PSK31 Guide: Frequencies, Setup and Clean Operating Habits
PSK31 is the mode that brought sound card digital communication to the amateur bands and made keyboard-to-keyboard chat on HF a daily activity for thousands of operators. It was introduced in 1998, long before FT8 existed, and it still has a faithful following among people who like to type to each other in real time instead of exchanging fixed messages. It works with a low power transmitter, a modest antenna and a computer, and it uses so little bandwidth that a dozen conversations can share the space of a single voice channel. This guide explains how PSK31 works, how BPSK31 differs from QPSK31, where to look for activity, how to set up the audio chain without splattering and how to behave on the band.
What PSK31 Is
The Wikipedia article on PSK31 describes it as a popular computer sound card generated radioteletype mode, used mainly for real-time keyboard-to-keyboard chat on the HF bands. Its data rate is tuned to be close to typing speed, and its bandwidth is extremely narrow. The mode was developed by the English amateur Peter Martinez, G3PLX, and introduced to the wider amateur community in December 1998. The 31 baud BPSK modulation system came from a program by Pawel Jalocha, SP9VRC. Because the symbol rate is 31.25 per second, one symbol lasts 32 milliseconds, and the same article explains why that number was chosen: a typing speed of about 50 words per minute needs a bit rate of about 32 bits per second, and 31.25 Hz is easily derived from the 8 kHz sample rate of many sound systems.
To the transmitter, the signal is just an audio tone, like somebody whistling into the microphone. The software changes the phase of this tone in step with the data, which is why the mode is called phase shift keying. The ITU emission designation is 60H0J2B, often shortened to J2B, according to the article. The information goes into the phase and not into the amplitude or the frequency, so the receiver compares the phase of each symbol with the one before it.
BPSK31 and QPSK31
When people say PSK31, they usually mean BPSK31, the binary version. In BPSK31 each symbol either keeps the phase of the carrier or reverses it by 180 degrees, and there is no error control. QPSK31 uses four phases instead of two and adds a convolutional code with a rate of one half, so it can correct some errors. The article says that QPSK31 has the same number of symbols per second, and so the same bandwidth, as BPSK31, and that switching is easy if difficulties arise during a contact. The price is that both stations must use the same sideband, because QPSK depends on the sense of the phase, and the two operators have to agree to switch.
| Mode | Symbol rate | Error control | Best for |
|---|---|---|---|
| PSK05 and PSK10 | 5 and 10 baud | None | Very weak signals, patient chat |
| BPSK31 | 31.25 baud | None | The standard mode, easy to find contacts |
| QPSK31 | 31.25 baud | Convolutional code, rate 1/2 | Stable paths, error correction when both sides agree |
| PSK63 | 62.5 baud | None | Faster exchanges, popular in contests |
The Wikipedia article notes that some software supports PSK10 and PSK05, which sacrifice throughput for greater resistance to noise, and that PSK63 is increasingly used for faster exchanges, especially in contest operating. For everyday use, BPSK31 is where the people are.
Why It Fits Low Power and Crowded Bands
The narrow bandwidth is the great strength of the mode. The same article says that PSK31 contacts can be conducted at less than 100 Hz separation, so with disciplined operation at least twenty simultaneous contacts can be held side by side in the 2.5 kHz that a single SSB contact needs. A signal that concentrates its energy in a small slice of the spectrum has a better signal-to-noise ratio in a narrow receiving filter than a wide signal with the same power, so a transmitter of only 5 watts can make contacts across continents when conditions are good. This explains why the mode became popular with operators who have a small antenna or are limited in power.
The limits are also clear. PSK31 has a limited error control and is not suited to the transmission of large blocks of data or critical data. It works well over paths that preserve phase and resists ordinary fading, but it can be disrupted on paths where auroral flutter or multipath disturbs the phase, as on a transpolar path. In these cases, QPSK is often beneficial.
Where to Find PSK31 Activity
The mode normally lives near the lower edge of the digital section of each band. The Wikipedia article lists commonly used frequencies, noting that there is no authoritative list and that some are based on observation. Among them are 3.580 MHz on 80 metres, 10.142 MHz on 30 metres, 14.070 MHz on 20 metres, 18.097 MHz on 17 metres, 21.080 MHz on 15 metres, 24.920 MHz on 12 metres and 28.120 MHz on 10 metres, together with 7.040 MHz on 40 metres in Region 1. Treat these as a starting point and check the current band plan for your region and the activity you see on the waterfall, since habits change. Operators generally use upper sideband for PSK31, even below 10 MHz where the convention is lower sideband, for the two reasons given in the article: signals then spread upwards into the digital segment from the base frequency, and QPSK requires both stations to use the same sideband.
Setting Up the Station
You need a transceiver with SSB, a computer with a sound card, the interface cables and a program. Fldigi is the most common choice and is described in our fldigi setup guide, which covers the sound card, rig control and macros. The connection between the radio and the computer, with its ground loops and isolation, is discussed in the article on the digital interface and isolation, and if you work with virtual audio devices, see the guide to the virtual audio cable setup.
- Select the data or USB mode on the radio, with the filters wide enough to pass the audio, typically 2.4 kHz or more, and with processing turned off.
