What Is PSK31? An Introduction to the Digital Mode
PSK31, also known as BPSK31 and QPSK31, is a popular computer-sound card-generated radioteletype mode used primarily by amateur radio operators to conduct real-time keyboard-to-keyboard chat, most often using frequencies in the high frequency amateur radio bands. PSK31 was the first widely adopted HF digital mode to use a computer sound card as the modem, opening the door to the explosion of digital modes that followed. It allows real-time keyboard-to-keyboard conversation on HF using only 31 Hz of bandwidth — narrower than a CW signal — making it extraordinarily efficient and capable of pulling readable signals out of noisy band conditions.
The History and Origin of PSK31
PSK31 was developed and named by English amateur radio operator Peter Martinez (call sign G3PLX) and introduced to the wider amateur radio community in December 1998. PSK31 was the brainchild of Peter Martinez, G3PLX — if the call sign seems familiar, you might recall Peter as the father of AMTOR.
PSK31 was created by Peter Martinez in the mid-1990s. In the early stages, PSK31 signals were sent using specialized DSP platforms. However, in 1999, Peter released software that allowed PSK31 to be sent and received using a computer soundcard. In an article that appeared in RadCom, the journal of the Radio Society of Great Britain, Peter explained why he developed PSK31 — simply put, he wanted to create a mode that was as easy to use as RTTY, yet much more robust in terms of weak-signal performance.
The 31 baud BPSK modulation system used in PSK31 was introduced by Pawel Jalocha (SP9VRC) in his SLOWBPSK program written for Motorola's EVM radio. Instead of the traditional frequency-shift keying, the information is transmitted by patterns of polarity-reversals, sometimes called 180-degree phase shifts. PSK31 was enthusiastically received and its usage spread rapidly worldwide. Due to the efficiency of the mode, it became especially popular with operators whose circumstances do not permit the installation of large antenna systems, the use of high power, or both.
How PSK31 Works: Phase Shift Keying Explained
PSK31 uses a form of modulation known as phase shift keying. This is rather different from the frequency shift keying used for modes such as RTTY. PSK involves reversing the polarity, or phase, of the signal through 180-degree phase shifts. Individual characters are represented by a binary sequence of 1s and 0s, the sequence being determined by a system called Varicode. The number of bits per character varies based on how commonly the letter is used — an "e" is represented by very few bits, achieving greater efficiency.
Varicode was designed so that the more frequently occurring characters had shorter encodings and the rarer characters used longer encodings, a coding scheme similar to Morse code. This elegant approach to encoding is one of the key reasons PSK31 is so efficient at matching human typing speed while keeping its signal extremely narrow.
Why PSK31 Became the Most Popular HF Digital Mode
PSK31 is distinguished from other digital modes in that it is specifically tuned to have a data rate close to typing speed, and has an extremely narrow bandwidth, allowing many conversations in the same bandwidth as a single voice channel. This narrow bandwidth makes better use of the radio frequency energy in a very narrow space, thus allowing relatively low-power equipment — as low as 5 watts — to communicate globally using skywave propagation.
When Peter released the sound card software in 1999, PSK31 quickly became one of the most popular digital modes in amateur radio due to the widespread availability of such a setup. Any operator with an SSB transceiver, a computer, and a simple interface could suddenly access a sophisticated digital mode without specialized hardware.
PSK31 vs Other Digital Modes: RTTY, FT8, and Olivia
RTTY is one of the oldest digital modes and remains widely used, especially in contests. PSK31 is known for narrow bandwidth and conversational capability. Olivia and similar modes provide excellent noise resistance and readability.
Olivia is a multi-tone FSK mode developed for keyboard conversations under very difficult conditions — it outperforms PSK31 in poor signal-to-noise ratios at the cost of wider bandwidth. Olivia 8/500 (8 tones, 500 Hz bandwidth) is a popular choice for difficult paths. For good band conditions, PSK31 is faster and narrower. For marginal conditions where PSK31 struggles, Olivia maintains readable copy.
How PSK31 Works: The Technical Breakdown
Phase Shift Keying and Varicode Encoding
PSK31 encodes text using phase shifts in a continuous carrier signal. A phase shift represents a "1" and no phase shift represents a "0." At 31.25 baud, the signal is slow enough that it fits in just 31 Hz of bandwidth — you can fit a hundred PSK31 signals in the space occupied by a single SSB phone signal.
The mode's design emphasizes simplicity and robustness, transmitting a single continuous tone that undergoes 180-degree phase shifts to represent binary data, resulting in a theoretical occupied bandwidth of about 31 Hz and a practical width of around 62.5 Hz at -60 dB.
Bandwidth and Spectral Efficiency
The "31" in PSK31 comes from its speed — 31.25 baud, which matches typical typing speed. The efficiency of PSK31 is remarkable: it only requires about 31 Hz of bandwidth, meaning you can fit up to 20 PSK31 conversations in the space needed for one SSB voice contact.
The narrow bandwidth means that atmospheric noise, which is distributed across the spectrum, contributes very little power to the received signal, giving PSK31 excellent performance in noisy HF conditions. This spectral efficiency is a defining feature that helped PSK31 achieve worldwide adoption and explains why a busy 20-meter segment can host dozens of simultaneous conversations in just a few kilohertz.
