Digital Signals FAQ Version: 2.0               +--------------------------+
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Last Update: Nov 15, 1995                      |         |     |          |
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Copyright (C) 1995 Stan Scalsky                |  |||  ||||| |||||  |||   |
Copyright (C) 1995 Mike Chace                  | |||||||||||||||||||||||  |                       | ------------------------ |                             | D I G I T A L  R U L E S |
Changes made since Version 1.0
   * Expanded modes, added Baud Summary, References, System Parameters
   * added PSK section/Multi-tone section update/RS-ARQ update
   * DUP-ARQ-2 added/G-Tor/ARQ-E3 updated/Coquelet updated
   * VHF Selcal modes added/ACARS on HF/CROWD36/CIS updated.
   * VHF Modes information updated/Artor updated

This Signals FAQ is maintained by Stan Scalsky (
Any questions, comments or corrections will gladly be accepted. The author
implies no guarantee on this information and does not claim to be an
expert in this field. All information has been gathered from public domain
sources, manufacturers documents, decoder documentation, real time analysis
and any Radio related publication that cares to write about digital signals.
I have tried to research for correctness each mode listed. Many thanks to 
those of you who post logs, information and answer stupid questions in the 
various forums that cover digital signals - you know who you are.

The decoder manufacturer portion of this document is provided courtesy 
of Mike Chace (

I would like to solicit material, to be included later, on any other
analysis techniques and/or DF techniques the utilities community is 
currently using.

  * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
  A word of Caution:  the rules  about listening  to signals not intended for
  you  applies  here.  The  contents  of  many  signals  might  be considered
  sensitive by  the party  sending and  the reception  of such signals may be
  illegal in your country. The authors neither condone or encourage such acts
  * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *


                             TABLE OF CONTENTS

Section 0 - Introduction
Section 1 - Modes on Shortwave
Section 2 - Modes on VHF
Section 3 - Baud Rate Summary Table
Section 4 - System Parameters Summary Tables
Section 5 - What decoders are available
Section 6 - References


Section 0 - Introduction

This Signals FAQ is designed to give utility listeners a sampling of the
kinds of signals and sounds available on shortwave/vhf radio today along 
with information on the available equipment needed to understand, analyze
or identify these signals.

With conditions being dismal over the last few years coverage of Utility  
Listening, especially Digital Utilities, has been dropped from most of
the main stream shortwave magazines. But in and of itself, Digital Utility
listening is inherently more difficult than regular shortwave listening.
The possibility of decoding the signal recieved adds another level of
complexity.  This FAQ is an attempt to let those beginners who are thinking 
of or wondering what digitial signals can be received and decoded and maybe 
provide the more advanced listener with a little more information to identify
those unknowns. 

Here in lies the basic problem with Digital Utility Listening - lack of 
information. Many systems are used by Military or Diplomatic Services 
and information on the specifics of a particular mode are impossible to 
find, even from the manufacturer. Many are considered proprietary, but that 
doesn't mean that a signal can not be identified! With the proper tools a 
given signal can be identified via the way it sounds (aurally) or how it 
looks (visually). Most decoders that include some kind of signal analysis 
can ID a signal by bit-pattern or baud rate. Many signals have a unique baud,
i.e. 300 baud packet, 240 baud HC-ARQ or 164.5/218.3 RUM-FEC. Once a signal is
identified there are many decoders that can print the traffic for you but keep
in mind various kinds of encryption are commonly found in use with these 
signals. Encryption types include figure group or letter group messages and 
even random bit-masking or bitstream encryption, which looks like a continuous
stream of random characters. 

And now a word about decoders... 

