NHK transmitted Hirohito’s surrender recording nationwide on August 15, 1945.
NHK was Japan’s national broadcasting organization, then known as the Japan Broadcasting Corporation. Its technicians recorded Hirohito inside the Imperial Palace on August 14 and prepared the recording for transmission the following day.
The broadcast was scheduled for noon Japan Standard Time. NHK announcers introduced the message, and Hirohito’s voice followed in a recording rather than a live speech. The recording had been protected during the Kyūjō coup attempt, when rebel officers tried to stop the announcement.
“Radio Tokyo” is a common wartime nickname for Japan’s international shortwave broadcasts, but it is not the best answer to this question. The domestic network carrying Hirohito’s message was NHK’s predecessor organization. The broadcast marked the first time most Japanese civilians heard the Emperor’s voice.