Editing Blue box (section)

==Frequencies and timings==
Each multifrequency tone consists of two frequencies chosen from a set of six, shown in the table on the left. The [[Touch Tone]] encoding is shown by the table on the right:
<div style="float:left">
{{Multi-frequency signaling tones}}
</div>
<div style="float:right">
{| class="wikitable" style="text-align:center"
|+Customer-dialed Touch-Tone ([[Dual-tone multi-frequency|DTMF]]) frequencies
|-
!
! 1209{{nbs}}Hz
! 1336{{nbs}}Hz
! 1477{{nbs}}Hz
! 1633{{nbs}}Hz
|-
! 697{{nbs}}Hz
| 1
| 2
| 3
| A
|-
! 770{{nbs}}Hz
| 4
| 5
| 6
| B
|-
! 852{{nbs}}Hz
| 7
| 8
| 9
| C
|-
! 941{{nbs}}Hz
| *
| 0
| #
| D
|}
The rightmost column is not present on consumer telephones.
</div>
{{Clear}}
Normally, the tone durations for passing numbers from machine to machine in a "speed dialing" format are on for 60{{nbs}}ms, with 60{{nbs}}ms of silence between digits. The 'KP' and 'KP2' tones are sent for 100{{nbs}}ms. KP2 (ST2 in the [[R1 standard]]) was used for dialing internal Bell System telephone numbers. However, actual tone durations can vary slightly depending on location, switch type, and the machine status.

For operators, technicians, and blue box phone phreakers, the tone durations would be set by how long the buttons were held down and, for silence, how long before manually pressing the next button.

A blue box could have been constructed which would send the tones with machine to machine timing, with the number either stored in digital memory or a matrix of switches. In the switch matrix, there might be 10 rows for digits, each with 5 switches. Two switches would be moved to on, selecting the 2 tones. (KP and ST would be hard wired.) The 5 switches could be labelled 0, 1, 2, 4, and 7, with the user selecting pairs of switches adding to each digit, with special case 4 plus 7 for digit 0.

Alternatively, the tones could be recorded on magnetic tape, which would be cut into pieces and spliced together, using a commercial splicer for accurate alignment. If the phreaker matched machine dialing and recorded at 7.5 ips (inches per second), the splices for tone and silence would be about 1/2-inch long., with KP 3/4-inch long. For more manageable splicing lengths, the phreaker could use a 15 ips tape recorder, which was less common, and double those lengths. For those without a 15 ips machine but having 2 tape recorders, the tones could be recorded an octave low at 7.5 ips, the pieces spliced together would be were double those lengths. The spliced tape would be re-recorded from a 7.5 ips machine to a 3.75 ips machine. The resulting recording could be played back at 7.5 ips. An interval of 2600{{nbs}}Hz, to disconnect the trunk, followed by an interval of silence, to give enough time for a digit receiver to connect, would be added to precede KP.

This set of MF tones was originally devised for [[Bell System]] [[switchboard operator|long-distance operators]] placing calls manually, as well as machine to machine dialing, and predates the DTMF ''Touch-Tone'' system used by subscribers. The leading 1 for customer dialed long-distance calls was not dialed. For operators, the line was muted during dialing, but, for customer telephones, it was only muted while a key was pressed. The Touch Tone frequencies were chosen to minimize the risk of customer talking while dialing, or background sounds, being registered as a digit or digits and resulting in a wrong number. Muting guarded against that happening during operator dialing, so the MF system did not have to be, and was not, so robust. The tones have a simple 200{{nbs}}Hz spacing. For Touch Tone, harmonic relationships and intermodulation products were taken into account in the choice of tones.