Editing SOS (section)

===Audio tone signals and automatic alarms===
Ships and coastal stations would normally have required quiet times twice an hour to listen for priority signals, for 3&nbsp;minutes, at different times for [[500 kHz]] and [[2182 kHz]].

[[File:Radioroom.png|alt=Ship's radio room clock|thumb|Ship's radio room clock, with 4-second-long red bands with 1-second white gaps around the circumference, so the CW alarm signal could be sent manually. The red and green wedges denote compulsory 3-minute [[Radio silence#Silent periods|silent periods]] for receiving weak distress signals.]]

Since many merchant vessels carried only one or two radio operators, no one might hear a distress signal when both operators were off-duty. Eventually, equipment was invented to summon operators by ringing an alarm in the operator's cabin, and on the bridge, and the only switch able to disable the alarm was only permitted to be in the wireless telegraph room.<ref name="m-ust000002-0782">{{cite web |url=https://www.loc.gov/law/help/us-treaties/bevans/m-ust000002-0782.pdf |title=International Convention for the Safety of Life at Sea |date=31 May 1929 |website=Library of Congress }}</ref> The alarm was sent by the operator on the ship in distress transmitting the radiotelegraph alarm signal (auto-alarm) signal—twelve extra-long dashes, each lasting four seconds with a one-second gap between them, and transmitted in A2 (modulated CW).<ref>{{cite web |url=https://www.radiomarine.org/reports-from-nmo/500kc-procedures |title=Reports from NMO - 500&nbsp;kc Procedures |website=RadioMarine.org|author=Jeffrey Herman|date=1994}}</ref> The alarm signal was normally sent with a mechanical or electronic timing circuit to ensure it was sent accurately. However, ships radio room clocks typically had markings on the dial to guide operators in sending the signal manually. The regulations for the auto-alarm were defined in the 1927 ''Safety of Life at Sea'' (SOLAS) international maritime regulations, and in Article&nbsp;19, §&nbsp;21, of the General Regulations annexed to the ''International Radiotelegraph Convention'', 1927.5&nbsp;5.<ref name="m-ust000002-0782" />

The Auto Alarm receivers were designed to activate upon receiving four such dashes. Once four valid dashes are detected, the automatic alarm is activated. The distressed ship's operator would then delay sending the SOS message itself to give off-watch radio operators time to reach their radio room.

The [[Radiotelephone|radiotelephony]] equivalent of the radiotelegraph alarm signal is the radiotelephony alarm signal, which is the transmission of alternating tones of 2200&nbsp;Hz and 1300&nbsp;Hz, with each tone having a duration of 250&nbsp;ms. Automatic alarm systems aboard ships must activate when such a signal is received and the receiving vessel is within {{cvt|500|nmi|km}} of the transmitting vessel's position, or if the distress position is in the polar areas (latitude greater than 70°&nbsp;N or 70°&nbsp;S). The alarm should also activate when the call is received and the distance between the vessel in distress and the receiving vessel cannot be determined.<ref>{{cite web |url=https://www.itu.int/rec/R-REC-M.493 |title=Recommendation ITU-R M.493 Digital selective-calling system for use in the maritime mobile service |website=itu.int |publisher=International Telecommunication Union}}</ref>