AFSK vs FSK vs Data Mode: RTTY and Digital Settings

Open the menu of a modern transceiver and you will find a small collection of mode names that look alike: USB, LSB, DATA, USB-D, RTTY, FSK, AFSK, PKT. They all deal with sending digital information, and they do it in different ways. Choose the wrong one and the signal may be wider than it should be, it may be distorted by the microphone circuits, or the other station may simply not decode you. This guide explains the real difference between AFSK vs FSK and the data mode setting, shows what each one does inside the radio and tells you which to use for RTTY, for sound card modes such as FT8 and PSK31 and for the packet and other modes where the answer is a little different.

What FSK Means

Frequency-shift keying is a simple idea. According to the Wikipedia article on frequency-shift keying, it is a frequency modulation scheme in which digital information is encoded on a carrier signal by periodically shifting the frequency of the carrier between several discrete frequencies. The simplest form is binary FSK, which uses two frequencies: one for a binary one, called the mark, and one for a binary zero, called the space. In radioteletype, the shift between the two frequencies is traditionally 170 Hz, and the transmitter’s carrier moves between the mark and space frequencies in step with the data.

In a transceiver with a true FSK mode, the data comes in as a simple on-off signal, often through a serial port line or a dedicated FSK input, and the radio itself shifts the frequency of its oscillator. There is no audio tone involved. The signal that goes to the antenna is a constant-amplitude carrier that moves between two frequencies, and the transmitter can be driven at full power with no concern for the linearity of the audio chain.

What AFSK Means

Audio frequency-shift keying does the same thing one step earlier. The article defines it as a modulation technique by which digital data is represented by changes in the frequency, or pitch, of an audio tone, producing an encoded signal suitable for transmission via radio or telephone. Normally the transmitted audio alternates between two tones, a mark for a binary one and a space for a binary zero. It differs from regular FSK in that the modulation is performed at baseband frequencies, and in radio applications the AFSK signal is then used to modulate an RF carrier in a conventional way, such as SSB or FM. The article also says that AFSK is still widely used in amateur radio because it allows data transmission through unmodified voiceband equipment.

This is the practical meaning. When you run RTTY with a sound card and an SSB transmitter, the computer makes two audio tones, for example a pair of tones with a 170 Hz difference, and the radio sends them as if they were a voice. The software that runs on the computer decides the baud rate and the tones, and the transmitter only needs a good SSB signal. The same principle underlies PSK31, FT8 and nearly all the sound card modes, although those use more complex waveforms than two tones. They are all AFSK-like in the sense that the audio signal that carries the data goes through the SSB modulator.

A practical detail is the choice of tones. Many RTTY programs use a mark tone of 2125 Hz and a space tone of 2295 Hz, which gives the 170 Hz shift, and some radios and interfaces let you change them. The values are not magic; what matters is that the two tones are inside the passband of the transmitter and receiver filters and that your software and the other station agree on which one is the mark. If you use AFSK on a radio with a narrow transmit filter, check that both tones pass through it without a loss of level, because an asymmetric response will make one tone weaker than the other and degrade the decoding at the receiving end.

What the Data Mode Setting Does

Many radios offer, in addition to USB and LSB, a mode called DATA, USB-D, LSB-D or PKT. This is not a different modulation. It is a variation of SSB in which the radio changes the audio path to suit a digital signal: the audio comes from the rear data connector or the USB audio device and not from the microphone, the microphone equaliser and the speech processor are switched off, the audio filtering may be flattened and the transmit and receive bandwidths may be set differently. In some radios, the data mode also allows the use of the data connector’s PTT line and changes the behaviour of the AGC.

The reason this matters is that a voice path is designed to make speech sound good, and that shapes the signal. A compressor or an equaliser changes the amplitude and the frequency response of the tones, which can distort a digital signal and widen it. The data mode removes these stages and gives the computer a clean path to the modulator. If your radio has such a mode, use it for FT8, PSK31 and other sound card modes, and set the audio drive so that the ALC shows no action. Our guide to SSB audio, mic gain, compression and ALC covers the controls that you must keep neutral, and the article on the digital interface and isolation covers the connection to the computer.

The Three Approaches Compared

RTTY: FSK or AFSK?

For RTTY, both ways work, and the choice is a matter of taste and equipment. The article on radioteletype gives the history of the mode, and our guide to RTTY contesting covers the software and the exchanges. Many serious RTTY operators prefer direct FSK because the transmitter generates the two frequencies itself, the signal has a constant amplitude and there is no audio chain to overdrive. The signal is clean, the bandwidth is tight and no ALC action is needed. The price is a more complicated connection: the program has to key the FSK input through a serial port or a USB interface, and you must make sure that the lines do not key the transmitter by accident. The method of using a serial control line for keying is the same as for CW, as discussed in the article on the CW keying interface and DTR/RTS lines.

