The text on this page describes the majority of sound-card digital modes available to amateur radio operators at the time it was written, as well as the software that implements it. As time passes, it is becoming obsolete (at least until I find the time to bring it up to date). If you are only looking for a simple list of links to these programs, try my digital software links page. For links to instructions for hardware interfacing, look here.

Radioteletype, or RTTY, is the original "digital" mode. While there are several possibilities, in HF ham radio RTTY most often means 170-Hz shift, 60 WPM Baudot radioteletype. Originally based on the use of surplus teletype equipment, RTTY began to expand in popularity in the early 1980s when microprocessor-controlled terminal units that could interface with a computer or a dumb terminal (remember those?) began to appear. For many years, these terminal units and multi-mode TNCs dominated the RTTY scene, although the HAMCOMM shareware program (DOS only) using a simple op-amp interface was another way to get started on RTTY.

When sound cards started becoming standard equipment in home computers, the option of using DSP techniques to encode and decode digital modes became open to everyone. Until recently, if you wanted sound-card RTTY, you could buy either the WriteLog contesting program for Windows, or RITTY for DOS by Brian Beezley, K6STI. During recent years, both MixW (demo and shareware versions), and TrueTTY (shareware) became available to support sound-card RTTY, but the majority of RTTY activity still used hardware TUs.

A major impetus to sound-card RTTY came in August, 2000 when MMTTY was made freely available. This program has quickly become very popular. HamScope, Logger32, Zakanaka and WinWarbler can now also be added to the list of available free RTTY software. Interestingly, however, while these programs offer other modes as well, they all rely on the MMTTY "engine" to decode and encode RTTY signals. RCKRtty, an RTTY program originally written for multi-mode TNCs, now supports sound-card RTTY using the MMTTY engine. N1MM Logger, a free contest logging program, also uses MMTTY to support sound-card RTTY, as does the YPLog logging program.

MMTTY was written by Makoto Mori, JE3HHT. It uses a sound card under Windows to do RTTY with user-selectable shift, speed and filter techniques; if preferred, it can also transmit using FSK on radios which have this capability. The user interface is quite complete, if somewhat cluttered, and there has been considerable effort put into making the on-line help file user-friendly. The current version may be downloaded from the MMTTY web site. There has also been some work done on combining MMTTY with WF1B's RTTY contesting program, which has recently been released as free software, although to date that requires either two computers or two unused COM ports on a single computer (here's how).

In addition to the standard 170Hz shift RTTY, there is now a 23Hz shift version (minimum-shift teletype, or MTTY, not to be confused with the MMTTY program). This version is supported by most of the current sound-card RTTY programs (HamScope, Logger32, MixW, MMTTY, N1MM Logger, RCKRtty, TrueTTY, WinWarbler, YPLog and Zakanaka). It offers the same speed and character set (uppercase only) as standard 170Hz RTTY, but takes up much less bandwidth.

Recently, another multi-mode program, MultiPSK by F6CTE, which supports RTTY at several speeds in addition to the standard 45-baud rate, has become available.

Probably the most popular activity on RTTY now is contesting. Almost every month of the year there are at least two RTTY contests scheduled.

PSK31 is the first of the "new" digital modes to become widely popular, and is now probably the most popular digital mode, except on RTTY contest weekends. The name stands for "Phase-Shift Keying, 31.25 baud". PSK31 was designed for keyboard-to-keyboard conversational use by Peter Martinez, G3PLX (who previously had introduced AMTOR to amateur radio). It is possible to conduct 40 WPM QSOs at low power within a bandwidth of only a few tens of Hz using this mode, although it can be susceptible to burst noise and especially to polar flutter (multipath). It was introduced to the amateur community at large in December 1998. For much more descriptive material, visit the "official" PSK31 home page. There are also various sites devoted to interface wiring for PSK31 and other sound-card modes, including PTT, which is usually performed (if needed -- you can also use VOX) by switching RTS or DTR on an unused COM port. Several of these sites, as well as sites where commercial interface kits are described, can be reached from www.psk31.com.

The original PSK31 program for Windows, PSK31SBW, is still available. Before it was supplanted by the many newer PSK31 programs, this program was often operated together with the PSKGNR front-end software. The PSKGNR program also introduced a PTT interface using the computer-control capabilities of recent Icom, Kenwood or Yaesu radios. This PTT interface is done through a separate stand-alone program (IPTT, KPTT or YPTT), which can also be used by some other software, including IZ8BLY's programs. During the early days of PSK31 (spring and summer of 1999), the PSK31SBW/PSKGNR combination was the "standard" software for PSK31.

