N1MM, General RTTY and PSK Information


2.6.1 General RTTY and PSK Information

2.6.1 General RTTY and PSK Information
1. RTTY Information
1.1. Common RTTY Frequencies
1.2. General RTTY Information
2. PSK Information
2.1. General PSK Information
2.2. Common PSK and Digital Frequencies

Digital mode contesting is growing rapidly. N1MM Logger supports digital mode contesting, not only RTTY but also other digital modes, with a flexible interface.

This section contains some general information about operating in RTTY that is not directly related to N1MM Logger. If you are new to digital mode contesting in general and RTTY in particular, this information may be helpful. If you are an old-timer at RTTY, you can probably skip this section.

Before the spread of personal computers, RTTY was the most prevalent digital mode (other than CW, that is!), and was done using surplus teletype equipment – mechanical teleprinters. This equipment posed severe constraints on the RTTY mode that are still evident today. Despite these constraints, RTTY has proven to be quite well-adapted to contesting, and it is still by far the most common digital contesting mode.

More recently, these mechanical teleprinters have been replaced by other devices. The first of these terminal units were self-contained microprocessor devices (modems) that converted the 5-bit Baudot code used in RTTY into ASCII characters to be sent to a terminal (keyboard and CRT display). The most popular of these units were multi-mode TNCs (Terminal Net Controllers) that supported RTTY in addition to packet radio and other digital modes. TNCs are still in use today, and N1MM Logger supports the use of TNCs for RTTY.

However, with the advent of the ubiquitous sound card-equipped PC, it became possible to perform the modulation and demodulation directly in a personal computer, and the majority of RTTY today is done using a sound card in a PC. N1MM Logger, of course, also supports this method of modulating and demodulating RTTY and other digital modes, using a sound-card based digital-mode “engine”.

An RTTY signal is a single carrier (like CW), but instead of being modulated on and off like CW, the transmitted power is kept constant, and modulation is imposed by changing the frequency by a preset amount; in amateur usage, the historical practice is to use a “shift” of 170 Hz. That is, RTTY is modulated using frequency-shift keying (FSK). The frequency shifting can be done either at RF, in radios which support this method, or at audio frequencies.

The first method (usually called FSK) requires an on-off keying signal to be applied to a keying input to the radio. This keying is very similar to CW keying, except that instead of turning the carrier on and off as in CW, closing the key input shifts the transmitter’s frequency. FSK therefore requires an on-off keying interface between the computer and the radio, and the radio must have the internal circuitry required to perform the frequency shifting. Radios that support this FSK mode usually have other features that assist RTTY operators, such as specialized filtering.

The second method, using audio tones fed into an SSB transmitter which converts the tones to RF in exactly the same way that SSB converts audio voice frequencies to RF, is called Audio Frequency Shift Keying (AFSK). AFSK can be used with any SSB transmitter. Because the optimum filtering and other settings for RTTY operation are different from those for voice communication, some transceivers offer special AFSK or digital-mode modes, but fundamentally these are the same as SSB.

Digital modes are harder on transmitting equipment than CW and SSB because of the higher duty cycle (sustained periods of full-power transmitting). As a result, it is important not to overstress the transmitter. It is also important to take steps to avoid transmitting extraneous noises or spurious signals.

Here are some tips for RTTY setup and operation:

Hardware interfacing:
Unless your radio has a USB Codec built in (e.g. IC7200 and 7600), in order to receive RTTY you will need to connect the audio output from your radio to the input of the sound card being used with your computer, or if you are using a TNC or TU, to its audio input (see the manual for your TNC/TU for details)
To transmit:
For AFSK, you need to connect the audio output from your sound card or TNC/TU to an audio input on your radio (exception: radios with a built-in USB Codec), either directly or via a sound card interface
For FSK, you need a keying circuit from a serial port to your radio’s FSK keying input. If you are using a USB-to-serial adapter, you will probably need to use the EXTFSK plug-in in MMTTY
For either AFSK or FSK, you need some way to control PTT (TX/RX switching). If you use PTT control from N1MM Logger in other modes, the same method can be used in digital modes. Alternatively, you can control PTT from a serial port with a keying circuit. In FSK, the same port can be used for PTT and FSK

