5 ms·

Two tones together are not illegal in general. However... think about 1950's cold-war era technology, and how much you could do with it. One of the conditions

Two tones together are not illegal in general. However... think about 1950's cold-war era technology, and how much you could do with it. One of the conditions of license of being a broadcaster was/is to listen to an upstream station, and with automated equipment, detect that tone and switch your own transmitter into emergency mode and propagate that tone pair if you are an intermediate station. So, not illegal in general, but illegal to transmit it on broadcast channel. I would even bet that if you searched E-Bay long enough, you could find a 1950's-era vacuum tube AM radio with tone-squelch tuned to those tones so that you could leave the radio on and just hear the duck-and-cover message without the distraction of that nasty Rock-n-Roll music that the kids are all listening to.

kstrauser 4y ago
They didn't play the tones through the announcements, though, did they? I don't remember them doing that. Wouldn't it need to be continuous for tone squelch to work?
- throwanem 4y ago
  They do not. An EAS message broadcast begins with a SAME header, which is followed by iirc about 30 seconds of the attention signal and then a verbal description of the alert (with onscreen text for TV broadcasts). After that, the message broadcast concludes with a SAME footer. The SAME header and footer are each repeated three times, and consist of BFSK-modulated data on a couple of frequencies I forget offhand. (If you're interested, they're specified in Section 31 of the regulation I linked elsewhere in this thread.) The header encodes the type and geographical applicability of the alert, and EAS receivers configured for the applicable area use it to automatically interrupt broadcast with the bulk of the alert. The SAME footer encodes no data but serves as an "end of message" flag; EAS receivers can use that to resume normal broadcast depending on configuration and the nature of the alert. (Sufficiently severe alerts may be repeated in entirety.) CONELRAD didn't use any in-band signaling of this type, in any case. The technology wasn't really there for it, and in any case the whole point of the scheme was not to continue transmitting from any one station for a significant length of time, in order to deny Soviet bombers navigation references which would help them drop nuclear bombs on American cities.
throwanem 4y ago
You're probably thinking of CONELRAD, the predecessor of the Emergency Broadcast System, which itself predeceased EAS. https://en.m.wikipedia.org/wiki/CONELRAD https://en.m.wikipedia.org/wiki/CONELRAD

kstrauser 4y ago

They didn't play the tones through the announcements, though, did they? I don't remember them doing that. Wouldn't it need to be continuous for tone squelch to work?

throwanem 4y ago
They do not. An EAS message broadcast begins with a SAME header, which is followed by iirc about 30 seconds of the attention signal and then a verbal description of the alert (with onscreen text for TV broadcasts). After that, the message broadcast concludes with a SAME footer. The SAME header and footer are each repeated three times, and consist of BFSK-modulated data on a couple of frequencies I forget offhand. (If you're interested, they're specified in Section 31 of the regulation I linked elsewhere in this thread.) The header encodes the type and geographical applicability of the alert, and EAS receivers configured for the applicable area use it to automatically interrupt broadcast with the bulk of the alert. The SAME footer encodes no data but serves as an "end of message" flag; EAS receivers can use that to resume normal broadcast depending on configuration and the nature of the alert. (Sufficiently severe alerts may be repeated in entirety.) CONELRAD didn't use any in-band signaling of this type, in any case. The technology wasn't really there for it, and in any case the whole point of the scheme was not to continue transmitting from any one station for a significant length of time, in order to deny Soviet bombers navigation references which would help them drop nuclear bombs on American cities.

throwanem 4y ago

You're probably thinking of CONELRAD, the predecessor of the Emergency Broadcast System, which itself predeceased EAS. https://en.m.wikipedia.org/wiki/CONELRAD https://en.m.wikipedia.org/wiki/CONELRAD