There are many kinds of data decoders available ranging from public domain
packages to professional dedicated units. Prices vary from free up to very
expensive and price is dependent on how much you want to be able to decode 
and what tools are available for signal analysis and identification. Public
domain packages, while good, can not compete with the capability provided
by the more expensive dedicated data decoder unit. It is safe to say that
price goes up with increased capability in this market - be prepared to
spend some big money if you want to cover a lot of modes. A good rule of
thumb is that a top-spec decoder will cost as much as a top-spec radio, ie
upwards of 2,000 dollars. You'll also need to decide upon whether to buy a 
stand-alone decoder or one that requires a computer to run.  The latter option 
will of course increase the cost if you don't already possess a machine, but 
does add flexibility to a decoder.  See the Decoders reference for unit 
specifics on capability and pricing.

What should you look for in a decoder? Some useful features include:

        * Signal Identification
        * Accurate baud rate measurement
        * Correlation Bit Analysis
        * Variety in modes decoded/identified
        * Ability to save captured text (disk and printer)

You can't beat a good Signal Identification Mode - both the Wavecom units
and Hoka units include this option. A good Signal Identification mode 
simplifies the task of figuring out what mode is currently tuned, but keep
in mind that even the best Identification mode is not always 100% correct.
They can be fooled by systems that share common idle characteristics (for 
example: SWED-ARQ, SITOR-A and TWINPLEX or SITOR-B and POL-ARQ), the presence
of interference or a noisy signal. Universal decoders do not include an 
Identification mode.

Accurate measurement of baud rate is another vital capability. Many modes can 
be accurately identified on baud rate alone because many rates are unique to 
a select signal. It also provides the opportunity to "fingerprint" a signal,
system or the user.  For example, the Hoka decoders can measure baudrate
accurately to 3 decimal places. See the Baud Rate Summary Table for further

Correlation Bit is a technique that samples the incoming digitized bit stream
and presents the data as a graph of bit occurences plotted against time. This
will show when patterns occur within a signal giving you another piece of
information when working out an unidentified system. This kind of analysis
tool can also tell when there are NO patterns to a signal indicating an
encrypted or random bit-masked signal, allowing you to move quickly onto
more productive signals. Hoka and Wavecom decoders include correlation bit

Mode variety is a personal preference. I would like to have a module for
any mode I can receive in the spectrum! While not possible or realistic I
will take as many as I can get.  I find there is nothing more frustrating
than being able to receive a clean signal and then not being able to 
identify or decode it (ignoring the problem of encrypted signals for the
moment). See the manufacturers listing for the modes decoded by various

The ability to save decoded output to a file and/or the printer should be
considered a very important feature of any decoder. Having some form of
hard copy, on disk preferrably, allows for archiving for later reference
or later analysis and independant printing and editing. Hoka decoders have 
the ability to save decoded text to disk or output to the printer. I believe 
Wavecom units have a similiar ability.

I also like a responsive manufacturer who regularly updates their decoder in 
line with developments "on the air".  Variations on existing systems and 
completely new systems are still appearing today.


Section 1 - Modes on Shortwave

What modes are currently on shortwave. This section attempts to present a 
little information about each kind of signal that can be heard within the 
shortwave spectrum. Signals are grouped together by the way they sound. 
This is an attempt to narrow the field of possible signals when trying to 
identify an unknown. The typical baud rate(s) of the signal is mentioned, 
if known, and any other synonyms or possible names are given. But ... don't
make the assumption that these are ALL the modes you will ever hear. There 
are many signals that remain unidentified.

1-A. SINGLE TONE system. 

  Yup - still there, didn't know where else to stick this. Can't miss it, 
  very distinctive sound but can it be decoded? Most decoders say they can 
  decode morse but fast machine generated or slow, uneven hand generated 
  morse can be difficult to decode clearly.

        CW              Morse code still used by the Amateur community and
                        Marine operations. Speed varies depending on whether 
                        hand generated or machine generated.


  Signals of this type generally sound like SITOR-A - a distinctive chirping 
  sound is their main characteristic. Short SWED-ARQ sounds and is exactly 
  like Sitor-A. Idling TWINPLEX is the same as Sitor-A. To identify these 
  signals by ear may be impossible depending on which mode they are currently
  in and even a decoder that can determine signal type may get confused unless 
  traffic is being sent.

