A blue box is an electronic device that produces tones used to generate the in-band signaling tones formerly used within the North American long-distance telephone network to send line status and called number information over voice circuits. This allowed an illicit user, referred to as a "phreaker", to place long-distance calls, without using the network's user facilities, that would be billed to another number or dismissed entirely as an incomplete call. A number of similar "color boxes" were also created to control other aspects of the phone network.
First developed in the 1960s and used by a small phreaker community, the introduction of low-cost microelectronics in the early 1970s greatly simplified these devices to the point where they could be constructed by anyone reasonably competent with a soldering iron or breadboard construction. Soon after, models of relatively low quality were being offered fully assembled, but these often required tinkering by the user to remain operational.
The long-distance network became digitized, replacing the audio call-control tones with out-of-band signaling methods in the form of common-channel signaling (CCS) carried digitally on a separate channel inaccessible to the telephone user. The audio-tone-based blue boxes were of limited use by the 1980s, and of little use today.
History
Automated dialing
Local calling had been increasingly automated through the first half of the 20th century, but long-distance calling still required operator intervention. Automation was deemed essential by AT&T. By the 1940s they had developed a system that used audible tones played over the long-distance lines to control network connections. Tone pairs, referred to as multi-frequency (MF) signals, were assigned to the digits used for telephone numbers. A different, single tone, referred to as single frequency (SF), was used as a line status signal.
This new system allowed the telephone network to be increasingly automated by deploying the dialers and tone generators on an as-required basis, starting with the busier exchanges. Bell Labs was happy to advertise their success in creating this system, and repeatedly revealed details of its inner workings. In the February 1950 issue of Popular Electronics, they published an advertisement, Playing a Tune for a Telephone Number, which showed the musical notes for the digits on a staff and described the telephone operator's pushbuttons as a "musical keyboard". Two keys on a piano would need to be pushed simultaneously to play the tones for each digit. The illustration did not include the tone pairs for the special control signals KP and ST, although in the picture the operator's finger is on the KP key and the ST key is visible. In the 1950s, AT&T released a public relations film, "Speeding Speech", which described the operation of the system. In the film, the tone sequence for sending a complete telephone number is heard through a loudspeaker as a technician presses the keys for dialing.
In November 1954, the Bell System Technical Journal published an article entitled "In-Band Single-Frequency Signaling", which described the signaling scheme used for starting and ending telephone calls for the purpose of routing over trunk lines. In November 1960, an article in the Bell System Technical Journal provided an overview of the technical details of signaling systems, and disclosed the frequencies of the signals.
The system was relatively complex for 1950s technology. It had to accurately decode the frequencies and ignore any signals where that frequency might be accidentally created; music playing in the background might randomly contain the SF tones and the system had to filter these out. To do this, the signaling unit compared the signal power from a bandpass filter centered on 2600 Hz to signal power in other parts of the audio band, and only triggered if the tone was the most prominent signal. The originating end of the call would play the tone into the trunk line when the call ended, and trigger the remote end to end the call. After a short time, the originating end reduced the tone level and continued to send tone as long as it received on hook status from its local equipment.