Soon after PSK31 started becoming popular, Nick Fedoseev, UT2UZ, released a program that combined several digital modes, including PSK31, RTTY, and several other modes in one package. This program, MixW, has been available since that time in both full-featured shareware version and a demonstration version. At the end of April 2001, a totally revamped version 2 of MixW was released. This version supports a huge range of digital modes (besides the other modes described on this page, it supports HF and VHF packet radio, SSTV, and can receive fax transmissions), a very good user interface (vastly improved over versions 1.x), transceiver control, a DX cluster window, and various other features. This version is a "must-see"!

Another popular PSK31 program is Digipan. This program introduced a panoramic display of 4000 Hz of received spectrum (if your radio's IF filters are set that wide), allowing the user to see all of the PSK31 activity on a given band at one time, with two simultaneous receive windows to decode two signals simultaneously.

Logger, a free Windows logging program by K4CY, also supports RTTY using a TNC and has its own sound-card PSK31 program built in. Logger can also control radios equipped with a computer control port, including controlling the PTT function using either software control or a hard-wired interface. This program has now been superseded by a new version, Logger32.

Another multi-mode program which includes PSK31 in its repertoire is UA9OSV's TrueTTY. This shareware program supports RTTY, ASCII, AMTOR-FEC, and MFSK16 as well as PSK31. Stream, HamScope and MultiPSK also support PSK31.

The remainder of the programs mentioned in this section all use the same "engine" for encoding and decoding PSK31 signals, although the user interfaces vary quite widely. Moe Wheatley, AE4JY, is the author of the Windows Dynamic Link Library PSKCore.dll, which he has made available to anyone who wished to incorporate it in a PSK31 program. The release of this DLL in May 2000 spawned a multitude of new PSK31 programs using the same PSK31 code with different user interfaces. The first of these was AE4JY's own WinPSK. This was followed by KA1DT's WinPSKse, featuring two receive channels and various other enhancements to the user interface.

K4CY's Zakanaka has a unique user interface with up to three simultaneous receive channels, in windows which can be moved independently and have several tuning aids including a panoramic waterfall or spectrum display as well as individual spectrum and phase scope displays for each receive channel. This program includes its own minilog, or it can be used together with Logger (also by K4CY) to make use of the latter's full logging and radio control features. In addition to PSK31, Zakanaka also supports sound-card RTTY by calling MMTTY as a helper program. Logger and Zakanaka have now been superseded by Logger32.

Other PSKCore-based programs include WinWarbler, DXPSK, and HamScope. Another program in this series is W1SQLPSK, which decodes up to 20 (yes, that's right, twenty!) simultaneous receive channels within the audio bandwidth of an SSB receiver. Several other programs, including MultiPSK, PSK31 Deluxe, QuikPSK and SmartPSK now also offer a similar capability to decode more than 20 signals simultaneously. Yet another PSK31 program is WO-PSK by ZS5WO. This one does not have multiple receive channels, instead using a "replay" feature which can be used to look at any signal in the waterfall display. Some commercial and shareware logging and RTTY programs, such as RCKRtty, YPLog and DX4WIN, also support PSK31 using PSKCore, as does the N1MM Logger free contest software. Note that WO-PSK uses an ActiveX version, developed by WA0TTN, instead of the basic PSKCore DLL.

The vast majority of PSK31 QSOs take place in BPSK mode. From the beginning, a QPSK mode has also been available, in which four phases rather than two are sent. This allows information to be transferred at twice the rate of BPSK; this speed is used to add forward error correction (FEC), which allows the same typing speed as BPSK with increased immunity to burst-type noise. All of the PSK31 programs described here except for the IZ8BLY Stream program support both BPSK and QPSK.

IZ8BLY's Stream program introduced two higher-speed variants on PSK31, called PSK63F and PSK125F. These modes have higher raw speed and use more bandwidth than PSK31, as suggested by their names. Both also make use of forward error correction, or FEC, to reduce the effects of errors. PSK63F therefore has the same text speed as PSK31 in twice the bandwidth, while PSK125F is both twice as fast and four times as wide as PSK31. Like QPSK, neither of these modes has really caught on.

However, in April 2003, Skip Teller, KH6TY, one of the co-authors of Digipan, introduced a modified version of the PSKCore dll which supports a mode called PSK63. Unlike PSK63F, PSK63 does not use FEC. Therefore its text speed is twice as fast as PSK63F's (i.e. the same as PSK125F's) in the same bandwidth (half of PSK125F's). Skip has suggested that PSK63 should be a good contesting mode, noting that it is both faster and uses less bandwidth than standard RTTY.

