[SOC] MCW - modulated continuous wave , A2 now A2A All you ever wanted to know and more.

[D.J.J. Ring, Jr.]

In case any are wondering why ICW would ever be used, the reason is that
immediately before the introduction of CW or Continuous Wave, radio (or
Wireless Telegraphy) signals were generated by spark or an high voltage
electric arc which both have a rough modulation which was extracted by
radio detectors like galena crystals which fed headphones.  When CW came on
the scene, it didn't produce any tone, so ICW was improvised to provide
modulation so CW would be detectable by these type detectors.  Ships
continued to carry spark transmitters and automatic distress alarm
receivers which had to be compatible with both modulated (spark, arc and
modulated continuous wave) and unmodulated (Continuous Wave) signals.

OK, Wake up! Your eyes are glazed over!

73

David N1EA
SOC Forever

On Fri, Aug 24, 2018, 14:58 g4jht <g4jht at eircom.net> wrote:

> All you ever wanted to know and more about MCW - modulated continuous
> wave or A2, now A2A.
>
> MCM stands for modulated continuous wave. was A2 now A2A.
> A2 was required to be used on Marine MF telegraph transmitters and
> lifeboat sets.
>
> How is it produced:
>                     The  RF  carrier is modulated with an audio tone in
> the range 500 to 800 cycles at a depth of 80 to 100% and then keyed.
> Usually using a Class C PA choke modulated with self oscillating modulator.
> Advantage of MCW is you can hear it on an ordinary (diode) envelope
> detector such as a crystal set without a BFO. The ITU state such a
> signal keyed at 35 WPM will not have a band width wider than 2.1 Kc/s
> (kHz). This was a development of ICW.
>
> There is another type of MCW used for MF D/F beacons. Here the carrier
> runs continuously and only the audio tone is keyed with the beacon
> callsign. Unfortunately this also covered by the ITU definition for A2A,
> but fairly obviously when you tune one in.
>
> ICW pre-dates MCW and was produced by interrupting the HT supply to the
> PA (again probably Class C) by means of a "tone wheel". The tone wheel
> had insulated segments and a brush, which chopped the supply to
> producing a square wave at an audio rate depending on the number of
> segments and the speed of the disk in revs per second. The disadvantages
> of this system were it required maintenance,  of the brush and segment
> hight like a commutator. the note produced was rather rough so difficult
> to listen to and the bandwidth would be wider than MCW, which replaced it.
>
> Hope that helps 73,
>                     Dave  G4JHT/EI0DB/VP8ART  2600 nautical miles East
> of the Brooklyn Bridge.
>
> Friends all over the globe by skywave W/T; CW the one true art; in an
> over digiefied world.
>
>
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