The ARC transmitters had long established the use of the expression Continuous Wave and that was adopted for the new generation of VALVE transmitters also. As you will have read on this site, the BACK SHUNTED ARC transmission achieved exactly the same as a VALVE transmitter transmitting an A1 (CW) Morse code signal. In both cases a receiver with a local oscillator (which today we call a BFO), don't confuse it with our modern day local oscillator which creates the Intermediate Frequency (IF) of the receiver was required to heterodyne to achieve the audio frequency of the Morse on/off tone.

  Many of these valve set were introduced in the early to mid 1920's and at that time Continuous Waves were considered different to how we now consider them.

  Type A1 Continuous Wave, un-modulated, key controlled. Continuous Waves of which the amplitude and/or frequency is varied by the operation of keying in telegraphic transmission. In a valve set the keying reduces the amplitude from a constant value to zero i.e., it gives amplitude variations. An example of frequency variation is the marking and spacing method employed in an ARC transmitter. A1 would have been the used almost continuously on the shore transmitters in the 20's Series.

  An alternative method to produce a kind of Type A1 is to impress the AF tone variation of the telegraphic signal (Morse signal) on the Amplitude or Frequency of the RF being radiated. An example of this has already been seen in a SPARK transmission. The damped waveform radiated from a Spark transmitter falls under the class known as 'Type B' waves, and defined as waves forming successive wave trains, in each of which the amplitude after reaching a maximum progressively decreases.

  Type A2 Continuous waves in which a variation of amplitude and/or frequency is made in a periodic manner at AF and key-controlled for the purposes of telegraphic communications. This type is normally referred to as Interrupted Continuous Waves (ICW) and in the particular case where the variation of amplitude or modulation is sinoidal, the kind of ICW is called 'TONIC TRAIN.'

  The production of Type A2 or ICW with a valve transmitter was variable and depended upon the type of transmitter being employed. It is not our intention to list the circuitry changes in each set necessary to produce the variations, suffice to say that our forebears would have been familiar with terms like:-

    Single Pulse ICW (See Type 36S transmitter for example)
    Double Pulse ICW (See Type 36S transmitter for example)
    Chopped CW

  and all produced by varying the HT (High Tension) voltage applied to the anode of the valve.

  ICW was a well known emission, it finally being used in the 601 Series of transmitters which were designed during WW2 leaving the Service in the 1960's. In the 601 Series (602E), ICW was used as an emergency keying emission, where the set was powered by a battery and there wasn?t enough power for components like smoothing capacitors, so an un-smoothed HT was keyed on and off. It produced a very distinctive Morse code note and took up a sizeable amount of bandwidth.

  Type A3 Continuous waves in which a variation of amplitude and/or frequency is made in accord with the characteristic vibrations of speech or music.