For a good many years, operators manuals and technical documents referred to modulation emissions as "A1A", "J3E", "F3E" etc. Most people only learned that "F3E" is "FM" and so on without finding out why. I have dug a little deeper and found the why behind it.

The International Telecommunication Union (ITU) uses a system for classifying radio frequency signals. Each type of radio emission is classified according to its bandwidth, method of modulation, nature of the modulating signal, and type of information transmitted on the carrier signal. It is based on characteristics of the signal, not on the transmitter used.

The full emission designation is of the form BBBB 123 45, where:


The bandwidth frequency is expressed as three digits and a letter. The letter indicates what unit of frequency is used to express the bandwidth: H indicates hertz, K indicates kilohertz and M indicates megahertz.

If the frequency is a whole number the letter would be at the end: "025K" means 25 kHz, if the frequency is not a whole number the letter occupies the position normally used for a decimal point: "12K5" means 12.5 kHz.

Type of modulation.

Letter  Type  

    A      Double-Sideband Amplitude Modulation (e.g. AM broadcast radio).

    B      Independent Sideband (two sidebands containing different signals).

    C      Vestigial Sideband (e.g. NTSC).  

    D      Combination of A and F or A and G.

    F       Frequency Modulation (e.g. FM broadcast, some PMR and Amateur Radio).

    G      Phase Modulation.  

    H      Single Sideband with full carrier.

    J        Single Sideband with suppressed carrier (e.g. Shortwave utility and Amateur Stations).  

    K       Pulse Amplitude Modulation.  

    L       Pulse Width Modulation.  

    M      Pulse Position Modulation.  

    N      Unmodulated carrier.

    P      Sequence of pulses without modulation.  

    Q      Sequence of pulses with phase or frequency modulation within each pulse.

    R      Single Sideband with reduced carrier.

    V      Combination of Pulse Modulation methods.   

   W      Combination of any of the previous.

    X       None of the previous.

Modulating Signal.

Number       Type  

     0            No modulating signal.

     1              One channel containing digital information no subcarrier.

     2              One channel containing digital information using a subcarrier.

     3              One channel containing analogue information.

     7              More than one channel containing digital information.  

     8              More than one channel containing analogue information.

     9              Combination of analogue and digital channels.  

     X              None of the previous.

Type of transmitted information.

 Letter             Type  

      A            Aural Telegraphy, intended to be decoded by ear, such as Morse Code.  

      B            Electronic Telegraphy, intended to be decoded by machine (RTTY and digital modes).  

      C            Facsimile (Still images).  

      D            Telemetry or Telecommand (Remote control or data collection).  

      E            Telephony (voice or music intended to be listened to by a human).  

      F            Video (television signals).  

      N            No transmitted information.  

     W            Combination of any of the previous.

      X            None of the above.

Details of information.

     Letter        Type

       A              Two-condition code, elements vary in quantity and duration

       B              Two-condition code, elements fixed in quantity and duration

      C            Two-condition code, elements fixed in quantity and duration, error-correction included

       D               Four-condition code, one condition per"signal element"

      E            Multi-condition code, one condition per "signal element"

      F            Multi-condition code, one character represented by one or more conditions

      G            Monophonic broadcast-quality sound.

      H              Stereophonic or quadraphonic broadcast-quality sound.

      J            Commercial-quality sound (non-broadcast).

      K             Commercial-quality sound -- frequency inversion and-or "band-splitting".employed.

      L            Commercial-quality sound, independent FM signals, such as pilot tones used to control the demodulated signal.

      M           Greyscale images or video.

      N           Full-color images or video.

      W          Combination of two or more of the above.

      X           None of the above.


 Letter  Type

    C       Code Division Multiplexing (excluding spread spectrum).  

    F       Frequency Division Multiplexing.  

    N       None used.

    T       Time Division Multiplexing.

   W       Combination of Frequency Division and Time Division.

   X       None of the above.

Common Examples.


Signalling by keying the carrier directly CW or On Off Keying currently used in Amateur Radio. This is often but not necessarily Morse Code.


Signalling by keying a tone modulated onto a carrier so that it can easily be heard using an ordinary AM receiver - as previously used for station idents of some RDF transmissions. This is usually but not exclusively Morse Code.


AM speech communication - as used for Aeronautical VHF communications.


FM speech communication - as used for Marine and many other VHF communications.


Continuous, unmodulated carrier - as previously commonly used for radio direction finding (RDF) in marine and aeronautical navigation.


SSB speech communication - as used on HF bands by marine, aeronautical and amateur users

A3E or A3EG

Normal AM broadcast - as found on MW radio


FSK telegraphy, such as RTTY

F8E or F8EH

Normal FM stereo broadcast - as found on VHF FM band

Note: There is some overlap, so a signal might legitimately be described by two or more designators. In such cases, there is often a traditionally preferred designator.

All above courtesy of Brian Perrett and Highfield Amateur Radio Club

Types of Modulation

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