On the radio make sure:
Audio processing is off
Speech processing is off
Hi boost is off (Kenwood radios)
If the radio does not have an RTTY-specific mode, it should be in LSB mode
RTTY-specific modes may also be called FSK, AFSK, LSB-D, PKT-LSB, …
The most often used speed is 45.45 Baud (60 words/min)
Some contests use 75 Baud (100 wpm)
The standard shift is 170 Hz
Some TNCs use 200 Hz shift
When using a TNC set ‘Mark’ to 2125 Hz and ‘Space’ to 2295 Hz
Don’t overrun your finals from the transceiver and/or amplifier
50 percent duty cycle is mostly ok, but some transmitters may require reduced power in digital modes
See the Interfacing chapter for url’s and tips on interfacing
The BAUDOT character-set does not have all ASCII characters so some special characters such as (~ * _ @ # =)? can not be printed/transmitted
During contests narrow filters are normally used (250 – 500 Hz)

1.1. Common RTTY Frequencies

Contests USA (kHz) USA (kHz) Europe/Africa (kHz) Japan (kHz)
Common Common DX frequency Common Common
1800 – 1810/1835 – 1845 1800 – 1810 1838 – 1843 1838 – 1843
3570 – 3600 3580 – 3600 3590 3580 – 3620 3520 – 3530
7025 – 7100 7025-7050/7080 – 7100 7040 7035 – 7045 7025 – 7040
10120 – 10150 10140 – 10150
14060 – 14120 14080 – 14100 14080 – 14100
18100 – 18110 18100 – 18110
21060 – 21150 21080 – 21100 21080 – 21120
24910 – 24930 24920 – 24930
28060 – 28150 28080 – 28100 28050 – 28150

1.2. General RTTY Information

There are two aspects of RTTY which are often confusing to newcomers to the mode.

The first of these is the “polarity” of the signal. In FSK, there are two frequencies, conventionally called “mark” and “space”. In amateur RTTY, the mark frequency is the higher of the two RF frequencies. Someone who is transmitting with the opposite polarity is said to be transmitting “upside down”. His signal will be gibberish at the receiving station, unless the operator there inverts his receive polarity. When first setting up for RTTY, if you appear to be unable to decode any signals you receive, try inverting your receive polarity (in MMTTY, use the “Rev” button; in MMVARI, switch between RTTY-L and RTTY-U settings).

In FSK, getting the polarity right involves arranging things so that the switching conventions (does closing the keying input result in mark or space?) match between the radio and the computer. Unfortunately, the switching conventions are not universal. Fortunately, almost all radios affected by this have a menu item in the radio to reverse the keying polarity. Once this option is set correctly, the radio’s transmit RTTY polarity will be correct from then on. On receive, most if not all radios in FSK mode receive RTTY on the lower sideband. If software is used to demodulate the received signal, it must be set so that the lower of the two audio tones is converted to mark and the upper tone to space. This is the default configuration in software that supports FSK keying (like the MMTTY engine used in N1MM Logger). Note that in FSK, the transmit and receive polarities are determined independently, i.e. it is possible to receive correctly and yet to transmit upside down.

In AFSK, getting the polarity right involves coordination between the choice of audio frequencies generated in the sound card and the choice of sideband on the radio. The most common combination is to use lower sideband on the radio, combined with an audio tone pair in which the mark tone is the lower of the two audio frequencies (e.g. the most common pair is mark = 2125 Hz, space = 2295 Hz). The use of the lower sideband inverts these tones at RF to match the standard amateur convention. Software that uses the opposite convention (mark tone higher than space) is used with the radio in upper sideband. Fortunately, once the receive polarity is correct in AFSK, the transmit polarity will also be automatically correct.

The second sometimes puzzling aspect is related to the RTTY character set. The digital code used in RTTY predates the ASCII code used by modern computers. Instead of 8 bits, which allows for 256 different characters, the Baudot or Murray code used in RTTY has only 5 bits. This 5-bit code only has enough different characters for 26 letters plus 6 control codes, so to get numbers and punctuation the text has to be preceded with a special “FIGS” character (one of the 6 control codes) to get a second set of 26 characters (10 numbers plus 16 punctuation marks). FIGS is “sticky”, so there is another special “LTRS” character to switch back to the letters case.

