In the age of sound card modems, it has never been easier to get started in amateur data modes—or so it seems...
All hams seem to run into almost exactly the same issues while trying to run data modes. This series should clear up some common roadblocks.
In modern amateur communications, several components are standard. Here is a basic overview of the process:
- An application produces data (text, images, etc.)
- A modem encodes data as an analog waveform (usually within the audible range)
- The signal is sent through an RF modulator
- The RF is transmitted through an antenna
- RF is received on an antenna
- The RF is processed by a demodulator circuit
- The resultant analog waveform is sent to the modem for decoding
- The receiving application interprets the inbound data
Sound Cards or Hardware Modems?
Why use a sound card when hardware modems have been around forever?
Because sound card interfaces can run multiple modes, work with multiple radios (if external), reduce software and setup complexity, allow for previously unfathomable speeds, and are highly portable.
Until about a decade ago, hardware modems were an excellent choice for hobby and emergency communications alike. If you needed something that was time-tested and "just worked," the hardware modem was your solution.
Today, served agency needs are significantly different and are constantly evolving. We need resilient, noise-rejecting modems with extremely high throughput. In most cases, something like PSK31 or RTTY-45 isn't going to cut it (realistically, even 1200 baud packet is obsolete).
How does the sound card interface work?
A sound card just adds a layer into the stack so that desktop computer software may act as a modem (instead of a dedicated microcontroller in a hardware modem).
- The ADC — In our case, RF signals are demodulated by our receivers. The resultant analog waveforms must be converted to digital bits through an Analog to Digital Converter (ADC), called the sound card "input."
- The DAC — If data can be received, the inverse process allows for transmission. Our sound card "output" acts as a Digital to Analog Converter (DAC), then our transmitters send analog waveforms through some RF modulation scheme.
- The Modem — While the sound card may handle digital information, it is just a digital representation of the analog waveform going to/from the radio. A software modem encodes and decodes this waveform to send or receive data. The way by which the modem sends and receives data determines speed and resistance to noise.
- The Application — Just the same as always, whether it be a keyboard-to-keyboard chat interface or WinLink Express. The connection to the modem is now done in software rather than a serial cable and is usually done over a network interface (here is the source of the "127.0.0.1" address within setup menus).
What do I need to make sound card modes work?
- A radio
- A method of getting audio to and from a PC
- Microphone and speaker connections are most common, but are not recommended
- A 6-pin mini DIN connector is the easiest external connection to use
- Some radios have an inbuilt sound card (IC-7100/7300, Yaesu FT-991A, Kenwood TH-D74A, etc.) which just requires a USB connection to your computer
- A sound card (if external)
- The SignaLink is an easy option, though others exist
- A computer
- Windows is unfortunately the friendliest option, though MacOS and Linux are useable (I will detail all three)
- Software modems
- WSJT-X suite
- Also Fldigi
- Custom (detailed later)