Radio Frequency by Steve Winder and Joe Carr - HTML preview

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16.4 Standard PSTN tones

Signal UK
USA

Number unobtainable 400 Hz equency ContinuousSequence 500^equency 600 Hz+ Number busy 400 Hz 0.75 s on, 0.75 s off 480+ 620 Hz System busy 400 Hz 0.4 s on, 0.35 s off, 480+ 620 Hz

0.225 s on, 0.525 s off

Dial tone 50 Hz 350+ 440 Hz 33.3 Hz

Ringing tone 400 Hz, or 0.4 s on, 0.2 s off, 440+ 480 Hz 400+ 450 Hz, 0.4 s on, 2 s off
modulated by
17, 25, or 50 Hz

Pay tone 400 Hz 0.125 s on, 0.125 s off

Time tones 900 Hz 0.15 s on, 0.85 s off. Three times every three minutes
Recorder warning 1400 Hz Interrupted every second SequenceInterrupted once per second Interrupted twice per second

Interrupted once per second One burst, every 15 s