At about 1302 UTC on 12th May 2024, I tuned in to some HF aircraft transmissions on 6673 kHz.

The map below shows the major World Air Route Areas-HF frequencies. The 6673 kHz falls in the Central-East Pacific area with the control transmissions being in San Francisco, USA.

The tones that can be heard in the short video below are known as SELCAL tones. Selective calling (SELCAL) is a radio system that can alert an aircraft’s crew that a ground radio station wishes to communicate with the aircraft. The SELCAL system uses a ground-based encoder and radio transmitter to broadcast an audio signal which is received by a decoder and radio receiver on board an aircraft. SELCAL allows an aircraft crew to be notified of incoming communications even when the aircraft’s radio is muted. (Wikipedia)

References.

1. Wikipedia. (2022). SELCAL. [online] Available at: https://en.wikipedia.org/wiki/SELCAL [Accessed 8 Jul. 2024].‌