Just like any other character, these FIGS and LTRS characters can be damaged by noise, QRM, QSB, etc., and if they are, the received info is displayed wrongly until the next LTRS or FIGS character (or in some situations, the next space character) comes along and sets things right. Sometimes the opposite happens – a text character is converted by noise into a FIGS or LTRS code, with similar results.

The most common problem that results is numbers being printed as letters, so with a bit of experience, many RTTY operators will get used to interpreting TOO as 599 and UE as 73. Serial numbers are slightly more difficult; PQW in the input data is most likely 012, and so on. You can see which letter corresponds to which number by comparing the top (QWERTY) row of letters on the keyboard with the numbers immediately above and to the left. Letters can also be printed as numbers and punctuation; for example, CQ TEST when converted to FIGS case becomes :1 53’5 .

Various software has different ways of helping out with this. When you run MMTTY stand-alone, if you right click on a “word” (delimited by spaces), the entire word changes to the opposite case. So, for example, VE4AEO is changed to ;3R-39 and vice versa. N1MM’s digital window has a box titled Letters/Figs for opposite-case display, that shows text that the mouse “hovers” over (no click necessary) in the opposite case. This requires you to move the mouse over the text that you want to convert; the unconverted text is displayed in the MouseOver box.Unshift on Space
There is a common feature called Unshift on Space (UOS or USOS) whose purpose is to deal with the lost {FIGS}/{LTRS} problem. It was designed for normal text, where the majority of information is alphabetic.

MMTTY has two UOS options. One of these is a button on the MMTTY main window that affects what you see in the receive window; the other is a setup option (under the Tx tab in the MMTTY setup) that affects what you transmit.

The receive option in the main window simply changes the receive window’s case back to {LTRS} at the beginning of every new “word”, i.e. after a space, unless of course the new “word” starts with {FIGS}. This takes no extra time, but improves reliability of receipt of alphabetic text.

The transmit option, on the other hand, actually transmits extra {FIGS} characters at the beginning of every numeric “word” to try to ensure greater reliability. It does not transmit an extra “LTRS” at the beginning of every alphabetic word, because using UOS on receive is a more efficient way to achieve the same end result.

When you are ragchewing, you should always use UOS on both receive and transmit. UOS assumes that the majority of “words” are alphabetic, which is true of normal text.

During contests, the receive UOS option is still helpful, especially when the exchange includes letters, and it does not cost anything. The N1MM Logger DI window’s “Letters/Figs” line can be used to deal with those cases where receive UOS converts an intended numeric field to letters.The transmit UOS option achieves greater reliability of numeric exchanges at the cost of some extra {FIGS} characters. If you are concerned about the slight speed penalty it imposes, you can leave transmit UOS on and use dashes (“-“) instead of spaces between all-numeric fields, e.g. 599-123-123 . The downside of using dashes in this way is that if the initial {FIGS} character is lost, the entire exchange will be in the wrong case, e.g. TOOAQWEAQWE. Sending spaces with transmit UOS on costs two extra {FIGS} characters but is more reliable (our example with an initial lost {FIGS} character becomes TOO 123 123). On the other hand, turning transmit UOS off results in 599 QWE QWE in any receiver using UOS, even with no errors at all. A compromise among all of these possibilities is to turn transmit UOS on and use a hybrid exchange: 599-123 123 (a dash instead of a space after the signal report, but spaces after that). A single {FIGS}/{LTRS} error will not prevent at least one copy of the exchange from being decoded correctly regardless of whether the receiving station is using UOS or not.

2.1. General PSK Information

PSK31 (and its higher-speed versions, PSK63 and PSK125) is an example of a “sound-card digital mode”, i.e. a digital mode that was made possible by the use of sound cards in PCs. The advent of sound cards in PCs made these sound-card modes available for anyone to use with a minimum of expense. All that is needed is an SSB transceiver, an audio interface (which can be as simple as patch cables, or can include isolation and attenuation controls) and a means of controlling PTT.

Conventionally, sound-card digital modes are communicated using USB, regardless of the band. Many PSK31 users set their radio’s dial to a standard frequency (14070.0 kHz is the most common) and then look for signals anywhere within their SSB filter bandwidth (e.g. from 250 Hz to 2750 Hz or so, which would correspond to transmitted frequencies from 14070.25 kHz to 14072.75 kHz). PSK31 signals are narrow-band, so there can be many different PSK31 signals simultaneously copyable within the available frequency range without changing the radio’s dial setting. Tuning is often done simply by clicking on the desired signal in the waterfall display.