AFSK on RTTY is the easy choice. It needs only the same audio connection that you use for the other digital modes. The Wikipedia article warns that AFSK is not always used for high-speed data because it is less efficient in both power and bandwidth than most other modulation modes. For a 45 baud RTTY signal this is not a serious drawback, if you keep the audio level low enough to avoid ALC and distortion. A drive that is too high produces intermodulation products that widen the signal and cause interference to neighbours in a crowded contest. In both cases, the sideband matters: RTTY is normally operated with the signal in the lower sideband sense, which is why many radios offer a separate RTTY mode that is effectively LSB with narrow filters. Our article on USB versus LSB explains the conventions, and a wrong sideband gives a signal with the mark and space reversed, so you decode nothing, unless the software has a reverse option.

FT8, FT4, PSK31 and Other Sound Card Modes

For these modes, you use the SSB data setting, upper sideband, and the audio from the sound card. There is no FSK input to use and no reason to look for one. The key point is cleanliness: the transmitter has to reproduce the audio tones without adding anything. Set the audio level in the computer so that the output power is clearly below the maximum, with no ALC reading and with the speech processor off. A rule often given for FT8 is to run the radio at no more than about half of its power rating, because the mode transmits for long periods and the transmitter must stay cool and linear. For the operating side of FT8, see our guide to FT8 operating etiquette, power and ALC.

Packet and FM Data

For 1200 baud packet on VHF, the radio is normally in FM, with a data setting that bypasses the microphone and uses a flat audio path. The Wikipedia article gives the background: 1200 baud AFSK is the same kind of modulation as the Bell 202 standard, used in early telephone modems and still used in North American caller ID. In amateur radio, this is the signal that a packet or APRS station sends, and it is an FM carrier modulated by the two tones. The deviation, set by the audio level, matters in the same way as the drive does on SSB, as does the pre-emphasis in the radio: in the data mode, it should be flat so that the tones keep their proper relative level. Check the settings of your radio against the manual of the software or the terminal node controller.

Common Mistakes

• Using USB and the microphone input with the processor on. The compression and equaliser can distort the tones and widen the signal.
• Overdriving the audio. Too high a level in AFSK gives a splattering signal that is wider than it should be, and it is the most common digital mode fault.
• Wrong sideband. A sideband that does not match the software’s expectation reverses the tones, and in modes where the sideband matters the signal cannot be decoded.
• Letting the serial line key the transmitter. In FSK, the software’s control lines must be set to the right state so that the radio does not transmit when the program opens.
• Forgetting to change the setting back. After you have used DATA, remember that the microphone is not active, so a voice contact needs the normal SSB mode again.
• Using wide filters. Narrow the receive filter to the width of the mode, which reduces noise and neighbours, and widen it again for voice.

The Bottom Line

FSK shifts the transmitter’s carrier between two frequencies directly, which gives a constant-amplitude signal and is favoured by many RTTY operators. AFSK sends audio tones through an SSB or FM transmitter, which is simpler and works with unmodified voiceband equipment but is less efficient and needs careful audio levels. The DATA or USB-D setting is SSB with the voice processing bypassed, and it is the right choice for FT8, FT4, PSK31 and other sound card modes. Keep the drive low enough for the ALC to stay quiet, use the right sideband and check your radio’s manual for the details of the data path. With the correct setting, your signal stays narrow, clean and decodable.

Frequently Asked Questions

What is the difference between AFSK and FSK?

FSK shifts the frequency of the transmitter’s carrier directly. AFSK shifts the frequency of an audio tone, which then modulates the carrier, usually in SSB or FM.

Should I use DATA or USB for FT8?

Use the DATA or USB-D setting if your radio has one. It bypasses the microphone processing and gives a cleaner signal. If it does not, use USB and keep the processor and equaliser off.

Is FSK better than AFSK for RTTY?

Many contest operators prefer FSK because the signal has a constant amplitude and there is no audio chain to overdrive. AFSK works well with correct levels and is simpler to connect.

Why is RTTY on lower sideband?

It is the usual convention, and radios often have an RTTY mode that is effectively LSB with narrow filters. What matters is that your software and the other station agree on which tone is the mark.

Why is my digital signal wide?

The most likely cause is too much audio drive, which pushes the transmitter into non-linear operation or ALC action. Reduce the level and turn off compression.

Does packet radio use AFSK?

Yes. 1200 baud packet uses AFSK tones, of the same kind as the Bell 202 modem standard, on an FM carrier.