BPSK31 vs QPSK31: Understanding the Variants
BPSK31 (Binary PSK) is the most common variant and the standard for casual contacts. QPSK31 (Quadrature PSK) encodes two bits per symbol using four phases, doubling the data rate but requiring better signal quality. PSK63 and PSK125 are faster variants useful when band conditions are good. Most PSK31 software supports all variants. For day-to-day contacts, BPSK31 is the right choice — it has the best sensitivity and widest software support.
QPSK31 adds two additional channels for error correction, but it is far less popular than BPSK31. Unless you have a specific reason to use QPSK31 — such as a known noisy path where forward error correction would help — stick with BPSK31 for general operating.
Signal-to-Noise Ratio and Weak Signal Performance
PSK31 achieves its weak-signal performance as a result of two factors. First, a PSK31 signal is narrow, being about 31 Hz wide. Second, the structure of Varicode allows the receiving computer to anticipate the times for each data bit. These factors combine to make PSK31 one of the most popular and high-performance digital modes.
Getting Started with PSK31: Equipment You Need
HF Transceiver Requirements for PSK31
Virtually any SSB transceiver will work for PSK31. The same is true of the antenna, as PSK31 will function with a few watts and a wire antenna. PSK31 performance is often virtually the same on both low-end and high-end equipment.
Set your transceiver to USB (Upper Sideband) mode — this is the convention for PSK31 regardless of which HF band you are operating on. Modern radios like the Icom IC-7300, Yaesu FT-991A, and Kenwood TS-590SG are all excellent choices because they include built-in USB audio interfaces, which simplify the computer connection considerably. Even older radios without built-in sound cards work perfectly well with an external interface.
Computer Interface and Sound Card Setup
PSK31 uses the same sound card interface connection as other HF digital modes — either a dedicated interface like a SignaLink USB or your radio's built-in USB audio. The only major requirement for the computer used to send and receive PSK31 signals is that it contain a sound card. A faster CPU and more memory are desirable, however PSK31 will work on virtually any PC equipped with an internal or external sound card.
The audio path is critical to PSK31 operation. Your computer's sound card output connects to the radio's audio input (either the microphone jack or accessory data port), and the radio's audio output connects back to the sound card input. Most modern transceivers have a dedicated data port on the rear panel that is designed exactly for this purpose and provides better isolation than the front-panel microphone jack.
USB Audio Interfaces and SignaLink Options
The SignaLink USB from TigerTronics is a popular external sound card that connects to the computer via USB and to your radio via its data port. It is plug-and-play compatible with minimal setup required. The SignaLink handles audio level adjustment via front-panel pots and provides VOX-based PTT (push-to-talk) keying, which means no additional PTT cable is required for most setups.
Other popular options include the RIGblaster series from West Mountain Radio, which offers models with and without built-in sound cards, and the MASTERS Communications DRA series. Many modern transceivers with USB connectivity — like the Icom IC-7300 — effectively have a built-in interface and require nothing more than a standard USB cable to get on the air with PSK31.
PTT Control and CAT Interface Basics
If the radio's CAT port is already connected to the computer, Ham Radio Deluxe and its multimode software Digital Master 780 will send the keying command via the CAT connection. CAT (Computer Aided Transceiver) control also allows your logging and digital mode software to read and set the radio's frequency automatically, which is especially useful for keeping your log accurate when you jump between bands.
For PTT control without CAT, most interfaces use a serial port (real or virtual) with the RTS or DTR line used to key the transmitter. Fldigi, DM780, and other software all support this method. Configure the COM port number in your software's settings to match whichever port your interface is connected to.
Best PSK31 Software for Ham Radio Operators
Fldigi: The Go-To Free PSK31 Program
Several excellent free programs support PSK31. Fldigi (Fast Light Digital Modem Application) is the most feature-rich and supports PSK31 along with dozens of other modes including RTTY, SSTV, Olivia, and more. Download it free at w1hkj.com.
Fldigi's waterfall display is clear and intuitive. The Fldigi waterfall display will fill with the vertical striped traces characteristic of PSK31 signals. Each pair of closely-spaced vertical lines is a PSK31 signal. Click on a signal to tune to it and Fldigi will start decoding the text. Fldigi also includes a built-in macro system, logging capability, contest support, and integration with Flrig for rig control. For most PSK31 operators, Fldigi is the only software they will ever need.
Ham Radio Deluxe Digital Master 780
Digital Master 780 (DM780) is part of the Ham Radio Deluxe program. Once installed, you will need to configure it for your particular radio, including setting up the audio input and output in DM780 as well as the radio control settings. In addition to the software, set up your radio for CAT control.
Ham Radio Deluxe/DM780 is a commercial product with a subscription fee, but it offers tight integration between the logbook, rig control, and digital modes in a single application. DM780 supports a wide variety of digital modes, including RTTY, Olivia, MFSK, and more. It's a strong choice if you want an all-in-one station management solution and are already using Ham Radio Deluxe for logging and rig control.
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