PSK31 is short for “Phase Shift Keying, 31.25 baud”. There are also higher-speed versions, PSK63 (62.5 baud – seen fairly often) and PSK125 (125 baud – rare). Actually, in addition to using phase shift keying for modulation, PSK31 also uses amplitude modulation (“waveform shaping”) to minimize the bandwidth occupied by a signal. As a result, PSK31 places great requirements on the linearity of the equipment used, from the sound card generating the signal to the transmitter, and also the receiver. The peak power of a PSK31 signal can be approximately twice as high as the average power. If a transmitter is operated near its power handling capacity, it can clip these peaks, resulting in “splatter”, which shows up on the waterfall as extra “tracks” in addition to the two main modulation tracks that are normally visible. To avoid having this happen, the audio levels in the sound card and in the transmitter’s input audio stages must be controlled to avoid reaching power levels that would result in clipping. In most transmitters, this is equivalent to keeping the power below the level that would result in ALC action, and often this means powers below approximately half the transmitter’s maximum power rating.

Standard PSK31 (sometimes also called binary phase shift keying, or BPSK31) is sideband-independent. There is a rarely-used variation called QPSK31 (or QPSK63 for the 62.5 baud speed) that uses four phases instead of two (quadrature phase shift keying). This allows for some error correction while still delivering the same text speed. QPSK31 is sideband-dependent, i.e. the transmitting and receiving station must both be using the same sideband in their radios (by convention, upper sideband).

PSK31 works well even at low powers. In fact, once the transmitted power is sufficiently high to give an acceptable level of copy, there is no advantage to be gained by increasing power further. Unlike analog modes, where increasing power may make your signal louder relative to QRM and therefore easier to copy, increasing the power in PSK31 does not improve your signal’s readability. It can even degrade copy by overloading the other station’s receiver and creating splatter within the receiver. More importantly, a very strong signal will affect the AGC in every receiver that picks it up, causing the receiver gain to decrease and making copy of signals on other frequencies more difficult. For this reason, high-power operation is unpopular in PSK31.

When you plan to run PSK:
Keep your macros short.
PSK is about 1/3 slower than RTTY; you can really impact your rates with wordy macros
Use lower case letters wherever possible
PSK is a varicode mode. That means that characters contain a variable number of bits, unlike ASCII characters that have a fixed number of bits. Most lower-case PSK characters have fewer bits in them than their upper-case equivalents, so lower-case (in general) transmits faster

On the radio make sure:
Audio processing is off
Speech processing is off
Hi boost is off (Kenwood radios)
Radio should be in USB mode
Some radios have modes designed for digital sound card modes, e.g. PSK, DATA A
Transmitter linearity is extremely important in PSK
Keep power below 1/2 the transmitter rating to avoid clipping peaks
Avoid any visible ALC action (except in radios with ALC designed for PSK, e.g. Elecraft K3)
See the Interfacing section of this help for url’s and tips on interfacing
Using lower case letters instead of all caps will increase speed and reduce TX time
Only 100 Hz is needed as channel separation
Example filter usage:
Available 2.4 kHz / 250 Hz /100 Hz filter bandwidths
2.4 kHz is used for monitoring the PSK area of band when in search and pounce mode
250 Hz and 100 Hz filter bandwidths are used when in run mode

2.2. Common PSK and Digital Frequencies

PSK31 activity generally starts from the bottom edge of the IARU RTTY bandplan, expanding upwards as activity increases.

Band Digital Frequencies (kHz) PSK Frequency (kHz) Remarks
160 meter 1800 – 1810 / 1838 – 1843 1807 / 1838 1807 in Region 2
80 meter 3575 – 3585 3580
40 meter 7030 – 7040 / 7060 – 7085 7035 / 7080 7080 in Region 2
30 meter 10130 – 10145 10142 WARC, no contesting
20 meter 14065 – 14090 14070
17 meter 18100 – 18110 18100 WARC, no contesting
15 meter 21060 – 21090 21080
12 meter 24920 – 24930 24920 WARC, no contesting
10 meter 28110 – 28125 28120