        ARQ6-70         6-character-block simplex ARQ using a different
                        alphabet and block timing from ARQ6-90/98.
        ARQ6-90/98      6-character-block simplex ARQ used by French and
                        Italian Diplo services, typically 200 bd. ARQ-6/90
                        and ARQ-6/98 differ in their inter datablock timing.
        SWED-ARQ        Swedish Adaptive simplex ARQ used by Swedish Diplo
                        services, typically 100 bd. Comes in the 3 packets
                        lengths: short, medium and long. Able to change 
                        mid transmission, depending on conditions, giving 
                        SWED-ARQ its adaptive capability. Also known as 
        TWINPLEX        4 frequency diplex system used by Interpol, Danish, 
                        Dutch, Pakistani MFAs, Spanish Diplo and UN services, 
                        typically 100 200 or 300 bd. This 2 channel system 
                        supports several different spacing/shift parameters
                        and word, bit, character or not-interleaved of the
                        channel characters. Reference Table 4-E.
        ARTOR           Adaptive Robust Transmission Over Radio - an adaptive,
                        error-free mode for HF using QPSK. Typical baud 
                        rates are 50, 100 or 200; automatically selected, ARQ
                        and FEC modes are supported.
        SITOR-A         The most common ARQ signal used by Amateur, Marine
                        and some Gov. Diplo services, typically 100 bd. Also 
                        Known as ARQ or TOR.
        SI-ARQ          Siemens Simplex ARQ used by Austrian and Indonesian
                        Diplo services, typically 96, 144, 192 or 200 bd.
                        Also known as ARQ-S or ARQ-1000-s.
        RS-ARQ/ALIS     Rohde & Schwarz simplex ARQ used by German and Italian
                        Diplo services, typically 228.7 bd but reports of 
                        457.0 have been noted. Now referred to in Klingenfuss
                        documents as ALIS. The marketing name for this system
                        is MERLIN and is a complete data-over-radio system and
                        can transparently handle many types of data. Con-
                        sequently it has many modes. The most commonly heard
                        mode is the 228.7 FSK system and a 7 tone 240 bd burst
                        ARQ mode. See Klingenfuss Radioteletype Code Manual
                        13th Ed. for more information.
        DUP-ARQ         A semi-duplex ARQ system used by Hungarian Diplomatic
                        services, typically 125 bd. If a DUP-ARQ system detects
                        interference it will change frequency in 400Hz steps. 
                        If a 3kHz channel is full of interference the system 
                        will select another frequency. Also known as ARTRAC, 
                        ARS-GUARD or 125-ARTRAC.
        HC-ARQ          Haegelin-Cryptos simplex ARQ used by UN and Red Cross
                        services, always 240 bd.
        IRA-ARQ         Duplex ARQ with IRA (ITA-5), used by Czech/Slovak
                        Diplo stations, typically 171.42, 200.2, or 300.3 bd
        PACTOR          A system designed with a combination of packet and
                        sitor techniques used by Amateurs, ICRC, UNO and 
                        MARS stations. The ICRC and UNHCR are the only users 
                        of a different form of pactor - this form, which 
                        modified part of the packet structure for privacy 
                        reasons is commonly referred to as UN-pactor or ICRC-
                        pactor. A Pactor Level II signal features 2 tones 
                        w/200Hz shift using baud rates of 100 or 200 fitting 
                        into a 500Hz channel. The system can handle raw 8 bit
                        data and ASCII compression. Depending on band conditions
                        the data throughput can be increased by changing the 
                        modulation form used: bad conditions start with BPSK 
                        (2-PSK) w/QRG diversity, then D-BPSK without diversity,
                        then D-QPSK (4-PSK) and finally D-QPSK at 200 baud. 
                        This puts the maximum throughput at 800 bits/sec.

                (continued on next section)

  • Detailed info on Decoder modes & features PART B