PSK63 is now supported directly in KH6TY's own QuikPSK software, as well as in Digipan, AA6YQ's WinWarbler, F6CTE's MultiPSK, AE4JY's WinPSK, HB9DRV's PSK31 Deluxe and DL4RCK's RCKRtty.Others are likely to follow, now that version 1.17 of the PSKCore dll supports both PSK31 and PSK63. QuikPSK, MultiPSK and PSK31 Deluxe can decode up to 24 signals simultaneously. QuickPSK has a unique additional capability to send colour thumbnail pictures (32x32 pixel, 16 colours) using the PSK63 mode.

A PSK63-only version of the PSKCore dll is also available at KH6TY's web site for use with any software that uses PSKCore to implement PSK31. Simply by replacing the original PSKCore file (it is suggested that you rename the original rather than deleting it) with the new version, the PSK31 software supports PSK63 instead of PSK31. This technique has been used during early experiments with the PSK63 mode, but it is not likely to continue to be very widely used now that software that supports PSK63 directly has become widely available.

In MixW, UT2UZ introduced another mode, called FSK31, which is said to use the 90/270 degree phase shift components also found in QPSK, but without BPSK's 0/180 degree phases. This mode is claimed to be much less susceptible to spurious sidebands than BPSK; these spurious sidebands, which can be easily caused by overdriving the sound card or the audio signal to the radio, are PSK31's equivalent to splatter in SSB or key clicks in CW. The FSK31 mode is supported by Digipan as well as by MixW. However, this mode is rarely encountered on the amateur bands.

MFSK stands for multi-frequency shift keying. MFSK16 has attracted quite a bit of interest (especially since an article in the January 2001 QST). It fits 16 audio tones into a bandwidth of 316 Hz, only slightly wider than the occupied bandwidth of a standard RTTY signal (but much wider than a non-overdriven PSK31 signal). MFSK16 is another conversational keyboard-to-keyboard mode, with similar throughput speed to PSK31 (around 42 WPM), but better resistance to ionospheric effects and burst noise. The first MFSK16 QSO took place on 18 June 2000, between Nino Porcino, IZ8BLY and Murray Greenman, ZL1BPU. The characteristics of this mode are described in detail at the ZL1BPU website.

The original MFSK16 program, Stream, was written by IZ8BLY. It also supports MFSK8, which is more robust, but only half the speed, as well as PSK31 (BPSK only) and two PSK variants, PSK63F and PSK125F. The waterfall display used for tuning permits point-and-click adjustment of the audio frequency. Like other IZ8BLY programs, it can be interfaced with the Logger program for both logging and radio control functions such as PTT, or with the WD5GNR PTT helper programs.

Another program supporting MFSK16, HamScope, is now also available. This program has RTTY and PSK31 modes in addition to MFSK16, and now also supports packet radio using the AGWPE software.

A third option for MFSK16 is version 2 of MixW. Like HamScope, MixW has a zoomable waterfall display for tuning, and integrates its user interface for MFSK16 very nicely with the wide range of other digital modes it supports. MFSK16 has recently been added to the repertoire of TrueTTY as well.

A new MFSK mode has recently been introduced by ZL1BPU, called domino. This mode has similar characteristics to MFSK16, but is much easier to tune. ZL2AFP has written a domino software program which you can use to try out this new mode.

Throb is another multi-frequency shift keyed mode, or rather group of modes. Unlike MFSK16, which transmits one of 16 audio tones in succession at a 15.625 baud rate, Throb sends two out of a possible nine tones simultaneously at a user-selectable rate of 1, 2 or 4 baud (10, 20 or 40 WPM) and a bandwidth of 72 Hz (144 Hz at 40 WPM). Thanks to its soft keying waveform, Throb has a distinctive throbbing sound. Because of the large number of tones within a narrow bandwidth, it can be difficult to tune in a signal in this mode. Another price paid for the combination of narrow bandwidth and robustness is a low typing speed and a reduced character set (uppercase only). The 4 throb/second version is the fastest and the easiest to tune. Both the Throb mode and (until recently) the only available software were developed by Lionel Sear, G3PPT, but DL5SWB has recently released an enhanced version of the original program. Version 2 of MixW now supports Throb as well, and is probably the easiest to use of these Throb programs.