- Set the receive level. In the program’s waterfall the noise should look dark blue, with the signals standing out in yellow or white. Avoid a level that fills the waterfall with colour.
- Set the transmit level. This is the step that decides whether your signal is clean. The Wikipedia article states that all amplification after the generation of the signal must be linear to keep the occupied bandwidth minimal, and that in practice you should limit the transmit audio below the level at which the transmitter generates ALC feedback, and disable audio compression and speech processing. Start with the audio drive at a low level, increase it until the power meter shows a small amount of output, and then stay below the point at which the ALC moves.
- Use low power. A 100 watt radio can be run at 20 to 30 watts for PSK31 without loss, since the mode is efficient and the transmitter works in a linear way for long periods. Our guide to SSB audio, mic gain, compression and ALC explains the same ideas for voice.
- Test the signal. Listen to yourself on a second receiver or a web SDR, or ask another station to report the look of your signal on the waterfall. A clean PSK31 signal shows one narrow trace, while a splattering signal has extra lines on both sides.
Making a Contact
A PSK31 conversation is typed text, and the habits are similar to Morse code and RTTY chat. Find a free frequency, set the waterfall to show about 100 Hz per signal, and call CQ with your call sign a couple of times. A typical CQ looks like “CQ CQ CQ de LY4A LY4A pse k”. When a station replies, click on its trace in the waterfall to lock the receiver on it, send a short exchange of name, location and signal report and then continue with your conversation. Use the macros of the program for the standard parts, and type the replies in real time, which is part of the charm. Keep the typing speed reasonable; the mode itself is limited to roughly typing speed.
- Click on the signal to tune, and do not move your own transmit frequency to follow a drifting station while you are transmitting if you can avoid it.
- Keep a distance of at least 100 Hz from the other signals, and more if the band is quiet.
- Do not call on top of an existing conversation. Listen first to see how long it has been going.
- Use the automatic frequency control carefully. It helps a drifting signal but can lock onto a neighbour.
- Do not use high power as a means to be heard. A cleaner signal is more useful than a stronger one.
Reading the Waterfall and Fixing Problems
The waterfall is your main instrument in PSK31. Each active signal appears as a narrow vertical trace, and a good signal is a single clean line with a slight flicker. If you see two or three parallel lines close to each other, the station is overdriving its transmitter. If your own received traces are faint but the text is clean, leave the receive level alone: the decoder works well with weak signals, and a stronger level only increases the noise. If the text on the screen is garbled when the trace looks good, check that you are not on the wrong sideband and that the frequency of the click is on the centre of the trace. A slow drift of the other station is normal with older equipment, and the automatic frequency control follows it. If the decoder shows text with a few mistakes now and then, that is usual for BPSK, which has no error correction, and you can ask the other station to repeat the part that matters.
Problems on your own transmit side usually have the same causes as in other sound card modes: too much audio, a computer that sends sound effects to the radio by mistake, or a power level that is too high for the transmitter in continuous use. Set the sound card volume so that system sounds are muted, use a separate sound device for the radio if you can, and watch the power output and the temperature of the transmitter during the first long conversation. A good PSK31 signal needs very little power, so there is no reason to run the radio hot.
Where PSK31 Stands Today
New modes with decoders, notably FT8, have taken most of the casual digital traffic, because they work at much lower signal levels and make contacts with little effort. PSK31 survives because it is conversational and because it needs little setup. Our overview of digital modes beyond FT8, FT4, JS8Call, PSK31 and RTTY compares the choices, and the answer depends on what you want from a contact. If you want a quick confirmation of a new country on a poor band, FT8 is better. If you want to talk to someone in real time with a few watts, PSK31 is a very good choice.
The Bottom Line
PSK31 is a narrow, efficient keyboard-to-keyboard mode created by Peter Martinez, G3PLX, in 1998. BPSK31 is the common version, QPSK31 adds error correction, PSK63 is faster and PSK10 and PSK05 are slower and more robust. Use upper sideband, look for activity near the lower edge of the digital segments, set the transmit audio below the ALC threshold with compression and processing off and keep your signal narrow so that many conversations can share the band. With a modest power and a clean signal, it is an effective and friendly mode.
Frequently Asked Questions
What is PSK31?
It is a sound card based digital mode for keyboard-to-keyboard chat on the HF bands, introduced in 1998. It has a symbol rate of 31.25 per second and a very narrow bandwidth.
What is the difference between BPSK31 and QPSK31?
BPSK31 uses two phases and has no error control. QPSK31 uses four phases and a convolutional code that corrects some errors, with the same bandwidth, but both stations must use the same sideband.
Which sideband should I use for PSK31?
Upper sideband, even on bands below 10 MHz where voice uses lower sideband. This keeps signals spreading upward from the base frequency and is required for QPSK.
How much power do I need for PSK31?
Very little. Many contacts are made with 5 to 30 watts. The mode is efficient, and a clean low-power signal is better than a distorted high-power one.
Why is my PSK31 signal wide and splattering?
The most common cause is too much transmit audio, which drives the transmitter into non-linear operation or ALC action. Reduce the audio level and turn off compression or processing.
Is PSK31 still used?
Yes, though FT8 has taken much of the casual digital activity. PSK31 remains popular for real-time chat, low-power operation and as an easy mode to learn.