | Points to watch |
|---|---|---|
| VOX | The radio, or the interface, transmits whenever it hears audio | Simplest wiring, but any computer sound, including system alerts, goes on the air |
| RTS or DTR | The software raises a control line on a serial port, which keys the radio | Fast and reliable; the software must be set to the right port and line |
| CAT | The software sends a transmit command to the radio | No extra wiring; the radio must be set to take its audio from the data or USB input when keyed this way |
Audio levels and ALC
On receive, set the audio level so the software shows a moderate reading on band noise alone, neither near zero nor close to clipping. WSJT-X, for example, suggests a background level of about 30 dB on its meter.
On transmit, the aim is a clean signal at the chosen power. ALC (automatic level control) is the radio's circuit that cuts back the gain when the audio drive is too high. On voice, some ALC movement is normal, but on digital modes ALC action distorts the signal and can produce unwanted signals on nearby frequencies. Set the radio's output power, then raise the audio from the computer until the power reaches that level, and back the audio off until the ALC meter reads zero. Turn off speech compression for data. Many modes, including FT8, transmit at full power for the whole transmission, so consider running less than full power.
Strong RF in the shack can disrupt a USB connection, causing dropouts, lockups or distorted audio. A common-mode choke or clamp-on ferrite on the USB and audio cables usually cures it. Modes such as FT8 and FT4 also need the computer's clock set accurately.