The MT63 mode is yet another multi-frequency shift keyed digital mode, whose main feature is its strong immunity to QRM. It was developed by Pawel Jalocha, SP9VRC, in 1999. This mode is relatively broadband (500 Hz, 1 kHz or 2 kHz bandwidth), and makes use of both frequency and time redundancy to achieve high immunity to interference and noise. It can be a bit difficult to spot an MT63 QSO in progress, as the audio sound of this mode resembles white noise. Until recently, the only readily-available software for this mode for Windows PCs was IZ8BLY's MT63 program. MT63 is now also supported by version 2 of MixW.

Because of its large bandwidth, it is suggested that this mode not be used under crowded conditions. While its own performance will shine under these conditions, it is considered "aggressive" and it is felt that it can cause unacceptable QRM to other more traditional modes. Nevertheless, I have more than once experienced situations with MFSK16 and MT63 QSOs taking place simultaneously on the same frequency, without any noticeable interference between the two QSOs! However, this relative tolerance between these two modes requires that neither signal be a lot stronger than the other, a condition which is hard to guarantee on HF. In addition, despite a very high maximum typing speed (100 WPM), there is a noticeable delay of several seconds between typing a character and its final transmission, caused by the strong forward error correction in MT63. This makes turnaround times from one station to the other very long by normal amateur radio standards. Copy is also something of an all-or-nothing proposition, meaning that you can lose contact with the other station without much warning.

Hellschreiber, named after its inventor, Rudolf Hell, has actually been around since 1929. In contrast to teletype machines, which use a digital code to operate a typewriter-like keyboard printer, Hellschreiber (or Hell) uses digital signals to control the printing of dots on paper (or your PC screen) in a pixel-type font, rather like a low-resolution facsimile system, at a speed of about 25 WPM. The biggest advantage of this mode is that it can use the operator's eyes and brain to dig out signals from the background noise. As any CW operator who has tried out digital decoding of Morse code can tell you, the human brain is better at making sense out of ambiguous noisy signal situations than any software yet available to the amateur community.

There has actually been some amateur radio activity on Hell since 1958, but the level of interest only started to increase when PC sound card software started becoming available in the late 1990s. Since that time, there has been experimentation with a number of variants, including not only the original CW-like Feld-Hell, but also PSK, FSK and multi-tone versions. DOS software for some of these versions has been written by G3PPT, LA0BX, and ZL1BPU, but the software most likely to be of interest for investigating these variants is IZ8BLY's Hellschreiber program for Windows. The basic Feld-Hell mode as well as FM (FSK) Hell are also supported by version 2 of MixW. MultiPSK also supports the Feld-Hell mode.

ZL1BPU's "Fuzzy modes" web site is loaded with information on Hellschreiber, as well as on the MFSK16 and MT63 digital modes.

If you are interested in VHF weak-signal communications, whether strictly terrestrial or EME (earth-moon-earth), you will want to know about WSJT. This program supports three new sound card digital modes, FSK441, JT6M and JT44. FSK441 and JT6M are designed to take advantage of meteor "pings", which permit brief contacts over 1000-km paths. FSK441 is intended for higher frequencies, while JT6M is optimised for 6 metres. JT44 is a much slower mode designed for EME and tropospheric scatter communications, where it can decode signals at astonishingly low signal-to-noise ratios. This program has made weak-signal VHF DX considerably more accessible than it once was.

Before the recent "sound-card revolution", much of the interest in development of digital modes in amateur radio was centred on ARQ (automatic repeat request) modes. These are modes which use a "handshaking" technique to ensure error-free communication of information between two stations. The first of these was AMTOR, introduced to amateur radio by G3PLX. It was followed by Pactor, G-TOR, Pactor-2 and Clover. While there are some DOS programs available for AMTOR and Pactor, the more recent ARQ modes are proprietary and are implemented only in commercially-available hardware.

HF packet radio still can be found on the air, although its performance on HF has always left a lot to be desired. Unlike the ARQ modes described above, however, the timing requirements for packet radio are not so stringent, and it is possible to use a sound card under Windows to do packet radio. MixW supports packet radio, and there is a suite of sound card software available from SV2AGW.

There is a new experimental sound card mode which offers the hope of better HF performance than packet. This mode is called Q15X25, and it is supported by the latest version of MixW. It shares some of the characteristics of MT63, including fairly high bandwidth. It is not suitable for ragchewing (the handshaking overhead is too high for normal ragchewing data rates), but may yet prove to be a suitable method for high-volume data transfers.

None of these ARQ modes has been widely used in contests or by DXpeditions, as the time required to initialize a handshaking link is not compatible with the short QSOs typical of these activities. On the other hand, ARQ modes are well-suited to automatic forwarding of messages and files between BBS stations, an amateur radio activity which can no longer compete with the Internet and is on the wane.